Evidence that mammalian sex ratios at birth are partially controlled by parental hormone levels at the time of conception.

Evidence is adduced for the hypothesis that mammalian (including human) sex ratios at birth are partially controlled by the hormone levels of both parents at the time of conception. Sex ratio variation is considered under three main headings viz that identified by Clutton-Brock & Iason (1986); that identified since that paper; and other variation mainly relating to human beings. Some of the evidence is strong.