Women who smoke or suffer from stress during pregnancy are more likely to give birth to a gay child, according to a neuroscientist.
Dr Dick Swaab, a professor of neurobiology at Amsterdam University, claims in his new book that a woman's lifestyle or area they live in could also have an effect on their child's IQ and development.
Swaab says in his book, entitled We Are Our Brains, that drinking and taking drugs during pregnancy can lower a child's IQ, while taking synthetic hormones or smoking while pregnant could increase the chance of girls becoming lesbian or bisexual.
He also claims that having older brothers increases the chances of boys being born gay as the mother's immune system develops stronger responses to male hormones with each son that is born.
"Pre-birth exposure to both nicotine and amphetamines increases the chance of lesbian daughters," Swaab told the Sunday Times.
"Pregnant women suffering from stress are also more likely to have homosexual children of both genders because their raised level of the stress hormone cortisol affects the production of foetal sex hormones."
Swaab points to previous studies which suggested woman who took synthetic oestrogen between 1939 and 1960 to reduce the chance of miscarriage increased their chances of giving birth to lesbian and bisexual daughters.
He adds: "The more older brothers a boy has, the greater the chance that he will be homosexual.
"This is due to a mother's immune response to male substances produced by boy babies in the womb, a response that becomes stronger with each pregnancy."
Swaab also claims mothers living in an area with high pollution levels increase the risk of their child being born with autism.
Ben Summerskill, chief executive of gay rights organisation Stonewall, condemned the study: "There does not seem to be a shred of evidence to support the idea that a mother's lifestyle changes a child's sexuality.
"Our feeling is that sexuality is probably caused by genetic inclination, but until the evidence is there, nobody can say either way," he said.