Research on the biological origins of human sexual orientation remains controversial (and poorly funded), and more than two decades of slow work is starting to reveal a much more complex process than anyone first suspected.
In an article for the Boston Globe Magazine last month, Neil Swidley examines the current state of the field, and discusses what's changed since his influential 2005 article on the subject.
There has been some real movement in the last decade, though, as usual for a field built on small, poorly funded and sometimes contradictory studies, the difficulty is in synthesizing all these fragments into a coherent framework.
The difference that jumps out at me right away is the new appreciation for "fluidity." The binary view of male sexual orientation that dominated the field a decade ago has softened. Back then, there was real scepticism about men who reported being anything other than heterosexual or homosexual. After all, lab data tended to suggest that their arousal -- which effectively defines sexual orientation in men -- was either to male erotica or to female erotica, but not to both.
Recent research based on more sensitive testing methods now suggests that sexual orientation is a complex biological trait for both sexes. Like height, where a group of people can range from the very short to the very tall, when it comes to sexual orientation individuals can fall on any point of a continuum that ranges from completely straight to completely gay. There's a lot of variation between those endpoints, as well as some people who don't experience desire at all.
The exact factors that make an individual straight or gay or some version of bi are still in question. Genes seem to play a role, but the diet of hormones that the brain is exposed to as it develops in utero is also important. And there may be yet other variables we don't yet understand. As evidence that sexual orientation is inborn piles up, the same research is also finding that there's probably more than one way for a brain to become straight or gay.
Picture: Alan Light via Flickr