Famine has been a recurrent problem in human evolutionary history. The onset of famine brings with it a sharp decrease in fertility, while the aftermath reliably includes a period of unusually high fertility. This phenomenon, in conjunction with female preferences for mates slightly older than themselves, eventually yields a population with low (male/female) operational sex ratio. The skewed sex ratio, in turn, leads to increased female intrasexual competition, diminished choosiness, more sexually permissive attitudes, and higher male reproductive success. Therefore, those males born or conceived during a famine have unusually high reproductive value.

Parents who are biased either toward the production of male as opposed to female offspring during famine (granting that production of any offspring during a famine is rare) or who show favoritism toward sons born or conceived during a famine will consequently be selectively advantaged compared to parents with no such bias. Historical reports of five famines––Ireland (1845-1850), China (1935), Holland (1944-1945), Bangladesh (1974-1975), and Ethiopia (1984-1985)––provide moderate support for the sex-biased parental investment hypothesis.