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Volume 59 2004/2
Can the Spanish Influenza Pandemic of 1918 Explain the Baby Boom of 1920 in Neutral Norway ?
Svenn-Erik Mamelund [*] Svenn-Erik Mamelund, Department of Economics, University of Oslo, P.O. Box 1095 Blindern, 0317 Oslo, Norway, Tel: + 47 22 85 51 26, Fax: + 47 22 85 50 35,
Two years after the First World War ended there was a surge in European birth rates, including in Norway that had been a neutral country. This paper tests the hypothesis that it was in fact the Spanish influenza that caused the Norwegian baby boom rather than the close of the war. The paper uses multivariate regression analysis, while previous studies have been univariate and largely descriptive. By using regional monthly data, the independent effect of the Spanish influenza morbidity on fertility over the years 1918-1920, net of the effect of mortality, is estimated. The fact that Norway was neutral was important in counterbalancing the influence of the war on fertility and nuptiality. Furthermore, the Norwegian data utilized in the analysis are of superior quality in a European context in that registration of population data, including vital statistics, continued normally in Norway undisturbed by the war.
Deux ans après la fin de la première guerre mondiale, les taux de natalité ont enregistré une vive progression en Europe, y compris en Norvège, pays resté neutre lors du conflit. Cet article tente de valider l’hypothèse selon laquelle la responsabilité du baby-boom en Norvège revient plutôt à la grippe espagnole qu’à la fin de la guerre. Alors que les recherches antérieures ont reposé sur des analyses univariées et essentiellement descriptives, cette étude s’appuie sur des analyses multivariées. L’effet propre de la morbidité due à la grippe espagnole sur la fécondité, en contrôlant l’effet de la mortalité, a été estimé pour la période 1918-1920 à partir de données mensuelles régionales. En raison de la neutralité de la Norvège, la guerre n’a pas interféré avec l’effet de la grippe espagnole sur la fécondité et la nuptialité. De surcroît, les données disponibles sont parmi les plus fiables d’Europe, étant donné que l’enregistrement des données démographiques, y compris d’état civil, n’a pas été perturbé par le conflit.
Dos años después del fin de la primera guerra mundial, las tasas de natalidad han aumentado considerablemente en Europa, incluso en Noruega, país neutro durante la guerra. Este articulo intenta verificar la hipótesis según la cual el baby-boom en Noruega fue provocado mas bien por la gripe española que por el fin del conflicto. Contrariamente a los análisis precedentes que se han basado en análisis univariados y esencialmente descriptivos, este estudio se apoya en análisis multivariados. El efecto propio de la gripe española sobre la fecundidad, controlando el efecto de la mortalidad, ha sido estimado para el periodo 1918-1920 a partir de datos regionales. Dada la neutralidad de Noruega, la guerra no ha interferido con el efecto de la gripe sobre la nupcialidad y la mortalidad. Los datos utilizados figuran entre los más fiables de Europa, ya que su registro no ha sido perturbado por el conflicto.
The baby boom of 1920 has received less attention than that following the Second World War, and was for long thought to be a simple catching up of the marriages and births that the war had prevented, as part of a gradual return to normalcy. Yet this baby boom occurred with similar intensity in nonbelligerent countries. Other lines of enquiry thus need to be explored. In this article dealing with the large number of extra births registered in 1920 in Norway, a country that was neutral during the war,Svenn-Erik Mamelund
examines the hypothesis, sometimes proposed but never verified, concerning an influence of the Spanish flu epidemic of 1918 on the decline in fertility in 1919 and its strong upswing in 1920. This particularly virulent epidemic is known to have touched nearly a quarter of the world’s population and to have caused between five and ten times as many deaths as the war. Because influenza was a notifiable disease in Norway, the author is able to examine on a monthly and regional basis the influence on fertility of the morbidity and mortality attributable to the flu. He concludes that the flu pandemic was indeed the main cause of the Norwegian baby boom of 1920 and suggests that it probably had a similar influence in other countries.
The European countries that were massively affected by the First World War, whether by sending soldiers to the front, fighting the war on their own soil, or both, experienced a sharp decline in fertility during the period 1914-1918 (Chesnais, 1992). Figure 1 shows a steep decline of fertility in Italy, France and Germany during the war. The fertility curves for other warring nations, for example AustriaHungary, England and Wales, Scotland, Ireland, and Bulgaria, not shown, were very similar. The low fertility in the belligerent countries during the war is probably explained by the separation of soldiers from their wives and by the war having hindered young adults from marrying. In addition, it is reasonable to assume that those who were not enrolled in the armed forces postponed marriages and births from fear of the war and its consequences. The war thus created a huge potential for a compensating later resurgence in marriages and births as well as a “moral obligation to replace the deceased”. Indeed, birth rates in most belligerent countries began to rise once again in 1919 (for France in 1917), so that by 1920 they had equalled or surpassed the pre-war level.
Figure 1
Crude death rates (CDR) and crude birth rates (CBR) for three belligerent countries, Italy, Germany, and France, 1905-1930 (per 1,000)
Source: Chesnais, 1992.
The fertility patterns of nations that remained neutral, Norway, Sweden, the Netherlands, Denmark, Switzerland and Spain, were not affected in the same way (Chesnais, 1992). Figure 2 shows that the birth rates in Norway, Sweden, and the Netherlands declined relatively smoothly and continuously throughout the war until 1919. (The pattern of these countries is also reasonably representative of Denmark, Switzerland, and Spain.) This decline, however, is typical of the transition from higher to lower birth rates that have marked all societies throughout Europe. Hence, no substantial decline in birth rates linked to the war seems to have occurred in neutral countries. Nonetheless, like the belligerent countries, neutral countries faced a baby boom in 1920 (Figure 2).
Figure 2
Crude death rates (CDR) and crude birth rates (CBR) for three neutral countries, Norway, Sweden, and the Netherlands, 1905-1930 (per 1,000)
Source: Chesnais, 1992.
Surprisingly, there are few studies that have analysed the baby boom in Europe after the First World War in any depth, in contrast to the more famous baby boom following the Second World War. Perhaps a natural explanation for the 1920 baby boom, especially for the warring nations, was the gradual return to normal patterns of life after the war (Henry, 1966; Winter, 1977). Yet despite this, the birth rates in 1919 seem to be far lower than prewar figures. Could there have been other factors at that time to cause people to postpone the start of a family ? There is also a clear discrepancy between birth rates in 1919 and those of 1920 that perhaps goes beyond the most immediate explanation, namely the First World War.
This article directs attention to a hitherto little appreciated factor, the Spanish influenza pandemic. It spread around the globe in three waves in 1918. The first bout of influenza appeared during the months from March to May, but at this point in time the disease was not very contagious and it claimed few lives. In mid-June, however, the influenza returned, spread fast, and reached pandemic proportions. Many came down with the flu during the summer wave from July through September, but still relatively few died from it. The Spanish influenza made a third appearance during the autumn from October through December. The influenza virus had mutated since the relatively mild summer outbreak and was now highly lethal to those who were infected. However, those with a previous infection were partially protected against later waves due to the acquisition of immunity.
Spanish influenza affected at least 500 million persons or over one fourth of the world’s population at that time (Laidlaw, 1935). According to the most recent revision and update the global death toll was between 50 and 100 million (Johnson and Mueller, 2002). Hence the number of victims of Spanish influenza exceeded the number of casualties from the First World War by five to ten times. People of all ages experienced an increase in mortality, but those who suffered the most were people in the fertile ages of 20 to 40 years who normally have little to fear from influenza. In addition to influenza, pneumonia was the most important cause of death during the pandemic. Figures 1 and 2 show for six European countries the great impact of the Spanish influenza on the crude death rates in 1918. Table 1 shows that the death rates from Spanish influenza were relatively homogeneous across Europe, ranging from 3.3 to 7.3 deaths per 1,000, and with no notable difference between neutral and belligerent countries. Exceptions are the relatively high death rates in neutral Spain and in belligerent Italy, Hungary, and Portugal.
Table 1
Estimated number of deaths and death rate (per 1,000) from the spanish influenza, selected european countries
Höijer (1959) was one of the first to suggest that the Spanish influenza of 1918 may have caused the baby boom in Europe in 1920. He specifically showed for neutral Sweden that there was a stunning decline in the monthly conception rates (birth rates set back nine months) when the death rates peaked during the autumn of 1918, resulting in low birth rates during the summer of 1919 and consequently high birth rates in 1920. However, the author did not undertake a detailed analysis of the monthly conception and death rates, and did not discuss the relationship between these factors in any greater detail. Höijer’s work did show, however, the importance of analysing this phenomenon on a monthly basis.
Other studies that have considered the hypothesis that Spanish influenza in 1918 caused the baby boom of 1920 have also been univariate and descriptive (Pool, 1973; Rice, 1983; Underwood, 1983; Mills, 1986; Johnson, 2002). Although previous studies have acknowledged that the increase in influenza morbidity and mortality may have affected fertility negatively, none was able to determine whether morbidity had an independent effect on fertility net of the effect of mortality between 1918 and 1920. Nor could previous studies determine whether morbidity had a specific effect on fertility during the three outbreaks of influenza in 1918. In contrast to previous studies, this article attempts to separate the effects of morbidity and mortality on fertility across time and space using cross-sectional data. The reason why this separation is important is that the time it takes for a couple to resume sexual relations after an interruption due to disease — and thus the time it takes to achieve a conception — would be shorter than for a person who has lost a spouse to influenza.
The main hypothesis that is tested in this article is that the 1918 flu caused the baby boom of 1920 in Norway. In order to do so, we use multivariate models to analyse monthly morbidity, mortality and fertility data for 37 rural and urban areas in 20 Norwegian counties. A study of Norway should be valuable for three reasons. First, the country was neutral throughout the war, and this is assumed to control for the influence of the war on fertility and nuptiality. Indeed, as the number of military casualties was insignificant in Norway (Mamelund, 2003b), the war did not cause a “lost generation” and thus a “demand” to replace the dead and to sustain the population by giving birth. In discussing the case of Great Britain, Johnson (2002, p. 230) states that “quantifying [the decline and subsequent increase in births], particularly the portion due to influenza and that due to the effects of war, is virtually impossible”. Johnson explains this by “the fact that both events fell so heavily on the same segments of the population, the young adults. Further, the disruption of population by war rendered the basic population data unreliable” (p. 231). The second asset of Norway is that registration of population data, including vital statistics, continued normally and was on the whole not affected by the war. The third asset is that influenza was a reportable disease in Norway in 1918 in contrast to most other countries. Neutral Norway is thus one of very few countries for which it is possible to carry out a multivariate study to estimate the effect of influenza morbidity on fertility net of the effect of war and influenza mortality over the years 1918-1920.
The present article first presents the framework of the demographic developments during a mortality-fertility crisis and discusses the underlying reasons why fertility was affected by the Spanish influenza, with special reference to the Norwegian case. After a description of the data and the indicators used in the study, we then proceed to describe the course of fertility and nuptiality from 1914 to 1918 in Norway. The effects on fertility and mortality that the outbreak of Spanish influenza may have had are thoroughly examined. Finally, we present the results of a multivariate analysis using least squares regression models.
I. The time profile of a combined mortality-fertility crisis
1. The general framework
The general framework that describes the demographic changes that commonly occur during and after a combined mortality-fertility crisis is based on work by Juhasz (1971), Preston (1978), Menken et al. (1981), Watkins and Menken (1985), Palloni (1988), Wrigley and Schofield (1989), Livi Bacci (2000), and in particular Lee (1989, 1990). There is fair agreement in the literature that four phases can be distinguished in connection with epidemic crisis mortality:
Phase 1: A sudden increase in morbidity and mortality is accompanied by a drop in coital frequency and conceptions below the normal level.
Phase 2: Coital frequency and conceptions drop further and reach the lowest relative levels when morbidity and mortality peak. The increase in the number of marriages dissolved by death leads to a further decline in coital frequency and conceptions.
Phase 3: A drop in morbidity and mortality either at or below precrisis level (due to decrease in the number of non-immune individuals and to negative selection of the frail) is followed by a surplus of conceptions compared to the normal level as couples compensate for the deficits in Phases 1 and 2 and “replace” those children who have died.
Phase 4: Fertility rebounds a year or two after the crisis to levels higher than the normal pre-crisis level (because of effects of compensation and replacement).
2. The Spanish influenza and fertility between 1918 and 1920 in Norway
Negative perceptions about the future may lead to voluntary postponements of births (Lee, 1989; Menken et al., 1981). It is conceivable that the fear induced by the pandemic itself (people could die within a three-day period) as well as a fall in social integration (the closings of schools, churches, theatres, and the banning of public meetings so as to prevent the spread of the disease) affected the desire for children negatively when pandemic mortality peaked during the autumn of 1918.
There was a widespread fear of death, and anxiety was intensified when it was realized that physicians were helpless. There were no vaccines or effective antiviral drugs. Two folk memory quotations describe the everyday atmosphere of fear. One woman recalled that “Aftenposten [a major newspaper of the capital] was full of death notices. I read them all every day, and it was awful”. Another woman remembered that “everyone was afraid of everyone else, the contagion was everywhere” (Mamelund, 1998). The terror and fear caused by the disease had a profound psychological impact on people and exerted a powerful influence on the choices they made. It seems reasonable therefore to believe that even people who were not struck down by the flu would have decided to wait before they had a baby.
There are two reasons to assume that a couple would reduce sexual intercourse if one of the spouses became infected (Phase 2). First, the physical condition of the patient (high fever, headache, etc.) inhibited sexual activity for 2 to 4 weeks (Mamelund, 1998). Second, in order to reduce the risk of infecting one another, couples not previously infected may have decided to abstain from sexual intercourse even though able and willing (Mills, 1986). This assumption is grounded in the belief that the Norwegian health authorities’ massive public information campaign through notices in newspapers and posters in public places was effective and had a preventive effect. People were urged to cover the mouth when coughing, to go to bed as soon as possible upon the onset of illness and to remain isolated from the rest of the family until they were free of symptoms. In light of this, it seems reasonable to assume that couples in which both spouses had survived would have decided to try for a pregnancy in 1919 (Phase 3), thereby leading to a baby boom in 1920 (Phase 4).
According to Norwegian marriage law at the time, a widow was not permitted to remarry before she had spent at least a year of mourning. Although the same law did not apply to men, it was nevertheless also customary for widowers to have a year of mourning. Assuming therefore that the widowed did not remarry in 1918 or 1919, that there was little sex outside of marriage, and that the widows were not pregnant before their husbands died, neither widows nor widowers would have been able to achieve a conception in 1918 or in 1919 to compensate for the postponed conceptions of 1918. If this mechanism works as described, it would also impede the rebound potential of fertility from being fully released until the first half of 1920.
Pregnant women, especially those in the last trimester, were extremely vulnerable to spontaneous abortion, stillbirth and maternal death if Spanish influenza was followed by pneumonia (Harris, 1919; Bourne, 1922). Two contemporary investigations report that anywhere from a fifth to half of pregnant women with pneumonia died (Bland, 1919; Harris, 1919). This certainly had a negative effect on the birth rate in 1918. Infant and child mortality on the other hand may have affected fertility positively (see Preston, 1978). Parents who lost infants and young children to Spanish influenza in 1918 may have wished to have another baby to achieve a certain family size, thereby increasing the number of conceptions in 1919. This may have contributed to the baby boom in 1920.
The probability of conception may have dropped in 1918 (Phases 1 and 2) due to stress imposed by the Spanish influenza. Support for this hypothesis is found in the literature. Biraben (1973) for example, reports that males, but not females, have become temporarily sterile in connection with influenza epidemics. If men also experienced temporary sterility during the Spanish influenza, this would inhibit the making-up in 1919 (Phase 3) of the conceptions postponed in 1918 (Phase 2). Changes in female fecundity associated with Spanish influenza may have stimulated conceptions, but the biological factors go in both directions. On the one hand, women who failed to conceive in Phase 2 were not pregnant in Phase 3. They were therefore at risk of another pregnancy after a short while, even during the peak of the crisis (Juhasz, 1971). In addition, women who failed to conceive would not have been breastfeeding. This is expected to give extra “power” to the making-up process. The net effect of high (male) morbidity rates (which is assumed to give relatively high temporary male sterility) and high spontaneous abortion and stillbirth rates for women in Phases 1 and 2, is assumed to be negative, thereby inhibiting the potential rebound of conceptions in Phase 3.
II. Available data and indicators used in the study
1. Conceptions and the crude conception rate
The regional monthly live births, stillbirths and population at risk that are used to calculate the decline in the crude conception rate associated with Spanish influenza come from the annual population reports Folkemengdens bevegelse published by Statistics Norway ( Statistisk sentralbyrå, cited henceforth as SSB). From 1919 onwards live births and stillbirths were compiled by place of residence of the parents ( de jure population), while the practice in previous years was to compile the data for the present population of parents ( de facto population). Unfortunately, stillbirths are probably underreported, but there have apparently not been large regional differences in their underreporting over time. The seasonal patterns of foetal mortality before the fifth month of pregnancy are not known. However, the seasonality of foetal mortality in the 5-6th month of pregnancy is available, but only for the whole nation. The crude conception rate is therefore defined as conceptions ending either in a stillbirth (7 9 months later) or a live birth (9 months later) per 1,000 people per month (Leridon, 1977; Wrigley and Schofield, 1989).
2. Influenza and pneumonia cases and the morbidity rate
The monthly reported influenza and pneumonia cases by region, taken from the annual health reports Sundhetstilstanden og medisinal-forholdene, published by Det civile medisinalvesen (cited henceforth as DCM), constitute the raw data input for calculating the increase in the in fluenza and pneumonia morbidity associated with Spanish influenza. The population at risk is taken from the annual population report Folkemengdens bevegelse published by SSB.
In contrast to most other countries, influenza was a reportable disease in Norway in 1918. After some time, however, it was realized that reported cases of influenza and pneumonia fell short of actual morbidity (DCM, 1922). The disease probably struck 1.2 million Norwegians whereas only a third of the cases were reported (Mamelund, 1998). The underreporting is explained by the short supply of doctors, and in some cases may be attributed to the long distances to doctors and hospitals, especially in the rural parts of Finnmark County in the high north. Furthermore, in many families doctors were only called for the breadwinner or the person who had to claim sick benefits when reporting unfit for work to a health insurance fund (Kristiania Sundhetskommision, 1919). More people employed in the secondary and tertiary sectors and living in cities had health insurance than people employed in the primary sector living in rural areas. It was the impression of the medical profession that underreporting was generally lower in cities than in rural areas (Mamelund, 1998).
With respect to underreporting over time, the physicians claimed that it was lowest during the autumn wave of 1918. This was explained by the increase in lethality of the Spanish influenza from the summer to the autumn, making reporting of a case more likely in the latter period. For Norway’s second largest city, Bergen, this assumption is confirmed. When comparing the figures from an “influenza census” of the Bergen population with official statistics, it is estimated that a third of all cases were reported during the summer and almost 100% during the autumn (Hanssen, 1923)
[1]. The “influenza censuses” are the only source that gives a reliable picture of morbidity. Similar “influenza censuses” carried out for several cities in England and in the United States generally confirm the finding from Bergen (Great Britain Ministry of Health, 1920; Vaughan, 1921; Collins, 1931; Sydenstricker, 1931; Britten, 1932).
The influenza and pneumonia morbidity rate is defined as the number of cases per 1,000 in the present ( de facto ) population reported to district physicians or to doctors in hospitals per month. The morbidity rate may be higher than 1,000 as a person might visit a physician because of influenza or pneumonia more than once a month. Unfortunately, the data needed to estimate duration of the disease for the 37 units of analysis do not exist. Obviously, the morbidity rate alone cannot satisfactorily explain the assumed decline in the conception rate. A sick leave of 12 weeks for mild cases as compared to a sick leave of 34 weeks for severe cases would clearly have different effects on the conception rates.
3. Deaths and the crude death rate
Unfortunately, monthly numbers of influenza and pneumonia deaths do not exist for the units of analysis used in the article. However, monthly numbers of deaths from all causes are available from the annual population report Folkemengdens bevegelse published by SSB. Mamelund (1998) has shown that the calculation of excess mortality for all causes of death, compared to the previous years, gives a good approximation of the mortality associated with the Spanish influenza. Therefore, excess in the monthly number of deaths from all causes in each of the 37 regions in 1918 is used to estimate the increase in mortality associated with the Spanish influenza. Although doctors were overworked and failed to report complete morbidity figures, every death was probably reported (Mamelund, 1998). The crude death rate in 1918 is defined here as the number of deaths per month for 1,000 persons in the present population, while in 1919 the crude death rate is the number of deaths per month per 1,000 people with permanent residence.
III. Combined fertility-mortality profiles during the 1914-1918 war and during the successive waves of influenza in Norway from 1918 to 1919
To analyse the effect of Spanish influenza on fertility, we will confront the observed seasonal fluctuations of fertility and mortality
[2] during the period 1918-1920 with the reference seasonal levels of the same demographic indicators calculated for the period 1915-1917. Let us first justify our choice of the 1915 to 1917 period as a reference.
1. 1915-1917 as reference years
The period 1915-1917 is chosen because the general level of fertility should not be very different from that of the years immediately following. Moreover, Cassel (2001) has shown for Sweden, which shares climate and cultural traditions with Norway, that the First World War did not affect the seasonality of conceptions. Both pattern and levels of conceptions, therefore, provide an appropriate standard of comparison with the time of the influenza pandemic.
In the period 1915-1917 there were no influenza pandemics. However, annual influenza epidemics were reported as usual in the months from October to March. The morbidity and mortality of the three influenza epidemics in the period 1915-1917 are representative of previous epidemics in that a large part of the population had immunity to fight the viruses, and that influenza and pneumonia mortality, which was generally low, was highest for the very young and the elderly. It seems therefore reasonable to use the years 1915-1917 as a baseline for comparing the pandemic of 19181919.
Some of the same arguments for selecting 1915-1917 as norm for fertility also apply for mortality. First, the death rates for the nation as a whole did not increase during the war, as shown in Figure 2 (Mamelund, 2003b). Second, because the period under analysis is part of a long period of mortality decline, the level of mortality would probably be exaggerated if years prior to 1915-1917 had been included to define normal conditions.
2. The Norwegian fertility-nuptiality-mortality regime from 1914 to 1919
Birth rates 1914-1918
There was no dramatic change in fertility in Norway between 1913 and 1918 (Table 2). Indeed, total fertility fell by 7% from 1914 to 1915, perhaps because of a decline in conceptions in the months following the outbreak of war in JulyAugust 1914, but in 1917 the total fertility rate rose slightly. The decline in fertility from 1914 to 1915 may have created a potential for a compensating rebound, although at first glance it was not realized in the following years. The relatively high and stable fertility in the years 19161918 occurs, however, during a long period of more or less constant decline in fertility from the 1890s to the mid 1930s (Brunborg and Mamelund, 1994). It is therefore likely that most of the potential for making up the conceptions postponed in 1914 was realized in the years 1915-1918.
Table 2
Live births and birth rates by legitimacy (per 1,000), and total fertility rate, Norway 1913-1923
Marriage rates 1914-1918
As may be seen from Table 3, there was a boom in marriages during the war. SSB (1926) explains the increase in marriages by the economic boom, by relatively large birth cohorts reaching marriageable age, and by low rates of emigration (emigration rates were around 4.0 per 1,000 population in the years 19131914, lowest in 1918 with 0.5 per 1,000, and around 2.0 per 1,000 in the period 1920-1922). This finding seems to support the hypothesis that the war did not lead people to postpone their marriages.
Table 3
Number of marriages and marriage rate (per 1,000 unmarried persons), Norway 1913-1923
The increase in the number of marriages concluded from 1917 to 1918 and the decline in marriages from 1918 to 1919 are explained by more restrictive marital legislation that went into effect on 1 January 1919, rather than by the armistice or Spanish influenza (SSB, 1926). The most important change in the law was a rise in legal age of marriage, from 16 to 18 years for women and from 18 to 20 years for men. According to SSB (1926), this resulted in an increase of 2,000 more marriages in December 1918 compared to normal figures, and 2,000 fewer marriages during the first half of 1919. If the figures in Table 3 are adjusted according to this displacement, it will be seen that there was no increase in the number of marriages from 1917 to 1918 and only a moderate decline by 640 marriages from 1918 to 1919. A reanalysis of monthly marriages for the purpose of this article (not shown here) led to the same conclusions as SSB. Hence, it seems safe to conclude, at least for the nation as a whole, that the baby boom in 1920 was not caused by a marriage boom after the armistice or by the withdrawal of Spanish influenza. Likewise, the result indicates that few marriages were postponed because of the disease. Moreover, since there were no apparent changes in the number of marriages because of Spanish influenza, one would not expect any indirect negative effects of marriage on the number of conceptions. As data on monthly marriages are published only at the level of the whole nation, marriage cannot be included in the multivariate analysis.
The 1918 spring wave of Spanish influenza
Approximately 6,000 people were off from work with influenza symptoms in the first week of April 1918 (Mamelund, 1998), but these cases were apparently not reported in the official statistics as influenza, as can be seen in Figure 3. A suspiciously high number of influenza cases were also reported in military camps, but the disease did not spread to the general population. At this stage of the pandemic, mortality was low and deaths were only reported among population groups where fatalities from influenza are most common, that is the very young and the elderly. The death rate from January to June 1918 was also lower than the “normal” seasonal levels. This is rather surprising as this was a time of relatively strict food rationing (Mamelund, 2003b). Nevertheless, the increased in fluenza activity in April may explain why the conception rate declined by 0.7 conceptions per 1,000 from March to April rather than increasing by 1.9 conceptions per 1,000 as it did on average in 1915-1917 (Figure 3). The fall in the conception rate by 1.0 conception per 1,000 from April to May might be expected, but it occurs at a rate of 2.5 conceptions per 1,000 fewer than normal.
Figure 3
Monthly influenza and pneumonia morbidity, and differences of the crude death rate and conception rate from the monthly average for 1915-1917, Norway 1918-1920 (per 1,000)
Source : DCM, 1922, 1923, 1924; and SSB, 1920a, 1920b, 1921a, 1921b, 1923a, 1923b.
On 28 May 1918, Norwegian newspapers printed a wire from the Reuters news agency revealing that a seemingly new type of influenza was spreading throughout Spain. Only two weeks later, on 15 June, the very first scattered cases of influenza, which later proved to be part of a pandemic wave, occurred in the capital of Kristiania (renamed Oslo in 1924) (Mamelund, 1998). It was not before the first week of July, however, that the number of reported cases skyrocketed to epidemic proportions (see Figure 3). One out of the four conceptions per 1,000 that were postponed in total between April and December 1918 would have occurred in the second quarter from April to June. It would appear that the decline in conceptions in this period can be attributed to the scattered cases of influenza in April and possibly also to a voluntary postponement of conceptions due to negative perceptions about the near future.
The 1918 summer wave of Spanish influenza
The conception rate normally rises and reaches the highest level of the year during the three summer months ending in August. This was not true in 1918. Instead of a normal seasonal increase of 1.4 per 1,000 from June to August, the conception rate decreased by 1.0 per 1,000. Of the decline of 1.7 conceptions per 1,000 from August to September, 40% is explained by normal seasonal fluctuations. The remaining 60% is probably explained by a decline in coital frequency in connection with the increase in the morbidity rate and the death rate during the first pandemic wave of Spanish influenza. Of the total decline in conceptions from April to December 1918, 44% would have occurred during the third quarter from July to September.
The third quarter of 1918 was not the quarter with the highest increase from the 1915-1917 norm in mortality in 1918, but according to the only “influenza census” in Norway, that of Bergen, as well as to reports from physicians from other parts of the country, actual morbidity was clearly highest in this quarter as the flu met a “virgin soil” population (Hanssen, 1923; Mamelund, 1998). Upon examination of the morbidity curve in Figure 3, however, it seems that reported morbidity was higher during the autumn of 1918 than during the summer of 1918 (574 cases per 1,000 in October versus 297 in July). As discussed above, this is due to underreporting of cases during the summer wave because of the mild character of the virus at this stage, while there was no or little underreporting during the highly virulent autumn wave. Actual morbidity for July 1918 is far higher than reported morbidity for October 1918, which is assumed to be close to actual morbidity. The increase in the death rate during the autumn wave was twelve times as high (8.6 deaths per 1,000) as the corresponding increase in the death rate during the summer wave (0.7 deaths per 1,000). To conclude, morbidity seems to have been more important than mortality in explaining the decline in the conception rate during the summer wave of relatively mild Spanish influenza.
The 1918 autumn wave of Spanish influenza
The average of 21.6 conceptions per 1,000 in the period from September to November 1918 is nearly 4.0 conceptions per 1,000 lower than normal, a fact that is probably due to the high morbidity and death rates during the autumn wave (Phase 2). Almost 60%, or 2.8, of the increase of 4.8 conceptions per 1,000 in the rate between November (21.7 per 1,000) and December (26.5 per 1,000) may be explained by normal seasonal fluctuations (Figure 3). The other 40% are probably explained by a strong decline in both the morbidity of influenza and mortality.
Of the total decline in conceptions from April to December 1918, it appears that 31% can be accounted for by the fourth quarter of 1918. The reason why a larger proportion of the decline in conceptions occurred during the third quarter (44%) than during the more lethal fourth quarter (31%), must be the fact that actual influenza morbidity for men and women in their most reproductive ages 20-40 was much lower in the fourth quarter than during the third quarter of 1918, thanks to a gain of relative immunity (Hanssen, 1923). Figure 3 indicates that the curve for reported morbidity was highest in the last quarter of 1918, but this is again probably due to the virus becoming more virulent, with more persons reporting themselves sick. To sum up, it seems that mortality was more important than morbidity in explaining the downturn in the conception rate during the lethal autumn wave.
The pandemic subsides: a rebound of conceptions in 1919
The increase in the conception rates in 1919 and during the first three months of 1920 (Phase 3) from the 1915-1917 norm more than compensated for the decline in the conception rates of the last nine months of 1918 from the 1915-1917 norm (Figure 3). Although the conception rate increased from November 1918 to March 1919 (positive excess rates compared to normal seasonal fluctuations in January-March 1919 only), births declined by 4,000 (0.26 births per woman) to 60,000 from 1918 to 1919 (Table 2, Figure 3). Obviously, conceptions that might have been expected to occur during the last nine months of 1918 were postponed to 1919 because of influenza (SSB, 1926 came to the same conclusion). Nearly 70,000 births were recorded in 1920 (Table 2). This was about 10,000 more, or 0.44 more births per woman, than in 1919. (In fact, the Spanish influenza baby boom cohort of 1920 is the second largest birth cohort ever registered in Norway, only exceeded by the Second World War baby boom cohort of 1946).
Spontaneous abortions and stillbirths in 1918
We have argued that Spanish influenza seems to have had a negative effect on fertility in 1919. Rising rates of spontaneous abortion and stillbirth due to the Spanish influenza probably had a negative effect on fertility in 1918. There was actually a significant increase from 1917 to 1918 in both the spontaneous abortion rate, defined as spontaneous abortions per 1,000 women who were 5-6 months pregnant, and the stillbirth rate, defined as stillbirths per 1,000 women who were 7-9 months pregnant. Based on data from DCM (1918-1923) and SSB (1920a, 1920b, 1921a, 1921b, 1923a, and 1923b), it is estimated that 45 excess spontaneous abortions or 4.0 per 1,000 women 56 months pregnant, and 335 excess stillbirths or 22.1 per 1,000 women 7-9 months pregnant, were due to the Spanish influenza. However, these calculations may be understatements, especially those for spontaneous abortions that may have been systematically underreported. Also, abortions that occurred between conception and the fifth month of pregnancy are omitted for lack of official statistics.
Deaths of pregnant women in 1918
Rising death rates from Spanish influenza among pregnant women are also thought to have negatively affected fertility levels in 1918. At the peak of the pandemic in November, maternal mortality, here defined as deaths of pregnant women, occurred at a rate that was five times as high as that in the average November month in the previous three years (see Mamelund, 2003a). Based on data from DCM (1918-1923), it is estimated that 67 excess maternal deaths from all causes, or 18.3 deaths per 1,000 pregnant women, were caused by the Spanish influenza. Based on the findings of Harris (1919) and Bourne (1922), higher maternal mortality rates should be expected for pregnancies in the last trimester. Unfortunately, the official data on maternal mortality are not broken down by month of pregnancy, and as such, this issue remains open to speculation.
Marriages dissolved by death in 1918
In a preliminary version of this article (Mamelund, 2003a), it was estimated that an excess of 3,528 marriages, or 7.7 per 1,000, were dissolved by death in 1918 compared to the annual average in the period 1915-1917. It was also found that newlywed women in their most reproductive ages ran the greatest risk of losing their husbands to Spanish influenza. Here it is assumed that wives who were not pregnant at the point of becoming widows in the latter half of 1918 would not give birth to a baby in 1919.
IV. Multivariate analysis
To better examine the relationship between the assumed increase in influenza and pneumonia morbidity and in mortality from all causes in 1918 on the one hand, and a decline in conceptions in 1918 on the other hand, in comparison with the 1915-1917 norm (models 1-4), we use ordinary least squares regressions. An analysis of the possibility that the assumed rebound in conceptions in 1919 can be explained by the decline in conceptions in 1918 and by the increase in morbidity and in mortality during the second half of 1918, again when compared to the norm (models 5 7), is also performed using the same type of regression analysis.
1. Dependent and independent variables
Four dependent variables (labeled 1 to 4) are used to test the hypothesis of a decline in the conception rate from the 1915-17 norm associated with Spanish influenza in 1918, while variable 5 is used to test the hypothesis of a makeup of conceptions in 1919. Variables 1 and 2 measure the decline in conceptions during the summer of 1918, and variables 3 and 4 the decline during the autumn. The decline is measured in two different ways. Variables 1 and 3 use the difference between the conception rate in the month with the largest decline and that in the corresponding month during the normal years of 1915-1917. Variables 2 and 4 use that difference for the month with the lowest observed level of conception in 1918. Dependent variable 5 is the difference in the conception rates from the norm of 1915-17 during the period from January through December 1919. The definitions and descriptive statistics for the five dependent variables and the fourteen independent variables included in the seven models can be found in Table 4.
Table 4
Definitions and descriptive statistics for the dependent and independent variables in 37 urban and rural parts of Norwegian counties
The denominators in the monthly conception, morbidity, death and stillbirth rates were adjusted for length of the month (see Cassel, 2001). The dependent and independent variables were not standardized, since some of the data needed to calculate standardized rates (age and sex distribution) exist only for the census of 1920 (and not for the years 1915-1919). It appears, however, that the regional differences in the age distribution of those in the 20-40 year age bracket, who had the highest morbidity, mortality and fertility rates, were too small to bias the computation. In order to account for spatial differences in underreporting of influenza and pneumonia cases, weights were applied to all observations used in the analysis. The weights were computed by multiplying the number of doctors per km 2, the percentage of population with health insurance, the number of physicians per 1,000 of present population, and the number of hospitals per 1,000 of present population (with average 3.5 and standard deviation 6.5).
Table 5 presents correlation coefficients between the independent variables in the models. The correlations between the independent variables included in most models are relatively low, and should therefore cause no multicollinearity problem. The correlations between variables indexing mortality and morbidity differences from the norm (variables 7 and 8 in model 3, and variables 9 and 10 in model 4) are high and may cause a multicollinearity problem. If so, although the parameter estimates will be correct, they will be unstable and have excessive standard errors. It may also be impossible to assess the independent effect of each of the two correlated covariates on the dependent variable. The issue of multicollinearity is taken up again when discussing the results from models 3 and 4.
Table 5
Correlations of independent variables
The 1918 summer wave of Spanish influenza
It appears necessary to include the assumption that a large previous decline in the conception rate in a region in early 1918 affected the conception rate in a negative direction in that region later in 1918. When controlling for the decline in the conception rate in the second quarter of 1918 from the 1915-1917 norm, the increase over the 1915-1917 norm in morbidity and mortality during the summer wave has a negative effect on the conception rate in the same period (Table 6, models 1 and 2). However, only the morbidity rate contributes significantly to explaining the decline in the conception rate compared to the norm, albeit in a very weak manner. An increase of one influenza and pneumonia case in a region leads to a later decline of only 0.006–0.007 conceptions.
Table 6
Results of linear regression analyses of the largest monthly difference of the conception rate over the norm (a) (model 1) and of the difference of the lowest monthly conception rate over the norm (model 2) between July and September 1918
This result seems to confirm the finding in the descriptive analysis that morbidity is more important than mortality in explaining the downturn in the conception rate during the summer of 1918. This is not surprising since the increase in morbidity was much larger than the increase in mortality in this period. The decline in the conception rate in the second quarter of 1918 compared to the 1915-1917 norm has a strong negative effect on the decline in the conception rate in the third quarter of 1918. When there is a decline of one conception in a region in the second quarter of 1918 compared to the norm, it leads to a decline of only 0.51–0.60 conceptions in that region in the third quarter, again compared to the norm. The result implies that the population living in regions in which large declines in the conception rates were observed in the second quarter of 1918 experienced a smaller decline in the conception rate in the third quarter of 1918, and vice versa.
The 1918 autumn wave of Spanish influenza
The results of the regression models for the effects of the increase in morbidity and mortality during the autumn wave on the one hand, and the decline in conceptions during the summer wave on the other hand are striking, and surprising with respect to the decline in conceptions during the autumn wave (Table 7, models 3 and 4). First, the size of the downturn in the conception rate in a region during the summer wave has a negative effect on the downturn in the conception rate in that region during the autumn wave, as expected. However, the effect here is weaker than the corresponding relationship found in models 1 and 2 (0.19–0.22 conceptions per 1,000 compared to 0.51–0.60), implying that, as a consequence of a previous decline in a region, the potential for further decline in conceptions in that region is decreasing. Second, but not surprisingly, the effect of mortality is highly significant and strongly negative on the downturn in conceptions during the autumn wave. An increase in the death rate by one death in a region during the autumn wave gives a reduction in the conception rate in that region of 0.46–0.49 conceptions, all other factors being equal. Third, and most surprisingly, the effect of morbidity goes in the opposite direction to what was expected. According to models 3 and 4, an increase in the morbidity rate by one influenza and pneumonia case gives an increase in the conception rate of 0.0095–0.0158 conceptions. This effect is not large, but it is highly significant.
Table 7
Results of linear regression analyses of the largest monthly difference of the conception rate over the norm (a) (model 3) and of the difference of the lowest monthly conception rate over the norm (model 4) between October and December 1918
In the section describing the independent variables, it was argued that the relatively high correlations between variables 7 and 8 in model 3 and variables 9 and 10 in model 4 may cause a multicollinearity problem. However, when model 3 is applied with and without, for instance, the increase in the morbidity rate, but with controls for previous decline in conceptions and the increase in the death rate, a substantial difference in the explanatory power occurs (uncorrected R2 increases by 30 percentage points to 62% when including the morbidity rate). In addition, the sign of the coefficients already included in the model does not change and the statistical levels of significance increase. It is worth noting that the two estimated coefficients were significant at the 1% level when the morbidity rate was not included in the model. The morbidity rate and the death rate together explain more than half of the variance in the conception rate — 52% in model 3 and 58% in model 4 — and independently each has a specific effect. More or less the same story goes for model 4. This reduces the possibility of a multicollinearity problem in models 3 and 4.
The pandemic subsides: a rebound of conceptions in 1919
If one does not control for the increase above the norm of morbidity, mortality, and stillbirths in the second half of 1918, a decline in the conception rate by one conception in a region in the last nine months of 1918 leads to an increase in the conception rate in that region of two conceptions in 1919 (Table 8, model 5). The positive coefficient implies that the larger the downturn in the conception rate was in a region in 1918, the larger was the rebound in that region in 1919 and vice versa.
Table 8
Results of linear regression analyses for the difference in the conception rate for 1919 from the average for 1915-1917
In model 6 (Table 8), however, two possible confounding factors are added. One is morbidity, in order to control for the possibility that the disease affected the fecundity of survivors negatively. The second is mortality, to control for the fact that the chances of those widowed in 1918 realizing a pregnancy in 1919 were low in view of the required year of mourning. In this case, the independent effect of a decrease from the norm by one conception in the last nine months of 1918 leads to an increase over the norm of 1.7 conceptions in 1919. This effect is lower than in model 5 by 0.3 conceptions. At the same time, the explanatory power (R 2) of model 6 increases by 16 percentage points to 73%. Surprisingly, the increase in morbidity and mortality in 1918 did not negatively affect the size of the rebound in the conception rate in 1919 as expected. This does not necessarily mean that the assumed effect was not present. More likely, other mechanisms may have worked simultaneously, positively affecting the dependent variable, and thus cancelling out the expected negative effect of morbidity and mortality on the assumed rebound of conceptions in 1919.
One reason why the effect of the death rate in 1918 (as a proxy for marital dissolution due to fatalities from Spanish influenza) on the rebound of conceptions of 1919 was positive may be that a larger proportion of the conceptions than usual took place out of wedlock. Indeed, the opportunity for remarriage in 1919 was limited due to the year of mourning imposed by the Norwegian marriage law. Data for the nation as a whole support this hypothesis: illegitimate births per 1,000 people increased by 20% in 1920 over the normal period of 1915-1917, and the legitimate births only by 8% (see Table 2). Remarriage rates for both widowers and widows, however, declined from 1918 to 1919, and increased from 1919 to 1920, as expected (see Table 9). This suggests that the original assumption of no sexual intercourse prior to remarriage was too strict. However, when running model 6 with only legitimate conceptions in both the dependent and the independent variables for the 37 areas (estimates not shown here), the death rate in 1918 still has a positive effect on the rebound, casting doubt on the explanation. Anyway, the result also begs the question whether there were any illegitimate-legitimate differences in the downturn in birth rates in 1919. When using 1915-1917 to exemplify a normal period, no significant differences were found, however, as the decline was 8.0 and 6.0% in legitimate and illegitimate birth rates respectively. This result is not surprising, as there is no reason to believe that the psychological or biological aspects of Spanish influenza would affect the fertility of married persons differently from that of the unmarried.
Table 9
Remarriages and remarriage rates (per 1,000) for widowers and widows, Norway 1913-1923
A second explanation of the positive effect of mortality in 1918 on the rebound in conceptions in 1919 is that those married couples living in regions where death rates and marriage dissolution rates due to Spanish influenza deaths were high, who did not lose their spouse to the disease, caught up relatively quickly with more conceptions than similar couples living in regions with low death rates and low marriage dissolution rates. The “underlying” explanation might be twofold. First, loss of a child due to the flu may have intensified the desire to have another child to replace the loss. Second, high community mortality in general may have fostered a desire to “replace” the dead.
In model 7 (Table 8), stillbirth rates in the second half of 1918 are included as a new independent variable to control for the assumed negative effect of high stillbirth rates in 1918 on the rebound of the conception rates in 1919. The coefficient is negative as expected, yet significant: an increase by one stillbirth in a region in 1918 reduces the rebound potential in that region in 1919 by 3.9 conceptions. This is explained by an increase in spontaneous abortions and stillbirths, making the women who experienced this event temporarily infecund. The independent effect of a decline in the conception rate in 1918 by one conception, all other factors remaining the same, in model 7 is now an increase of 1.5 conceptions in 1919. In total, the best model (model 7) explains nearly 80% of the variance in the rebound stage of the pandemic. Note also that the effect of the death rates in 1918 is no longer significant on the rebound in model 7, though the sign is still positive. The reason for a decline of one conception in 1918 leading to an increase of more than one conception made up in 1919 may be due to two inseparable factors: first, but not necessarily the more important, joy that the Spanish influenza and the First World War were over; second, a desire to replace the dead, especially small children.
Conclusion
This article has tested the hypothesis that the 1918 Spanish influenza pandemic, which infected 1.2 million Norwegians (45% of the population) and killed 15,000 (5.7 deaths per 1,000 population), with most victims in the fertile ages of 20-40, caused the baby boom of 1920 in Norway. The descriptive analysis showed that the observed time profile of Spanish influenza fits well with the demographic changes that occur commonly during and after a general mortality crisis. The course of the 1918 influenza and the ensuing effects on fertility and nuptiality also correspond fairly well to the experience reported for other countries (Höijer, 1959; Pool, 1973; Rice, 1983; Underwood, 1983; Mills, 1986; and Johnson, 2002). However, previous studies have been univariate and descriptive and thus could not investigate whether Spanish influenza morbidity had an independent effect on fertility net of the effect of mortality, or whether morbidity had a different effect on fertility than mortality had during the summer and autumn waves of 1918.
The study applied multivariate analysis to shed light on these questions, and used detailed and reliable cross-sectional demographic data available for Norway that have not been analysed before. The data on morbidity are particularly valuable for the analysis. Such data are very difficult to obtain for other countries and when available, are often of poor quality. Another advantage of Norway is that it remained neutral throughout the First World War. Registration of population data including vital statistics was therefore on the whole relatively undisturbed by the war. Moreover, any assumed effect of the Spanish influenza on fertility and nuptiality was not affected by the war.
There are two key hypotheses concerning why Spanish influenza is believed to have caused a decline of conceptions in 1918. First, the disease probably restrained the ability and willingness to have sexual intercourse for some weeks during each of the waves. Second, those who lost their spouses to the Spanish influenza were bound by the legal and moral customs of the time to desist from remarriage at least for a year. The regression analysis showed that the higher the increase in influenza morbidity was in a region during the relatively mild summer outbreak in 1918, the larger the decline in the conception rate was in that region. The increase in mortality also had a negative effect on conceptions during the summer outbreak, but the effect was not statistically significant. Likewise, it was found that the higher the increase in Spanish influenza mortality was in a region during the highly lethal autumn outbreak in 1918, the larger the decline in the conception rate was in that region. As mortality increased much more during the autumn wave than during the summer wave, the effect of mortality on conceptions was also strongest during the former. It was also found that mortality had a stronger effect than morbidity on conceptions, probably because those who came down with influenza but did not die had a shorter period of sexual abstinence than those who lost their spouse. Surprisingly, the effect of the increase in morbidity on the conception rates during the autumn outbreak — net of the effect of mortality — was positive and highly significant.
The regression analysis also showed that the size of the rebound of conceptions in 1919 surpassed the compensating rebound potential — created by the decline of conception in 1918 — by 50%. This means that a decline of one conception in a given region in 1918 resulted in 1.5 conceptions made up in that region in 1919. The reason for this is probably that, in addition to making-up of the previous decline in conceptions, additional conceptions were realized to replace the dead, especially lost children. These resulted in births during 1920. The conclusion of this article is thus that the Spanish influenza pandemic of 1918 created the baby boom in Norway in 1920.
One important speculative issue that arises from the article, but which is not addressed in the analysis, is why the baby boom of 1920 appeared to be modest in the belligerent countries. The potential for a rebound was certainly large given the great fertility decline during 1914 1918 that was associated with the First World War, to which was added a possible decline in fertility associated with the onset of Spanish influenza in 1918. The reason for this is perhaps that both the war and Spanish influenza took their heaviest toll among males aged between 20 and 40 years, thereby lowering the chances of marriage for spinsters and of remarriage for widows. This in turn may explain why the potential making-up of births seems to have been far from fully exploited.
Acknowledgements
Comments to this article from Nico Keilman, Øystein Kravdal, Alberto Palloni, Susan De Vos and two anonymous reviewers are gratefully acknowledged. Thanks are also due to Halvard Skiri, Kirsten Dybendal and Britt Elin Bråten who assisted in finding sources of historical data in Statistics Norway. I am grateful to the Center for Demography and Ecology at the University of Wisconsin-Madison who provided excellent working conditions while I wrote a preliminary version of this paper in the autumn of 2001. Last but not least, thanks to Jennifer Chisholm-Høibråten for proofreading. The article is part of the research project Spanish influenza and beyond: The case of Norway, financially supported by the Norwegian Research Council and Department of Economics, University of Oslo.
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NOTES
[*]
Department of Economics, University of Oslo, Norway.
Department of Economics, University of Oslo, Norway.
[1]
The census covered 10,000 persons or one-ninth of the population and was carried out by nurses who asked heads of household whether any of the members were sick or died from influenza during the different waves of 1918.
The census covered 10,000 persons or one-ninth of the population and was carried out by nurses who asked heads of household whether any of the members were sick or died from influenza during the different waves of 1918.
[2]
Seasonal fluctuations in total mortality, incidence of and mortality from influenza and pneumonia, spontaneous abortions, stillbirths, maternal deaths, live births, and marriages dissolved by death.
Seasonal fluctuations in total mortality, incidence of and mortality from influenza and pneumonia, spontaneous abortions, stillbirths, maternal deaths, live births, and marriages dissolved by death.
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