Science writer Jess Bering says that, though other animals are known to play with their genitalia or mount an occasional leg here or there, only humans masturbate to orgasm as far as we know. And it may be one of the defining characteristics of human sexuality. While some suspect males do this to keep their sperm fresh — by essentially rotating their reproductive stock — Bering suggests a different, surprising reason: Our advanced cognitive systems allow us to play back arousing scenes in our memories when we’re alone, or even during intercourse.
Bering’s assertion that many people aren’t really with the person they’re really with certainly explains the stories of a lover calling out the wrong name in an intimate moment. It would also explain the sometimes inexplicable couples who emerge from the doors of a bar as it closes for the night. More significantly — and this may be where it becomes an evolutionary advantage — it could enable the maintaining of relationships between long-time partners for whom mutual attraction has faded, or otherwise well-suited couples who were never physically attracted to each other in the first place.