Environmental Influence on Gender

There are many well-established natural factors that bias the sex ratio of human offspring, proving that it is at least possible. The most famous is the returning-soldier effect. During and immediately after major wars, more sons are born than usual in the belligerent countries as if to replace the men that died. (This would make little sense; the men born after wars will mate with their contemporaries, not with those widowed by the war). Older fathers are more likely to have girls, but older...