Evidence of adaptive intergenerational sex ratio adjustment in contemporary human populations

A test of oscillation in the human secondary sex ratio

Background and objectives

The sex ratio of human birth cohorts predicts the health and longevity of their members. Most literature invokes natural selection in support of the argument that heritable tendencies to produce male or female offspring induce oscillation in the sex ratio and its sequelae. Tests of the argument remain exceedingly rare because they require vital statistics describing many generations of a population both unaffected by migration and exposed to an exogenous stressor virulent enough to change the sex ratio at birth. We contribute to the literature by using time-series modeling to detect oscillation in the best data currently available for such a test.

Methodology

We apply rigorous time-series methods to data describing Sweden from 1751 through 1830, a period when the population not only aged in place without migration, but also exhibited the effects of an Icelandic volcanic eruption including a historically low secondary sex ratio. That very low sex ratio should have induced oscillation if heritable mechanisms appear in humans.

Results

We detected oscillation in the ratio but not that predicted by heritable tendencies to produce males or females. We found peak-to-trough oscillation at 14 rather than the approximately 32 years expected from the heritable tendencies argument.

Conclusions and implications

Our findings suggest that mechanisms other than perturbation of heritable tendencies to produce males or females induce oscillation in the human secondary sex ratio. These other mechanisms may include reproductive suppression and selection in utero.

LAY SUMMARY

The male to female ratio in human birth cohorts predicts longevity but its variation over time remains unexplained. We test the long-held theory that the ratio oscillates due to heritable tendencies to produce males or females. We find oscillation, but it appears due to social processes rather than heritable mechanisms.

China's Great Leap Forward: An estimate of total and additional excess male birth losses

INTRODUCTION

Stress reduces the male to female ratio at birth (M/T). China's Great Leap Forward (11/1957-1/1961) had disastrous consequences with a catastrophic national famine that lasted between 1959 and 1961. This resulted in a reduction in births and an additional male birth deficit. This study was carried out in order to estimate the total pregnancy losses and M/T, along with these value's perinatal mortality rates equivalents.

METHODS

Estimated M/T and births for 1956-61 and for the aggregates of 1956-58 and 1959-61 were calculated. Expected values for the latter period were also estimated.

RESULTS

There was a sharp decline in total births after 1958 as well as a reduction in M/T. There were an estimated 18,286,000 less births in 1959-61 than anticipated (chi=2147, p<0.0001). Based on M/T for the period 1956-8, for the 42,318,000 births observed in 1959-61, with an expected M/T (0.5246≡52.5%), there was an additional deficit of 196,221 male births (chi=1823, p<0.0001). The total birth deficit was circa 301.7 per 1000 with an additional male deficit of 3.2 per 1000.

DISCUSSION

The perinatal mortality rate is an important public health indicator and is expected to be ≈6/1000-these values greatly exceed perinatal mortality figures. Political decisions should be taken with caution since they can effect enormous impacts on history and on human biology, including massive effects on births and M/T. MESH: Birth rate/*trends; Sex ratio; China; Infant, newborn; Starvation.

PARTIALLY CONSTRAINED SEX ALLOCATION AND THE INDIRECT EFFECTS OF ASSISTED REPRODUCTIVE TECHNOLOGIES ON THE HUMAN SEX RATIO

Infertility affects around 15% of human couples and in many countries approximately 1-4% of babies are born following Assisted Reproductive Technologies (ART). Several ART techniques are used and these differentially affect the sex ratio of offspring successfully produced. These direct effects on sex ratio also have the potential to influence, indirectly, the sex ratios of offspring born to untreated couples. This is of concern because human sex ratio bias may adversely affect public health. Here the extent of indirect effects of ART that could operate, via Fisherian frequency-dependent natural selection, on the progeny sex ratio of unassisted members of a population is heuristically modelled. Given the degrees to which ART techniques bias sex ratios directly, it is predicted that well over 20% of couples would have to reproduce via ART for there to be any discernible effect on the sex ratios produced, in response, by the remainder of the population. This value is greater than the estimated prevalence of infertility problems among human couples. It is concluded that providing ART to couples with fertility problems does not currently generate significant ethical issues or public health concern in terms of indirect effects on the offspring sex ratios of untreated couples.

Neonatal outcomes in relation to sex differences: a national cohort survey in Taiwan

Background

An extensive assessment investigating the association between sex differences and neonatal outcomes is lacking. In the current study, we estimated the correlation of gender with adverse birth outcomes in a large cohort population.

Methods

National population-based data containing maternal and neonatal information in 2001 to 2010 were derived from the Health Promotion Administration, Taiwan. Singletons without high-risk pregnancy were further analyzed for the sex ratio of live births in relation to neonatal outcomes—including preterm birth, birth weight, neonatal death, delivery mode, and congenital anomaly. A multivariate logistic regression model was used to adjust for possible confounders.

Results

In total, 2,123,100 births were valid for the analysis. Overall, the sex ratio at birth (male/female) was 1.096. Compared to multiple births, the sex ratio was significantly higher with singleton births (p < 0.001). Among multiple births, the incidence of stillbirths was significantly higher in males than in females (p < 0.05). The sex ratio at gestational age (GA) <37 weeks was 1.332, and it declined proportionally with a rise in the GA to 0.899 at GA of ≥41 weeks. In contrast, the sex ratio was 0.850 at birth weight <3000 g, and it rose proportionally with a rise in the birth weight to 1.902 at birth weight ≥4000 g (macrosomia). Operative delivery was more common in males than in females (p < 0.001). The regression analysis showed greater risks of preterm birth, macrosomia, operative delivery, neonatal death, and congenital anomaly among male newborns.

Conclusions

Male gender carried higher risks of adverse neonatal outcomes, including preterm birth, macrosomia, operative delivery, neonatal death, and congenital anomaly. The data have clinical implications on health surveillance for plotting strategies in response to the unbalanced sex ratio in relation to the boy preference.

Proximate causes of the variation of the human sex ratio at birth

There is evidence that the human sex ratio (proportion males at birth) is the result of two processes. First, the sexes of zygotes (from which the primary sex ratio would be calculated) are thought to be partially controlled by the hormone levels of both parents around the time of conception. Second, this primary sex ratio is apparently modified downwards by male-sex-selective spontaneous abortion caused by high levels of maternal stress-induced adrenal androgens, thus yielding the sex ratio at birth (the secondary sex ratio). Since maternal stress is one cause of spontaneous abortion (and of other forms of reproductive sub-optimality), and since some forms of pharmacological treatment of maternal stress are deleterious to the foetus, best practice would suggest non-pharmacological treatment (e.g. psychotherapy, hypnosis or massage) for pregnant women who have a previous history of spontaneous abortion, preterm birth or low-birth-weight infants.