Contributed by Kirk R. Smith, November 15, 2007 (received for review September 15, 2007)
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.
Author contributions: R.C. and K.R.S. designed research; R.C. and T.B. performed research; R.C., T.B., and K.R.S. analyzed data; and R.C., T.B., and K.R.S. wrote the paper.
The authors declare no conflict of interest.
Freely available online through the PNAS open access option.
