One explanation is that women's sexual behaviour is driven more by relationships. Long discredited are theories that parenting - one midth century theory held that boys raised by a domineering mother with a distant father were more likely to be gay - has anything to do with sexual orientation. It does not make them straight. By researching sets of brothers, by far the largest study of its type, the Northwestern study is searching for the specific genes that influence some brothers to be gay and others to be heterosexual. The likelihood that if one identical twin is gay, the other will be also be gay is much higher than the "concordance" of homosexuality between fraternal twins, indicating that genes play a role in sexual orientation, but are not the entire cause.
Alison. Age: 31.
Women may have more fluidity of sexual expression than men, but that doesn't mean they don't have a specific sexual orientation, said Lisa Diamond, a professor of psychology and gender studies at the University of Utah who studies female sexual orientation.
Adrianna. Age: 32.
US researchers find evidence that homosexuality linked to genetics
What scientists call "the fraternal birth order effect", the fact that each successive boy born to the same mother has a greater chance of being gay, may be due to an increasing immunological response by a mother's body to each male foetus in her womb. Until , CNN polls had found that a majority of Americans believed gay people could change their sexual orientation if they chose to; it was only last year that a majority for the first time said homosexuality was an inborn trait. By researching sets of brothers, by far the largest study of its type, the Northwestern study is searching for the specific genes that influence some brothers to be gay and others to be heterosexual.