Changing sex ratio in Iran, 1976-2000

A preliminary analysis of the secondary sex ratio decline after the COVID-19 pandemic in Japan

Objectives

The secondary sex ratio (SSR, i.e., the number of male births per 100 female births) has long been proposed as a sentinel health indicator. Studies have suggested that exogenous environmental stressors reduce SSR after 3 to 6 months (via disproportionate male fetus deaths) and after 9 months (via reduced male conception). We aimed to examine whether SSR declined after the coronavirus disease 2019 (COVID‐19) pandemic in Japan.

Methods

We used monthly vital statistics records collected between January 2013 and April 2021 (the ‐number of male and female live births, in particular). Using information reported before the pandemic struck Japan (i.e., January 2013 to January 2020), we predicted SSRs for the months after the pandemic (i.e., February 2020 to April 2021) and compared reported and predicted SSRs. We also stratified the analysis by including two groups of prefectures with different degrees of possible influence from the pandemic.

Results

We observed a significant reduction in SSR during December 2020 of 102.81 (i.e., 9–10 months after the pandemic affected the country), which was below the lower bound of the 95% prediction intervals (103.12–106.33). This was the lowest SSR observed during the 100‐month study period. In addition, the reduction in SSR during December 2020 was more pronounced in the more seriously affected prefectures.

Conclusion

In Japan, we found a significant reduction in SSR (i.e., fewer male live births) 9–10 months after COVID‐19 was declared a pandemic. This suggests the onset of the COVID‐19 pandemic was a significant population‐level stressor.

Maternal stress and sex ratio at birth in Sweden over two and a half centuries: a retest of the Trivers-Willard hypothesis

STUDY QUESTION

Is there a negative relationship, as predicted in the Trivers–Willard hypothesis (TWH), between the intensity of maternal stress and sex ratio at birth (SRB)?

SUMMARY ANSWER

Using a comprehensive data set with multiple indicators of maternal stress, most measures of stress show no statistically significant association with SRB over a period spanning 243 years, indicating no support for the TWH.

WHAT IS KNOWN ALREADY

Evolutionary biologists have proposed a widely discussed hypothesis that women in poor and stressful conditions during pregnancy are more likely to give birth to girls, and exposure to stressful events may therefore lead to a reduction in sex (male-to-female) ratio at birth. The empirical evidence so far is mixed.

STUDY DESIGN, SIZE, DURATION

Annual time series data, spanning 243 years between 1749 and 1991 for Sweden at the national level, were drawn from multiple sources. The outcome is defined as the percentage of male births relative to all births in Sweden in a given year. The covariates include a set of economic and climatic variables as proxies for maternal stress.

PARTICIPANTS/MATERIALS, SETTING, METHODS

We conduct a series of ARIMA (autoregressive integrated moving average) models to examine the relationship between maternal stress and SRB during three periods: 1749–1991, 1749–1861 and 1862–1991.

MAIN RESULTS AND THE ROLE OF CHANCE

In 1749–1991, economic proxies for maternal stress showed no statistically significant association with SRB. In 1749–1861, two indicators were significantly associated with SRB, but the coefficients were opposite in direction to the TWH. In 1862–1991, five out of six covariates showed no significant association with SRB. An additional analysis found no significant correlation between sex ratio of stillbirths and all covariates in 1862–1991. Our results are incompatible with the TWH and suggest that previous findings in support of the TWH are not robust.

LIMITATIONS, REASONS FOR CAUTION

This study provides population-level evidence that may not necessarily reflect the nature of all individuals due to the ecological fallacy. The time series analysed in this study are annual data, and we cannot examine the potential seasonality due to the lack of disaggregated monthly data. Our findings may not be generalised to the contexts of extreme maternal stress conditions such as famine and war.

WIDER IMPLICATIONS OF THE FINDINGS

The results from existing studies in this topic may be speculative, and additional research with more comprehensive design, data and covariates is needed to reconsider the robustness of previous findings.

STUDY FUNDING/COMPETING INTEREST(S)

The author receives no external funding and has no conflict of interest to declare.

TRIAL REGISTRATION NUMBER

N/A.

Live birth sex ratios and father's geographic origins in Jerusalem, 1964-1976

OBJECTIVE

To examine whether ancestry influenced sex ratios of offspring in a birth cohort before parental antenatal sex selection influenced offspring sex.

METHODS

We measured the sex ratio as the percent of males according to countries of birth of paternal and maternal grandfathers in 91,459 live births from 1964 to 1976 in the Jerusalem Perinatal Study. Confidence limits (CI) were computed based on an expected sex ratio of 1.05, which is 51.4% male.

RESULTS

Of all live births recorded, 51.4% were male. Relative to Jewish ancestry (51.4% males), significantly more males (1,761) were born to Muslim ancestry (54.5, 95% CI = 52.1-56.8, P = 0.01). Among the former, sex ratios were not significantly associated with paternal or maternal age, education, or offspring's birth order. Consistent with a preference for male offspring, the sex ratio decreased despite increasing numbers of births over the 13-year period. Sex ratios were not affected by maternal or paternal origins in North Africa or Europe. However, the offspring whose paternal grandfathers were born in Western Asia included fewer males than expected (50.7, 50.1-51.3, P = 0.02), whether the father was born abroad (50.7) or in Israel (50.8). This was observed for descendents of paternal grandfathers born in Lebanon (47.6), Turkey (49.9), Yemen & Aden (50.2), Iraq (50.5), Afghanistan (50.5), Syria (50.6), and Cyprus (50.7); but not for those from India (51.5) or Iran (51.9). The West Asian group showed the strongest decline in sex ratios with increasing paternal family size.

CONCLUSIONS

A decreased sex ratio associated with ancestry in Western Asia is consistent with reduced ability to bear sons by a subset of Jewish men in the Jerusalem cohort. Lower sex ratios may be because of pregnancy stress, which may be higher in this subgroup. Alternatively, a degrading Y chromosome haplogroup or other genetic or epigenetic differences on male germ lines could affect birth ratios, such as differential exposure to an environmental agent, dietary differences, or stress. Differential stopping behaviors that favor additional pregnancies following the birth of a daughter might exacerbate these lower sex ratios.

THE ENIGMA OF ETHIOPIAN SEX RATIOS AT BIRTH

This study analysed sex ratios at birth (defined as the number of male births per 100 female births) using data on children ever-born from three censuses conducted in Ethiopia in 1984, 1994 and 2007. The results showed very high values by any standard, with an average of 108.4 for a sample of some 8.2 million births, with somewhat lower values in urban areas. Analysis of socioeconomic correlates revealed that the sex ratio varied very much by household wealth, from about 110 for very poor women to about 102 for wealthier women. The high value of the sex ratio at birth in Ethiopia could be explained by poverty, used as a proxy for poor nutritional status. In multivariate analysis, the effects of living in urban areas and of maternal education were less important than household wealth. Among the many ethno-linguistic groups, the Nilotic family had higher sex ratios than other groups. The results were confirmed using data from DHS surveys conducted in the country, and by the analysis of children still living at time of census.

Civil conflict, gender-specific fetal loss, and selection: a new test of the Trivers-Willard hypothesis

A sizeable economics literature explores the effect of prenatal shocks on later health or socioeconomic status. Work in other disciplines, following the seminal contribution of Trivers and Willard (1973), suggests that prenatal shocks may increase fetal loss and reduce the number of boys relative to girls at birth. This has been largely ignored in the economics literature and could affect the interpretation of estimates of the effect of prenatal shocks and that of gender in other applied economics contexts. This paper analyzes the effect of in utero exposure to a shock - civil conflict in Nepal - on (i) fetal loss, and (ii) gender and (iii) health at birth. Maternal fixed effects estimates show that exposed pregnancies are more likely to result in a miscarriage and in a female birth, but exposed newborns are neither smaller nor more subject to neonatal mortality.

Economic stress or random variation? Revisiting German reunification as a natural experiment to investigate the effect of economic contraction on sex ratios at birth

BACKGROUND

The economic stress hypothesis (ESH) predicts decreases in the sex ratio at birth (SRB) following economic decline. However, as many factors influence the SRB, this hypothesis is difficult to test empirically. Thus, researchers make use of quasi-experiments such as German reunification: The economy in East, but not in West Germany, underwent a rapid decline in 1991. A co-occurrence of a decline in the East German SRB in 1991 has been interpreted by some as support for the ESH. However, another explanation might be that the low SRB in 1991 stems from increased random variation in the East German SRB due to a drastically reduced number of births during the crisis. We look into this alternative random variation hypothesis (RVH) by re-examining the German case with more detailed data.

METHODS

Our analysis has two parts. First, using aggregate-level birth register data for all births in the period between 1946 and 2011, we plot the quantum and variance of the SRB and the number of births and unemployment rates, separately for East and West Germany, and conduct a time series analysis on the East German SRB over time. Second, we model the odds for a male birth at the individual level in a multiple logistic regression (1991-2010, ~13.9 million births). Explanatory variables are related to the level of the individual birth, the mother of the child born, and the regional economic context.

RESULTS

The aggregate-level analysis reveals a higher degree of variation of the SRB in East Germany. Deviations from the time trend occur in several years, seemingly unrelated to economic development, and the deviation in 1991 is not statistically significant. The individual-level analysis confirms that the 1991-drop in the East German SRB cannot directly be attributed to economic development and that there is no statistically significant effect of economic development on sex determination in East or West Germany.

CONCLUSION

Outcomes support the RVH but not the ESH. Furthermore, our results speak against a statistically significant effect of the reunification event itself on the East German SRB. We discuss the relative importance of behavioral and physiological responses to macro-level stressors, a distinction that may help integrate previously mixed findings.

Declined sex ratio at birth in Fallujah (Iraq) during Iraq war with Iran

The main objective of this study was to investigate whether the sex ratio at birth (SRB, male proportion) alter in Iraq during its war against Iran (1980-1988). Here we compared 785 births (348 males, 437 females) during 1980-1989 with 1144 births (655 males, 489 females) before 1980 and 2914 births (1496 males, 1418 females) after 1989 in Fallujah, Iraq. The SRB significantly decreased during 1980-1989 in comparison with the ratios before 1980 (OR=0.595, 95 % CI: 0.495-0.714, P<0.001) and after 1989 (OR=0.755, 95 % CI: 0.644-0.884, P=0.001). Considering that the Iran-Iraq war began at 1980 and ended in 1988, therefore it seems that the SRB decreased in Fallujah (Iraq) during the war.

Selection in utero contributes to the male longevity deficit

The literature offering evolutionary explanations of the male longevity deficit does not address temporal variation in the deficit. This circumstance appears attributable to the fact that natural selection intuitively explains the deficit's pervasive and persistent nature, while social processes more parsimoniously explain its temporal variability. I offer consilience of these perspectives by speculating that selection in utero, a mechanism both conserved by natural selection and affected by social processes, could induce deviations around trend in the male longevity deficit. I describe the mechanism and offer an empirical test of its possible effect among Swedes - a population with the longest continuous record of sex-specific longevity in annual birth cohorts. I replicate the test with data from England and Wales. Results support the hypothesis that selection in utero against less fit males may explain part of the difference in longevity between males and females in modern populations.

Offspring sex ratio at birth and maternal breast cancer risk: A case-control study and meta-analysis of literature

To investigate whether offspring sex ratio at birth (SRB) was associated with maternal breast cancer risk, the present case-control study and a meta-analysis of literature were done. A total of 389 female breast cancer patients who had at least one offspring participated in the study. From general population 356 healthy female were selected as a control group. Control subjects were frequency matched with patients according to age and number of offspring. Among control group, 1324 offspring (666 males, 658 females) and within families of the breast cancer patients 1326 offspring (648 males, 678 females) were identified. There was no difference for SRB between breast cancer patients and control subjects (P>0.05). Meta-analysis was performed using 7 eligible studies. There was significant heterogeneity between studies (P<0.05). In overall the SRB was negatively associated with the maternal breast cancer risk (OR=0.95; 95 % CI: 0.92-0.99, P=0.02). Menopausal status of the patients was not associated with offspring SRB (P=0.07). Further researches are needed to determine the possible association between gender of offspring and maternal breast cancer risk.

The changing sex ratios at birth during the civil war in Tajikistan: 1992-1997

Sex ratios at birth are known to change during wars or shortly after. This study investigated changes in sex ratios during the civil war that occurred in Tajikistan after the dismantling of the Soviet Union. This civil war was particularly bloody and long lasting, and had many demographic consequences. According to vital registration data, some 27,000 persons died in excess of previous trends during the civil war period (1992-1997), and total mortality was sometimes estimated to be three times higher by independent observers. Birth rates dropped markedly during the war, and sex ratios at birth increased significantly from 104.6 before the war to 106.9 during the war, to return to baseline values afterwards. The change in sex ratio is investigated according to demographic evidence (migration, delayed marriage, spouse separation), substantiated with qualitative evidence (difficulties with food supply), and compared with patterns found in Europe during World War II, as well as with recent wars in the Middle East.

Sex ratio at birth following prenatal maternal exposure to severe life events: a population-based cohort study

BACKGROUND

A reduction in the sex ratio at birth has been linked to maternal condition during and before pregnancy. A recent study reported an association between maternal exposure to severe life events and sex ratio at birth using the Danish national register. We attempted to replicate that study using a new Danish cohort.

METHODS

Mothers of all singleton live births (n = 1.35 million births) in Denmark, between 1 January 1980 and 31 December 2002, were linked to data on their children and partners. The old cohort consisted of babies born between 1980 and 1992 (n = 699 362), whereas the new cohort consisted of babies born between 1993 and 2002 (n = 633 451). We defined exposure as death or serious illness in older children and partners in the first trimester or in the 6 months before conception. Sex ratio at birth was defined as the proportion of male live births.

RESULTS

During the study period, there were 1,349,099 singleton live births (692,870 boys and 656,229 girls). The sex ratio at birth in the new cohort was 0.5134. In the new cohort, prenatal exposure to severe life events was not associated with a reduction in the sex ratio at birth [relative risk = 1.00 (95% confidence interval: 0.95-1.05)].

CONCLUSIONS

In the new cohort, we did not find strong evidence that, in a stable western population, prenatal exposure to severe life events is associated with a reduction in the sex ratio at birth.

Sex ratio patterns according to Asian ethnicity in Québec, 1981-2004

Elevated ratios of male to female births are emerging in Asian countries due to selective abortion of female foetuses. Little research has evaluated the possibility of sex selection among Asians in the West. We evaluated patterns in the secondary sex ratio (SSR, number of male per 100 female births) according to ethnicity in Québec, Canada. We performed a population-based retrospective analysis of live and still births in urban Québec, 1981-2004 (N = 1,612,614 live births). Outcomes included: (1) first and second order SSR over four consecutive 6-year intervals analysed according to parental mother tongue, parental birth region, and mothers still speaking her mother tongue at home, (2) adjusted relative risk (RR) of female birth by parental ethnicity, and (3) estimated number of unborn females. For the period 1987-1992, first order SSRs were elevated (range 118-121) for Indo-Pakistani parental mother tongue and mothers currently speaking Indo-Pakistani at home. Second order SSRs were not elevated. For Indo-Pakistani mother tongues, the RR of female birth was lower than French/English in the same period (adjusted RR 0.96, 95% CI 0.92-0.99). SSRs were not imbalanced in the late 1990s among Indo-Pakistani parents, or among Chinese ethnicity in any study period. The SSR in Québec was elevated in the 1980s and early 1990s among firstborn relative to subsequent born infants of Indo-Pakistani descent. The reason for this imbalance is unclear. Further research in other Western settings is necessary to evaluate the possibility of sex selection.

Effect of 1-month war in Lebanon on sex ratio

Our study showed no effect of 33-day war in Lebanon on sex ratio. More research is needed to explore other modifying factors for a better understanding of the complex effect of wars on sex ratio changes.

Effect of the Lebanese civil war on sex ratio

Sex ratio is a subject of scientific interest but little is known about the factors that affect the sex ratio of humans. The aim of this study was to assess the effect of the Lebanese civil war on sex ratio. Data on all live births delivered at a large university hospital for the years 1977-2005 were used in this study. Study periods were defined as wartime (1977-1992) and post-war (1993-2005). The sex ratio in the study time period was calculated as the male proportion, i.e. males/males + females in live-born infants. Sex ratio during the war was compared with that of the post-war period. The sex ratio was similar in the war and post-war period (0.515 versus 0.513; OR = 1.007; 95% CI 0.98-1.04). The annual variation in the sex ratio during the study period did not show any significant change in any of the years. In conclusion, the Lebanese civil war did not cause a detectable change in sex ratio at birth. Factors that might have affected the sex ratio include the nature of the study population (civilians), the variable intensity of war in different periods, and the effect of stress and environmental toxins.

Did the 1991-1995 wars in the former Yugoslavia affect sex ratio at birth?

Proportion of males at birth (commonly referred to as the sex ratio) has been investigated for countries of former Yugoslavia that were affected by the 1991-1995 wars. Number of live births for Slovenia, Croatia, Bosnia and Herzegovina with Serbian Republic, and Serbia and Montenegro were obtained from the official vital statistics data and analysed with the Chi-square test. Results yielded no difference in the sex ratio associated with the war for the entire data set. However, country level data analysis revealed a significant increase in Bosnia and Herzegovina with the Serbian Republic, where proportion of male births during the wartime reached as high as 0.523 (compared to 0.516 in the pre-war and 0.515 in the post-war period). Countries that were involved in either mild or intermediate level of warfare did not exhibit a significant increase in the sex ratio (e.g. Slovenia and Croatia). Although war in Croatia lasted a year longer than in Bosnia and Herzegovina, analysis of the most intensive wartime periods in Croatia did not yield significant change. It seems that a hypothetical threshold of the warfare intensity combined with duration has to be reached (e.g. as in case of Bosnia and Herzegovina), in order for war to influence the sex ratio.

Abnormal sex ratios in human populations: causes and consequences

In the absence of manipulation, both the sex ratio at birth and the population sex ratio are remarkably constant in human populations. Small alterations do occur naturally; for example, a small excess of male births has been reported to occur during and after war. The tradition of son preference, however, has distorted these natural sex ratios in large parts of Asia and North Africa. This son preference is manifest in sex-selective abortion and in discrimination in care practices for girls, both of which lead to higher female mortality. Differential gender mortality has been a documented problem for decades and led to reports in the early 1990s of 100 million "missing women" across the developing world. Since that time, improved health care and conditions for women have resulted in reductions in female mortality, but these advances have now been offset by a huge increase in the use of sex-selective abortion, which became available in the mid-1980s. Largely as a result of this practice, there are now an estimated 80 million missing females in India and China alone. The large cohorts of "surplus" males now reaching adulthood are predominantly of low socioeconomic class, and concerns have been expressed that their lack of marriageability, and consequent marginalization in society, may lead to antisocial behavior and violence, threatening societal stability and security. Measures to reduce sex selection must include strict enforcement of existing legislation, the ensuring of equal rights for women, and public awareness campaigns about the dangers of gender imbalance.

Declining sex ratio in a first nation community

Members of the Aamjiwnaang First Nation community near Sarnia, Ontario, Canada, voiced concerns that there appeared to be fewer male children in their community in recent years. In response to these concerns, we assessed the sex ratio (proportion of male births) of the Aamjiwnaang First Nation over the period 1984-2003 as part of a community-based participatory research project. The trend in the proportion of male live births of the Aamjiwnaang First Nation has been declining continuously from the early 1990s to 2003, from an apparently stable sex ratio prior to this time. The proportion of male births (m) showed a statistically significant decline over the most recent 10-year period (1994-2003) (m = 0.412, p = 0.008) with the most pronounced decrease observed during the most recent 5 years (1999-2003) (m = 0.348, p = 0.006). Numerous factors have been associated with a decrease in the proportion of male births in a population, including a number of environmental and occupational chemical exposures. This community is located within the Great Lakes St. Clair River Area of Concern and is situated immediately adjacent to several large petrochemical, polymer, and chemical industrial plants. Although there are several potential factors that could be contributing to the observed decrease in sex ratio of the Aamjiwnaang First Nation, the close proximity of this community to a large aggregation of industries and potential exposures to compounds that may influence sex ratios warrants further assessment into the types of chemical exposures for this population. A community health survey is currently under way to gather more information about the health of the Aamjiwnaang community and to provide additional information about the factors that could be contributing to the observed decrease in the proportion of male births in recent years.

Sex ratio at birth and war in Croatia (1991–1995)

BACKGROUND

We have investigated sex ratio at birth (expressed as the proportion of males) in Croatia before, during and after the war (1991-1995).

METHODS

Data for each of 21 counties in Croatia (861 516 births) were collected and pooled into two groups: the first, consisting of the counties unaffected by the war, and the second, comprising the counties affected by war events. Odds ratios of being born as a male were calculated, with being born in a county exposed to war defined as the risk factor.

RESULTS

No significant deviations from the expected ratio of 0.514 were found in pre-war, wartime or post-war period at the national level. The ratio was 0.515 during the pre-war and wartime periods, and 0.514 in the post-war period. Comparison of the ratios in the three periods in both affected and unaffected counties revealed no significant increase in the sex ratio. The only significant increase in the sex ratio was registered in two counties unaffected by the warfare.

CONCLUSIONS

This study indicates that warfare did not cause a detectable increase of the sex ratio at birth in Croatia, in contrast to what might have been predicted based on earlier reports in the literature.

Sex ratios in California following the terrorist attacks of September 11, 2001

BACKGROUND

Natural and man-made disasters as well as declining economies appear to coincide with reduced odds of male live births among humans (i.e. lower secondary sex ratio). This association has been attributed to excess death of males in gestation and to reduced conception of males. We attempt to empirically discriminate between these two attributions by testing the hypotheses that the attacks of September 11, 2001 were followed in California first by higher fetal death sex ratios and later by lower sex ratios among very low weight births and total live births.

METHODS

We apply interrupted time-series methods to the fetal death, very low birth weight, and secondary sex ratios. The methods control for trends, seasonal cycles, and other forms of autocorrelation that could induce spurious associations.

RESULTS

Findings support the excess death explanation in that the fetal death sex ratio reached its highest level in the 6 year test period in October and November of 2001, while the very low weight birth sex ratio dropped to its lowest level in 14 years in December of 2001. The secondary sex ratio exhibited its second lowest value in 14 years in December of 2001. No support was found for the reduced conception explanation in that the sex ratio did not differ from expected values 9, 10 or 11 months after the attacks.

CONCLUSIONS

We infer support for the excess death explanation at the expense of the reduced conception explanation. We also describe the implications of our findings for public health planning.