NO ONE BELIEVES IN DARWINISM ANY MORE:

For many years, scientists believed that humans had sex for a few simple reasons: to reproduce, experience physical pleasure or relieve sexual tension. Then a 2007 study from the University of Texas identified 237 expressed motives for sex. The reasons ranged from the mundane (stress reduction) to the spiritual (to get closer to God) and from the altruistic (to make the other person feel good) to the spiteful (to retaliate against a partner who cheated by cheating).

Now, two studies by University of Toronto researchers published this month in the journal Personality and Social Psychology Bulletin, have divided the most common reasons why people have sex--and the ones most relevant to long-term relationships--into two broad categories of motivation: approach and avoidance. Approach motives pursue a positive outcome. ("I want to increase intimacy with my spouse" or "I want to feel closer to my partner.") Avoidance motives aim to evade a negative outcome. ("I want to avoid conflict" or "I don't want to feel guilty.")

Each category is also divided into subcategories: self-focused or partner-focused.

The researchers paid particular attention to partner-focused goals. "They have the greatest impact on the outcomes of a relationship," says Amy Muise, a postdoctoral fellow at the University of Toronto and lead researcher on the study.

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