Strangely, it is true.

Until quite recently there was no scientific explanation as to why. There have been many reasons suggested from 'natural balance being restored' to 'divine intervention'.

However Corry Gellatly at Newcastle University in the UK developed an interesting theory. He noticed (I say 'noticed', but a lot of hard work went into this 'noticing'!) that fathers with more brothers than sisters tended to have sons, those with more sisters tended to have daughters - there is only a small tendency but it is there. The same is not true of women.

Using this information he argues that in a situation where three brothers go off to war leaving a single sister behind, at least one is likely to come back and procreate and in doing so has more boys than girls. However if a man goes to war leaving his three sisters behind he is less likley to come back and and have babies who would tend to be daughters - in not coming back the number of girls born reduces.

www.ncl.ac.uk/press.office/press.release/item/boy-or-girl-it