Proximate causes of the variation of the human sex ratio at birth

There is evidence that the human sex ratio (proportion males at birth) is the result of two processes. First, the sexes of zygotes (from which the primary sex ratio would be calculated) are thought to be partially controlled by the hormone levels of both parents around the time of conception. Second, this primary sex ratio is apparently modified downwards by male-sex-selective spontaneous abortion caused by high levels of maternal stress-induced adrenal androgens, thus yielding the sex ratio at birth (the secondary sex ratio).
Source: Early Human Development - Category: Perinatology & Neonatology Authors: Source Type: research