When it comes to the matter of desire, evolution leaves little to chance. Human sexual behavior is not a free-form performance, biologists are finding, but is guided at every turn by genetic programs.

Desire between the sexes is not a matter of choice. Straight men, it seems, have neural circuits that prompt them to seek out women; gay men have those prompting them to seek other men. Women’s brains may be organized to select men who seem likely to provide for them and their children. The deal is sealed with other neural programs that induce a burst of romantic love, followed by long-term attachment.

So much fuss, so intricate a dance, all to achieve success on the simple scale that is all evolution cares about, that of raisingthe greatest number of children to adulthood. Desire may seem the core of human sexual behavior, but it is just the central act in a long drama whose script is written quite substantially in the genes.

Read more www.nytimes.com/2007/04/10...10gene.html