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Researchers estimate that a third of American males experiment sexually with each other during their teen years, even though approximately 9 out of 10 eventually settle into relationships with girls. But both men and women may switch gears later on or be bisexual throughout life. "There are some people in whom sexual orientation does not maintain itself," says June Reinisch, director of the Kinsey Institute for Research in Sex, Gender and Reproduction at Indiana University. "It's not a matter of what they prefer, it's whom they fall in love with." She cites the example of a woman who fell in love with and was married to a man for 10 years, then at the age of 30 fell in love with a woman and spent 11 years in that relationship, and at 41 fell in love with a man. Clearly, even if sexual orientation does have a biological basis in the brain, it is not necessarily fixed. "All of us believe that genetic and hormonal influences are involved in homosexuality," says Reinisch, "but there's also an interaction with the environment."
Over the years much research on homosexuality has been motivated by a desire to eradicate the behavior rather than understand, let alone celebrate, diversity. (A notorious German biologist, for instance, claims that prenatal hormone injections could act as a "vaccine" against homosexuality.) LeVay and others hope their work will enable humans to view homosexuality the way other species seem to see it: as a normal variation of sexual behavior.