Partnership status and the human sex ratio at birth

Ovarian Hyperstimulation Syndrome Is Associated with a High Secondary Sex Ratio in Fresh IVF Cycles with Cleavage-Stage Embryo Transfer: Results for a Cohort Study

The sex ratio at birth is defined as the secondary sex ratio (SSR). Ovarian hyperstimulation syndrome (OHSS) is a serious and iatrogenic complication associated with controlled ovarian stimulation (COS) during assisted reproductive technology (ART) treatments. It has been hypothesized that the human SSR is partially controlled by parental hormone levels around the time of conception. Given the aberrant hormonal profiles observed in patients with OHSS, this retrospective study was designed to evaluate the impact of OHSS on the SSR. In this study, all included patients were divided into 3 groups: non-OHSS (n=2777), mild OHSS (n=644), and moderate OHSS (n=334). Our results showed that the overall SSR for the study population was 1.033. The SSR was significantly increased in patients with moderate OHSS (1.336) compared to non-OHSS patients (1.002) (p=0.048). Subgroup analyses showed that increases in the SSR in patients with moderate OHSS were observed in the IVF group (1.323 vs 1.052; p=0.043), but not in the ICSI groups (1.021 vs 0.866; p=0.732). In addition, the elevated serum estradiol (E2) and progesterone (P4) levels in OHSS patients were not associated with SSR. In this study, for the first time, we report that a high SSR is associated with OHSS in patients who received fresh IVF treatments. The increases in SSR in OHSS patients are not attributed to the high serum E2 and P4 levels. Our findings may make both ART clinicians and patients more aware of the influences of ART treatments on the SSR and allow clinicians to counsel patients more appropriately.

Vulnerable Boys: Short-term and Long-term Gender Differences in the Impacts of Adolescent Disadvantage

The growing gender gap in educational attainment between men and women has raised concerns that the skill development of boys may be more sensitive to family disadvantage than that of girls. Using the National Longitudinal Study of Adolescent to Adult Health (Add Health) data we find, as do previous studies, that boys are more likely to experience increased problems in school relative to girls, including suspensions and reduced educational aspirations, when they are in poor quality schools, less-educated neighborhoods, and father-absent households. Following these cohorts into young adulthood, however, we find no evidence that adolescent disadvantage has stronger negative impacts on long-run economic outcomes such as college graduation, employment, or income for men, relative to women. We do find that father absence is more strongly associated with men's marriage and childbearing and weak support for greater male vulnerability to disadvantage in rates of high school graduation. An investigation of adult outcomes for another recent cohort from the National Longitudinal Survey of Youth, 1997 produces a similar pattern of results. We conclude that focusing on gender differences in behavior in school may not lead to valid inferences about the effects of disadvantage on adult skills.

Biological, environmental and socioeconomic determinants of the human birth sex ratio in the Czech Republic

The Trivers-Willard Hypothesis (TWH) states that parents in good conditions bias the sex ratio towards sons and parents in poor conditions bias the sex ratio towards daughters. This study used data from a large nationwide population dataset (N=1,401,851) from the Czech Republic - a modern contemporary society. The study included air pollution and property prices in the TWH estimation, and had a more detailed focus on stillbirths than previous studies. Using official natality microdata from the Czech Statistical Office for years between 1992 and 2010 and data on levels of air pollution in the country over the same period, the study assessed whether the biological and socioeconomic status of mothers and environmental factors affected the sex of children. The results were largely insignificant and not robust across specifications. The presented epidemiological evidence suggests that stillbirths are randomly distributed in the Czech Republic and that the sex ratio is not affected by the socioeconomic status of mothers or by environmental characteristics.

Sex of children and family structure in Brazil: father & grandmother bias?

Brazil has a high rate of mothers living without a partner as well as a high intergenerational co-residence rate, including parents and grandparents. These family types may influence a child's well-being. Even though there is no evidence of sex-selective abortion in Brazil, sex preference could still be subtly present. This paper tests the composition of family structure (father co-residence, grandmother co-residence, and birth of siblings) in Brazil associated with the sex of the child by using a nationally representative household survey, treating sex of the first and second child as exogenous variables in OLS regressions. I found women with lower birth-order daughters are less likely to live with a partner. I also found suggestive evidence that maternal grandmothers are more likely to live with granddaughters than with grandsons. Women with lower birth-order daughters are more likely to have additional children. Evidence suggests that in Brazil, fathers show a preference for sons over daughters, while grandmothers show a preference for granddaughters over grandsons. Additionally, mothers of girls, without co-residing partners may compensate for the economic loss caused by their lack of partner by living with their own mother. This contributes to the literature on child sex preferences, which has mostly focused on males (fathers); I have analyzed data on grandmothers to include females.

Social determinants of adult sex ratios and racial/ethnic disparities in transmission of HIV and other sexually transmitted infections in the USA

In Black population centres in the USA, adult sex ratios (ASRs) are strongly female-biased primarily due to high male incarceration and early mortality rates. I explore the system of social determinants that shape these ASRs, and describe their apparent consequences. Evidence suggests that female-biased ASRs play a role, along with racial residential segregation, to increase mixing between core and peripheral members of sexual networks, facilitating transmission of human immunodeficiency virus and other sexually transmitted infections. Unique historical factors underlie Black male incarceration and mortality rates in the USA, making comparisons with other groups or other countries challenging.This article is part of the themed issue 'Adult sex ratios and reproductive decisions: a critical re-examination of sex differences in human and animal societies'.

Relation between maternal antenatal anxiety and infants' weight depends on infants' sex: A longitudinal study from late gestation to 1-month post birth

OBJECTIVE

To test for gender-differences in the relation between mothers' antenatal anxiety and infants' body weight during gestation, at birth, and at 1-month of age.

METHODS

Two hundred and twelve randomly-recruited women were divided into two groups: Controls (n=105) and Anxious Group (n=107) based on a standard cut-off of the Beck Anxiety Inventory. Outcome measures were Fetal Weight derived from biometrics obtained from an ultrasound scan in the 3rd trimester and infants' weight at birth and at 1-month of age, both obtained from medical records.

RESULTS

Multivariate analyses showed main effects of Gender on infants' birth weight (P=.001) and on infants' weight at 1-month of age (P=.004), but no main effects of Anxiety Group at any time-point. Gender x Anxiety Group interactions at all three time points (Fetal weight: P=.05; Birth weight: P=.03; 1-month of age: P=.10) reflected gender differences (males>females) among infants in the anxious group, but not among controls. Distinct trends regarding same sex comparisons across groups (Control vs. Anxiety) were in line with predictions (male controls<male anxious; female controls>females anxious). Controlling for Postpartum Anxiety and Antenatal and Postpartum Depression in the models did not affect primary results.

CONCLUSION

Gender differences in fetal and birth weight were more substantial among infants of anxious mothers than among controls due to the seemingly accelerated growth of "anxious" males and the diminution of weight among "anxious" females.

Where have all the young men gone? Using sex ratios to measure fetal death rates

Fetal health is an important consideration in policy formation. Unfortunately, a complete census of fetal deaths, an important measure of overall fetal health, is infeasible, and available data are selectively observed. We consider this issue in the context of the Clean Air Act Amendments of 1970 (CAAA), one of the largest and most influential environmental regulations in the history of the United States. We discuss a model of potential bias in measuring observed fetal deaths, and present the sex ratio of live births as an alternative fetal health endpoint, taking advantage of the finding that males are more vulnerable to side effects of maternal stress in utero. We find the CAAA caused substantial improvements in fetal health, in addition to previously identified reductions in post-natal mortality.

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.

The impact of children's sex composition on parents' mortality

BACKGROUND

This study explores the relationship between children's sex composition and parents' mortality in a contemporary western society. It improves on earlier research by using a larger and more representative dataset - constructed from registers and encompassing the entire Norwegian population.

METHODS

The analysis is based on discrete-time hazard models, estimated for the years 1980-2008 for women and men born after 1935.

RESULTS

When operationalising sex composition as the "number of boys", coefficients are insignificant in all specifications. However, when considering the three categories "only boys", "only girls" and "mixed sex", I find a small but significant disadvantage of having only girls, compared to having at least one child of each sex, for mothers of two or more children. Having only daughters is associated with a mortality disadvantage compared to having only sons for mothers of two children, but a mortality advantage among mothers with four children. Among women who gave birth to their first child as teenagers, those who have only sons have relatively high mortality. I also find an excess mortality both for mothers of only girls and mothers of only boys in the period 1980-1989.

CONCLUSION

These results lend some support to the notion that there is a larger benefit of the first son or daughter than the later children of the same sex, and especially in the earliest decade of the study period.

Hormone-mediated adjustment of sex ratio in vertebrates

The ability to adjust sex ratios at the individual level exists among all vertebrate groups studied to date. In many cases, there is evidence for facultative adjustment of sex ratios in response to environmental and/or social cues. Because environmental and social information must be first transduced into a physiological signal to influence sex ratios, hormones likely play a role in the adjustment of sex ratio in vertebrates, because the endocrine system acts as a prime communicator that directs physiological activities in response to changing external conditions. This symposium was developed to bring together investigators whose work on adjustment of sex ratio represents a variety of vertebrate groups in an effort to draw comparisons between species in which the sex-determination process is well-established and those in which more work is needed to understand how adjustments in sex ratio are occurring. This review summarizes potential hormone targets that may underlie the mechanisms of adjustment of sex ratio in humans, non-human mammals, birds, reptiles, and fishes.

Geographic clustering of the secondary sex ratio in Japan: association with demographic attributes

The secondary sex ratio (SSR) has been suggested to decrease with adverse physical and psychological environments. Previous studies have focused on reduced SSR under adverse conditions, such as war, terrorism attack and earthquake, but few studies have investigated fluctuations in SSR in moderately adverse environments. This study analysed municipality-level vital statistics records in Japan collected between 1998 and 2002 to identify high-SSR clusters and low-SSR clusters with spatial-scan statistics. In 999 runs of simulation, high- and low-SSR clusters were detected but fewer than 950 times, indicating that SSR was not geographically clustered in Japan if type I error of 5% was adopted. Explorative analyses comparing demographic attributes between high-SSR clusters and low-SSR clusters that were detected more than 500 times in 999 runs of simulation, showed that rate of spontaneous abortion, rate of artificial abortion and divorce rate were higher in low-SSR clusters, while male life expectancy, female life expectancy and total fertility rate were higher in a high-SSR cluster.

Revisiting a sample of U.S. billionaires: how sample selection and timing of maternal condition influence findings on the Trivers-Willard effect

Based on evolutionary theory, Trivers & Willard (TW) predicted the existence of mechanisms that lead parents with high levels of resources to bias offspring sex composition to favor sons and parents with low levels of resources to favor daughters. This hypothesis has been tested in samples of wealthy individuals but with mixed results. Here, I argue that both sample selection due to a high number of missing cases and a lacking specification of the timing of wealth accumulation contribute to this equivocal pattern. This study improves on both issues: First, analyses are based on a data set of U.S. billionaires with near-complete information on the sex of offspring. Second, subgroups of billionaires are distinguished according to the timing when they acquired their wealth. Informed by recent insights on the timing of a potential TW effect in animal studies, I state two hypotheses. First, billionaires have a higher share of male offspring than the general population. Second, this effect is larger for heirs and heiresses who are wealthy at the time of conception of all of their children than for self-made billionaires who acquired their wealth during their adult lives, that is, after some or all of their children have already been conceived. Results do not support the first hypothesis for all subgroups of billionaires. But for males, results are weakly consistent with the second hypothesis: Heirs but not self-made billionaires have a higher share of male offspring than the U.S. population. Heiresses, on the other hand, have a much lower share of male offspring than the U.S. average. This hints to a possible interplay of at least two mechanisms affecting sex composition. Implications for future research that would allow disentangling the distinct mechanisms are discussed.

Newborn's condition at birth does not depend on maternal sexual strategy--evidence against the "hunting for good genes" hypothesis

OBJECTIVES

Neonatal weight and health depend on many epigenetic and environmental factors and also on a child's genes as inherited from the mother and the father. The "Hunting for Good Genes" (HfGG) hypothesis claims that women pursue short-term mating strategies in order to obtain good genes for their progeny. If this is true, one should expect that in comparison to children born in long-term relationships (LR), children born in short-term relationships (SR) should have a larger neonatal size and be healthier.

METHODS

To test the HfGG hypothesis, and whether sexual strategy influences neonatal parameters, we used an on-line questionnaire aimed at mothers. Totally, 1,558 women took part in this study, and among them 130 conceived their first-born child in a short-term relationship (SR mothers) and 1,428 in a long-term relationship (LR mothers).

RESULTS

There was no difference between those two groups of children in terms of birth weight and Apgar score. Although we found no difference in the occurrence of genetic diseases and birth defects between SR and LR boys, the girls born by LR mothers were healthier than those born by SR mothers. We also did not find any difference in the sex ratio of the children born to SR and LR mothers.

CONCLUSIONS

The results of our study do not support the "Hunting for Good Genes" hypothesis for the first born child. In the contemporary western society of the study, women do not seem to derive genetic benefits expressed as child condition at birth from short-term relationships.

Estimating a Dynamic Model of Sex Selection in China

High ratios of males to females in China, which have historically concerned researchers (Sen 1990), have increased in the wake of China’s one-child policy, which began in 1979. Chinese policymakers are currently attempting to correct the imbalance in the sex ratio through initiatives that provide financial compensation to parents with daughters. Other scholars have advocated a relaxation of the one-child policy to allow more parents to have a son without engaging in sex selection. In this article, I present a model of fertility choice when parents have access to a sex-selection technology and face a mandated fertility limit. By exploiting variation in fines levied in China for unsanctioned births, I estimate the relative price of a son and daughter for mothers observed in China’s census data (1982–2000). I find that a couple’s first son is worth 1.42 years of income more than a first daughter, and the premium is highest among less-educated mothers and families engaged in agriculture. Simulations indicate that a subsidy of 1 year of income to families without a son would reduce the number of “missing girls” by 67% but impose an annual cost of 1.8% of Chinese gross domestic product (GDP). Alternatively, a three-child policy would reduce the number of “missing girls” by 56% but increase the fertility rate by 35%.

Why does the Great Chinese Famine affect the male and female survivors differently? Mortality selection versus son preference

Evidence shows that exposure to nutritional adversity in early life has larger long-term impacts on women than on men. Consistent with these findings, our paper shows a higher incidence of disability and illiteracy among female survivors of the Great Chinese Famine (1959-1961). Moreover we find that the better health of male survivors most plausibly reflects higher male excess mortality during the famine, whereas the observed gender difference in illiteracy rate is probably better explained by the culture of son preference.

Exploring the causal machinery behind sex ratios at birth: does hepatitis B play a role?

The causal machinery underlying sex determination is directly relevant to many questions relating gender and family composition to social and economic outcomes. In recent work, Oster highlighted a correlation between parental hepatitis B carrier status and sex of the child. One of her analyses went further, speaking directly to causality. That analysis appeared to have answered an important question that had remained unresolved in medical and biological literatures—namely, does chronic infection with hepatitis B cause male‐skewed sex ratios at birth? Oster’s creative empirical analysis appeared to suggest that it does; however, in this article I reassess the result and present evidence that, at the very least, the question remains open. Further investigation into questions around the causal machinery of sex determination is warranted in the social science literature, as well as in that of biology and medicine. However, my results suggest that it is extremely unlikely that chronic hepatitis B infection plays a biologically significant role.

Extra-pair young in house wren broods are more likely to be male than female

Sex-allocation theory predicts that females should preferentially produce offspring of the sex with greater fitness potential. In socially monogamous animal species, extra-pair mating often increases the variance in fitness of sons relative to daughters. Thus, in situations where offspring sired by a female's extra-pair mate(s) will typically have greater fitness potential than offspring sired by the within-pair mate, sex-allocation theory predicts that females will bias the sex of offspring sired by extra-pair mates towards male. We examined the relationship between offspring sex and paternity over six breeding seasons in an Illinois population of the house wren (Troglodytes aedon), a cavity-nesting songbird. Out of the 2345 nestlings that had both sex and paternity assigned, 350 (15%) were sired by extra-pair males. The sex ratio of extra-pair offspring, 0.534, was significantly greater than the sex ratio of within-pair offspring, 0.492, representing an increase of 8.5 per cent in the proportion of sons produced. To our knowledge, this is the first confirmed report of female birds increasing their production of sons in association with extra-pair fertilization. Our results are consistent with the oft-mentioned hypothesis that females engage in extra-pair mating to increase offspring quality.

Reproductive parameters over a 37-year period of free-ranging female Borneo orangutans at Sepilok Orangutan Rehabilitation Centre

We analysed the reproductive parameters of free-ranging female orangutans at Sepilok Orangutan Rehabilitation Centre (SORC) on Borneo Island, Sabah, Malaysia. Fourteen adult females produced 28 offspring in total between 1967 and 2004. The average censored interbirth interval (IBI) (i.e. offspring was still alive when mother produced a next offspring) was 6 years. This was shorter than censored IBIs reported in the wild but similar to IBIs reported for those in captivity. The nonparametric survival analysis (Kaplan-Meier method) revealed a significantly shorter IBI at SORC compared with wild orangutans in Tanjung Putting. The infant (0-3 years) mortality rate at SORC of 57% was much higher than rates reported both in the wild and captivity. The birth sex-ratio was significantly biassed toward females: 24 of the 27 sex-identified infants were females. The average age at first reproduction was 11.6 years, which is younger than the age in the wild and in captivity. The high infant mortality rate might be caused by human rearing and increased transmission of disease due to frequent proximal encounters with conspecifics around the feeding platforms (FPs). This young age of first reproduction could be because of the uncertainty regarding estimated ages of the female orangutans at SORC. It may also be affected by association with other conspecifics around FPs, which increased the number of encounters of the females with males compared with the number of encounters that would take place in the wild. Provision of FPs, which improves the nutritional condition of the females, caused the shorter IBI. The female-biassed birth sex-ratio can be explained by the Trivers and Willard hypothesis. The female-biassed sex ratio could be caused by the mothers being in poor health, parasite prevalence and/or high social stress (but not food scarcity) due to the frequent encounters with conspecifics around FPs.

Sex-of-Offspring Differences between Mothers

Evolutionary biologists and psychologists have long been aware of maternal effects on the secondary sex ratio in mammals. Up till now these have appeared inconsistent but re-analysis of the evidence, focusing on normally distributed maternal testosterone and its physiological and behavioral sequelae, reveals some support for the existence of sex-of-offspring differences between mothers. In addition to the animal studies showing a relationship between maternal, testosterone-based dominance and the secondary sex ratio, research on human mothers shows sex-of-offspring differences in a range of testosterone—related attributes (physiological, behavioral and cognitive) measured at varying times from before the child was conceived to the mother's maturity. Further exploration of these differences might help elucidate the problems surrounding sex determination in mammals.

Female genital cutting and mother's age at birth are associated with the sex of offspring in Africa

Female genital cutting (FGC) is widespread in Africa. Its short and long term medical consequences are widely reported. Here we report that FGC is associated with and may contribute to increases in the sex ratio at birth (Odds Ratio = 1.019; 95% C.I. = 1.007, 1.032) while mother's age at birth is inversely associated with probability of male birth (Odds Ratio = 0.998; 95% C.I. = 0.997, 0.999) in a generalized linear, logistic model with the probability of a male birth the dependent variable and eight potential explanatory variables applied to 413,384 births in 22 African countries. While these two associations are statistically significant, their odds ratios are close to 1.0 and they only slightly decrease the null deviance of the model. Therefore sex determination in these data is still a mostly stochastic process determined by the random variation in X or Y sperm production, transport, and conception.

Trivers-Willard at birth and one year: evidence from US natality data 1983-2001

Trivers & Willard (TW) hypothesized that evolution would favour deviations from the population sex ratio in response to parental condition: parents in good condition would have more sons and parents in poor condition would have more daughters. We analyse the universe of US linked births and infant deaths to white mothers 1983-2001, covering 48 million births and 310,000 deaths. We find that (i) married, better educated and younger mothers bore more sons and (ii) infant deaths were more male if the mother was unmarried and young. Our findings highlight the potential role of offspring sex ratio as an indicator of maternal status, and the role of infant mortality in shaping a TW pattern in the breeding population.

Possible constraints on adaptive variation in sex ratio at birth in humans and other primates

There is general agreement that adaptive variation of sex ratio at birth has not been decisively demonstrated in primates (including human beings). So some workers have questioned whether it actually exists. Others have conjectured that it exists but is subject to as yet unidentified 'constraints' (factors opposing the modifying influences of selection in the phenotype). Meanwhile though most workers have called for research to reveal the proximate causes of sex ratio variation, few (if any) have directed studies toward that end. Here it is argued that hormonal action is responsible both for much adaptive and non-adaptive sex ratio variation, and for constraints on the adaptive variation. My hypothesis proposes that levels of steroid hormones (testosterone and oestrogen) of both parents around the time of conception are positively associated with offspring sex ratio (proportion male at birth) of mammals including man. Testosterone in men and oestrogen in women are also known to be positively associated with the health, attractiveness and fertility of individual human beings. However, high levels of testosterone in women are frequently associated with adverse medical conditions. It is suggested that for these reasons (and contrary to some adaptive theory) some classes of people (particularly women) in suboptimal health ("condition") produce excesses of sons. It seems that gonadal hormones are responsible for adaptive variation; and that maternal adrenal hormones are responsible for maladaptive variation. In evolutionary terms, gonadal hormones precede adrenal hormones.