At the end of war, and other times of both chronic and acute stress, remarkable changes occur in the human secondary (birth) sex ratio. At the end of a long war, significantly more boys are born; after a short war, or disaster, fewer boys than usual are born six to nine months later. Since it is commonly held that the sex of the offspring is a matter of chance, these data provide an intriguing problem; but new findings in reproductive physiology, and an increased understanding of male vulnerability, could help resolve it. It appears the sex ratio of offspring may be influenced by variations in the mother's follicular testosterone. Under conditions of chronic stress, maternal testosterone rises, resulting in an increase in male conceptions; but these same stressful conditions also exacerbate differential male vulnerability, so more males are lost during pregnancy. At the end of war, improving conditions temper male vulnerability, leaving higher sex ratios at birth. Conversely, normal conditions at conception followed by a severe stressor during pregnancy result in lower secondary sex ratios.