"Pre-birth exposure to both nicotine and amphetamines increases the chance of lesbian daughters," writes neurobiologist Dick Swaab in the book We Are Our Brains. He also claimed stressed-out women were more likely to have queer newborns: "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 fetal sex hormones."

While these claims are highly contentious, at least they point to a biological origin for sexual orientation.

Related: Homosexuality Is Hereditary, Says Scientist

In the 1990s, Swaab reported that the brains of homosexuals and heterosexuals differ, a controversial finding now widely accepted in scientific communities. He's also among those who subscribe to the theory that the more older brothers a male has, the more likely he is to be gay.

Richard Lane of the British LGBT group Stonewall says he's not convinced by the study, and worries it "might encourage expectant mothers to take up smoking in the hope that they'll end up with a gay child."

Yeah, but what brand?