Boys with older brothers 'more likely to be gay'

Boys are more likely to grow up gay if they have older brothers - because of biology, rather than upbringing.

Ten years ago researchers made the startling discovery that the more elder brothers a boy has, the greater chance he has of being homosexual.

For each additional brother that precedes him, a boy's likelihood of growing up gay increases by a third.

Having older adopted or stepbrothers with a different mother made no difference to whether a boy turned out gay or straight. However, brothers sharing the same mother but raised in a separate family still exerted an influence.