Reproductive suppression follows threats to child survival

Income, stress, and sex ratios over 1895-2010

The medical literature has shown that populations under high stress have a lower sex ratio at birth (i.e. number of males for every 100 females). In this paper, I examine the relationship between income, as a source of economic stress, and the sex ratio at a subnational level for the 1895-2010 period. For this, I use census microdata from Argentina -a developing country that experienced rapid growth at the end of the 19th century and stagnated in recent decades- and I estimate from a two-way fixed effects model that exploits the wide temporal and geographic variability in income. The results show that as per capita income increases, the sex ratio at birth also increases. In particular, for every US$ 1,000 increase in per capita income, the sex ratio increases between 0.3 and 0.6 points. These findings make it possible to quantify the lost boys (i.e. those boys who were not born due to high economic stress on their parents) and constitute a call for attention in favor of the implementation of prenatal care policies -especially in periods of stagnation or income decline- to maintain a more balanced sex ratio.

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.

Reproductive suppression and longevity in human birth cohorts

OBJECTIVES

Reproductive suppression refers to, among other phenomena, the termination of pregnancies in populations exposed to signals of death among young conspecifics. Extending the logic of reproduction suppression to humans has implications for health including that populations exposed to it should exhibit relatively great longevity. No research, however, has tested this prediction.

METHODS

We apply time-series methods to vital statistics from Sweden for the years 1751 through 1800 to test if birth cohorts exposed in utero to reproductive suppression exhibited lifespan different from expected. We use the odds of death among Swedes age 1 to 9 years to gauge exposure. As the dependent variable, we use cohort life expectancy. Our methods ensure autocorrelation cannot spuriously induce associations nor reduce the efficiency of our estimates.

RESULTS

Our findings imply that reproductive suppression increased the lifespan of 24 annual birth cohorts by at least 1.3 years over the 50-year test period, and that 12 of those cohorts exhibited increases of at least 1.7 years above expected.

CONCLUSIONS

The best available data in which to search for evidence of reproductive suppression in humans support the argument that populations subjected to environments dangerous for children yield birth cohorts that exhibit unexpectedly great longevity.

Male-mediated prenatal loss: Functions and mechanisms

Sexually selected infanticide has been the subject of intense empirical and theoretical study for decades; a related phenomenon, male-mediated prenatal loss, has received much less attention in evolutionary studies. Male-mediated prenatal loss occurs when inseminated or pregnant females terminate reproductive effort following exposure to a nonsire male, either through implantation failure or pregnancy termination. Male-mediated prenatal loss encompasses two sub-phenomena: sexually selected feticide and the Bruce effect. In this review, we provide a framework that explains the relationship between feticide and the Bruce effect and describes what is known about the proximate and ultimate mechanisms involved in each. Using a simple model, we demonstrate that male-mediated prenatal loss can provide greater reproductive benefits to males than infanticide. We therefore suggest that, compared to infanticide, male-mediated prenatal loss may be more prevalent in mammalian species and may have played a greater role in their social evolution than has previously been documented.

Can sex ratios at birth be used in the assessment of public health, and in the identification of causes of selected pathologies?

This paper will consist of two parts. In the first, further support is given to the proposal that offspring sex ratios (proportions male) may usefully be regarded as indicators of public health. In the second, it is shown that sex ratios may help in the identification of the causes and effects of several pathologies that seriously impinge on public health viz. autism, testicular cancer, hepatitis B and toxoplasmosis.

Offspring sex ratio: Coital rates and other potential causal mechanisms

In recent years, scientists have begun to pay serious attention to the hypothesis that human parental coital rates around the time of conception causally influences the sexes of subsequent births. In this paper, the grounds of the argument are outlined. The point is important because, if the hypothesis were credible, it can potentially explain one of the best established (and otherwise unexplained) epidemiological features of sex ratio at birth - its rises during and just after World Wars 1 and 2 insofar as increased coital rates increase the ratio. Moreover, the greater the understanding of the variations of sex ratio at birth, the greater will be the understanding of the causes of those selected diseases associated with unusual sex ratios at birth (testicular cancer, hepatitis B, Toxoplasma gondii, and, perhaps, prostatic cancer).

Understanding periviable birth: A microeconomic alternative to the dysregulation narrative

Periviable infants (i.e., those born in the 20th through 26th weeks of gestation) suffer much morbidity and approximately half die in the first year of life. Attempts to explain and predict these births disproportionately invoke a "dysregulation" narrative. Research inspired by this narrative has not led to efficacious interventions. The clinical community has, therefore, urged novel approaches to the problem. We aim to provoke debate by offering the theory, inferred from microeconomics, that risk tolerant women carry, without cognitive involvement, high risk fetuses farther into pregnancy than do other women. These extended high-risk pregnancies historically ended in stillbirth but modern obstetric practices now convert a fraction to periviable births. We argue that this theory deserves testing because it suggests inexpensive and noninvasive screening for pregnancies that might benefit from the costly and invasive interventions clinical research will likely devise.

Separating the Bruce and Trivers-Willard effects in theory and in human data

OBJECTIVES

Theories of reproductive suppression predict that natural selection would conserve mechanisms that abort the gestation of offspring otherwise unlikely to thrive in prevailing environments. Research reports evidence among humans of at least two such mechanisms-the Trivers-Willard and Bruce Effects. No literature, however, compares the mechanisms nor estimates their relative contribution to observed characteristics of human birth cohorts. We describe similarities and differences between the Trivers-Willard and Bruce Effects and explore high quality historical data from Sweden to determine which mechanism better describes temporal variation in the ratio of males to females in birth cohorts.

METHODS

We measure Trivers-Willard exposures with the death rate among women of reproductive age. We measure Bruce exposures with the death rate among children. We use time-series regression methods to estimate the relative contribution of the Trivers-Willard and Bruce Effects to temporal variation in historical Swedish secondary sex ratio data.

RESULTS

We find that the Bruce Effect appears to be a better predictor of the secondary sex ratio than does the Trivers-Willard Effect.

CONCLUSIONS

Attempts to identify mechanisms by which reproductive suppression affects fetal loss and characteristics of human birth cohorts should consider the Bruce Effect as an alternative to the Trivers-Willard Effect.

Maternal residential proximity to unconventional gas development and perinatal outcomes among a diverse urban population in Texas

OBJECTIVE

To assess associations between unconventional natural gas development (UGD) and perinatal outcomes.

METHODS

We conducted a retrospective birth cohort study among 158,894 women with a birth or fetal death from November 30, 2010-November 29, 2012 in the Barnett Shale, in North Texas. We constructed three UGD-activity metrics by calculating the inverse distance-weighted sum of active wells within three separate geographic buffers surrounding the maternal residence: ≤½, 2, or 10-miles. We excluded women if the nearest well to her residence was >20 miles. Metrics were categorized by tertiles among women with ≥1 well within the respective buffer; women with zero wells ≤10 miles (the largest buffer) served as a common referent group. We used logistic or linear regression with generalized estimating equations to assess associations between UGD-activity and preterm birth, small-for-gestational age (SGA), fetal death, or birthweight. Adjusted models of fetal death and birthweight included: maternal age, race/ethnicity, education, pre-pregnancy body mass index, parity, smoking, adequacy of prenatal care, previous poor pregnancy outcome, and infant sex. Preterm birth models included all of the above except parity; SGA models included all of the above except previous poor pregnancy outcome.

RESULTS

We found increased adjusted odds of preterm birth associated with UGD-activity in the highest tertiles of the ½- (odds ratio (OR) = 1.14; 95% confidence interval 1.03, 1.25), 2- (1.14; 1.07, 1.22), and 10-mile (1.15; 1.08, 1.22) metrics. Increased adjusted odds of fetal death were found in the second tertile of the 2-mile metric (1.56; 1.16, 2.11) and the highest tertile of the 10-mile metric (1.34; 1.04-1.72). We found little indication of an association with SGA or term birthweight.

CONCLUSIONS

Our results are suggestive of an association between maternal residential proximity to UGD-activity and preterm birth and fetal death. Quantifying chemical and non-chemical stressors among residents near UGD should be prioritized.

Maternal residential proximity to unconventional gas development and perinatal outcomes among a diverse urban population in Texas

OBJECTIVE

To assess associations between unconventional natural gas development (UGD) and perinatal outcomes.

METHODS

We conducted a retrospective birth cohort study among 158,894 women with a birth or fetal death from November 30, 2010-November 29, 2012 in the Barnett Shale, in North Texas. We constructed three UGD-activity metrics by calculating the inverse distance-weighted sum of active wells within three separate geographic buffers surrounding the maternal residence: ≤½, 2, or 10-miles. We excluded women if the nearest well to her residence was >20 miles. Metrics were categorized by tertiles among women with ≥1 well within the respective buffer; women with zero wells ≤10 miles (the largest buffer) served as a common referent group. We used logistic or linear regression with generalized estimating equations to assess associations between UGD-activity and preterm birth, small-for-gestational age (SGA), fetal death, or birthweight. Adjusted models of fetal death and birthweight included: maternal age, race/ethnicity, education, pre-pregnancy body mass index, parity, smoking, adequacy of prenatal care, previous poor pregnancy outcome, and infant sex. Preterm birth models included all of the above except parity; SGA models included all of the above except previous poor pregnancy outcome.

RESULTS

We found increased adjusted odds of preterm birth associated with UGD-activity in the highest tertiles of the ½- (odds ratio (OR) = 1.14; 95% confidence interval 1.03, 1.25), 2- (1.14; 1.07, 1.22), and 10-mile (1.15; 1.08, 1.22) metrics. Increased adjusted odds of fetal death were found in the second tertile of the 2-mile metric (1.56; 1.16, 2.11) and the highest tertile of the 10-mile metric (1.34; 1.04-1.72). We found little indication of an association with SGA or term birthweight.

CONCLUSIONS

Our results are suggestive of an association between maternal residential proximity to UGD-activity and preterm birth and fetal death. Quantifying chemical and non-chemical stressors among residents near UGD should be prioritized.