New brain imaging research suggests that cerebral differences, developed in the womb or early infancy, hardwire sexual orientation:

The scans reveal that in gay people, key structures of the brain governing emotion, mood, anxiety and aggressiveness resemble those in straight people of the opposite sex. The differences are likely to have been forged in the womb or in early infancy, says Ivanka Savic, who conducted the study at the Karolinska Institute in Stockholm, Sweden. "This is the most robust measure so far of cerebral differences between homosexual and heterosexual subjects," she says.

Previous studies have also shown differences in brain architecture and activity between gay and straight people, but most relied on people's responses to sexuality driven cues that could have been learned, such as rating the attractiveness of male or female faces. To get round this, Savic and her colleague, Per Lindström, chose to measure brain parameters likely to have been fixed at birth. "That was the whole point of the study, to show parameters that differ, but which couldn't be altered by learning or cognitive processes," says Savic.