Intrauterine stress and male cohort quality: The case of September 11, 2001

Empirical research and the theory of natural selection assert that male mortality more than female mortality responds to ambient stressors in utero. Although population stressors may adversely damage males that survive to birth, the rival culled cohort hypothesis contends that males born during stressful times may exhibit better health than males in other cohorts because fetal loss has “culled” the frailest males. We tested these hypotheses by examining child developmental outcomes in a U.S. birth cohort reportedly affected in utero by the September 11, 2001 attacks. We used as outcomes the Bayley cognitive score and child height-for-age from the Early Childhood Longitudinal Study-Birth Cohort. Previous research demonstrates a male-specific effect of 9/11 on California infants born in December 2001. We, therefore, compared cognition and height of this cohort with males born prior to the 9/11 attacks. We controlled for unobserved confounding across gender, season, and region by using triple-difference regression models (N = 6950). At 24 months, California males born in December scored greater than expected in cognitive ability (coef = 9.55, standard error = 3.37; p = 0.004). We observed no relation with height. Results remained robust to alternative specifications. Findings offer partial support for the culled cohort hypothesis in that we observed greater than expected cognitive scores at two years of age among a cohort of males affected by 9/11 in utero. Contemporary population stressors may induce male-specific culling, thereby resulting in relatively improved development among males that survive to birth.