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We find support for the hypothesis that changes in the monthly odds of a twin among live-born males predict subsequent and opposite changes in the odds of a twin among live-born females. The hypothesis arises from the long standing argument that natural selection has conserved mechanisms by which pregnant women in stressed populations spontaneously abort fetuses least likely to yield grandchildren. Previous attempts to empirically test this argument focus almost entirely on males. We contribute to the literature by showing that, consistent with the logic of natural selection, maternal adaptations to environmental changes likely have effects on the survival of both male and female conceptuses and fetuses.

The theory that natural selection has conserved mechanisms by which women subjected to environmental stressors abort frail male fetuses implies that climate change may affect sex ratio at birth and male longevity. Using time series methods, we find that cold ambient temperatures during gestation predict lower secondary sex ratios and longer life span of males in annual birth cohorts composed of Danes, Finns, Norwegians, and Swedes born between 1878 (earliest year with complete life tables) and 1914 (last birth cohort for which male life span can be estimated). We conclude that ambient temperature affects the characteristics of human populations by influencing who survives gestation, a heretofore unrecognized effect of climate on humanity.

We examined pregnancy outcomes in New York City (NYC) and upstate New York after the September 11, 2001 World Trade Center disaster.Using birth certificate data for NY residents (n = 1,660,401 births), we estimated risk of low birthweight (LBW:

The literature implies a 'diminished entelechy' hypothesis in which birth cohorts subjected to relatively many or relatively virulent environmental insults early in life do not realize their otherwise expected lifespan. No direct test of this hypothesis appears in literature.We test the hypothesis directly by measuring the association between mortality in the first 5 years and life expectancy at age 5 for male and female cohorts born in Sweden (1751-1912), Denmark (1835-1913), and England and Wales (1841-1912). The methods control for trends, seasonal cycles, and other forms of autocorrelation that could induce spurious associations.Our results support the hypothesis in that life expectancy at age 5 fell below the values expected from history in cohorts in which child mortality before age 5 increased over its expected value. We find no evidence for culling effects in which a cohort remaining after suffering relatively many environmental insults may be smaller but hardier than expected.These findings converge with individual-level studies and suggest that suffering relatively virulent or many environmental insults during childhood reduces the subsequent lifespan of birth cohorts.

Population stressors reportedly reduce the human secondary sex ratio (i.e., the odds of a newborn's being male) by, among other mechanisms, inducing the spontaneous abortion of males who would have been born live had mothers not been stressed. Controversy remains as to whether these abortions result from reduced maternal tolerance of males at the low end of a relatively constant distribution of survivability (i.e., the “culled cohort” explanation) or from shifts in the whole distribution of survivability such that more males fall below a relatively constant criterion of maternal tolerance for low survivability (i.e., the “damaged cohort” explanation). These alternatives make opposing predictions regarding the relationship between the secondary sex ratio and lifespan of male birth cohorts. We test the hypothesis that the secondary sex ratio among Swedish cohorts born in the years 1751 through 1912 predicts male cohort life expectancy at birth (i.e., realized lifespan). Our results support the culled cohort argument. We argue that these findings have implications for the basic literature concerned with temporal variation in the secondary sex ratio, for more applied work concerned with the fetal origins of adult health, and for pubic health surveillance.

The ratio of male to female live births (i.e. the sex ratio) reportedly falls when populations suffer rare and extreme ambient stressors such as the collapse of national economies. This association has been attributed to the death of male fetuses and to reduced conception of males. We assess the validity of the first of these mechanisms by testing the hypothesis that the fetal death sex ratio varies positively over time with the unemployment rate. Using the unemployment rate also allows us to determine if ambient economic stressors less extreme than collapsing national economies affect the fetal death sex ratio.We test our hypotheses by applying time-series methods to monthly counts of fetal deaths and the unemployment rate from the state of California beginning January 1989 and ending December 2001. The methods control for trends, seasonal cycles, and other forms of autocorrelation that could induce spurious associations.Results support the fetal death mechanism in that male fetal deaths increased above the values expected from female deaths and from history in months in which the unemployment rate also increased over its expected value.Our findings suggest that ambient stressors as common as increasing unemployment elevate the risk of fetal death among males. We discuss the social, economic, and health costs borne by parents and communities afflicted with these fetal deaths.

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.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.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.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.

Objectives: We aim to contribute to the literature reporting tests of selection in utero. The theory of reproductive suppression predicts that natural selection would conserve mechanisms, referred to collectively as selection in utero, that spontaneously abort fetuses unlikely to thrive as infants in the prevailing environment. Tests of this prediction include reports that women give birth to fewer than expected male twins, historically among the frailest of infants, during stressful times. The onset of the COVID-19 pandemic in the United States in Spring 2020 demonstrably stressed the population. We test the hypothesis that conception cohorts in gestation at the onset of the pandemic in the United States yielded fewer than expected live male twin births. Methods: We retrieved deidentified data on the universe of live births in the United States from the National Center for Health Statistics birth certificate records. We applied Box-Jenkins time-series methods to the twin secondary sex ratio computed for 77 monthly conception cohorts spanning August 2013 to December 2019 to detect outlying cohorts in gestation at the onset of the pandemic. Results: The twin secondary sex ratio fell below expected values in three conception cohorts (i.e., July, September, and October 2019, all p < .05) exposed in utero to the onset of the pandemic. Conclusions: Our results add to prior findings consistent with selection in utero. The role of selection in utero in shaping the characteristics of live births cohorts, especially during the COVID-19 pandemic, warrants further scrutiny. [ABSTRACT FROM AUTHOR]