One thing we're proud of here at A Moment of Science is that there isn't a lot of sex and violence on our program.

Although, did you ever stop to wonder why sex and violence so often go hand in hand? How come nobody complains about violence and gardening, say, or sex and bowling?

There may be a biological basis for this, as we can see when we look at other species. In his book Shadows of Forgotten Ancestors, Carl Sagan points to the blue heron when considering this question. The male blue heron will emit a call that attracts females. When one arrives and shows interest in mating, however, the male drives her away and may even attack her. Once she is gone he cries out again for her return. This behavior continues until eventually the male is able to mate without fighting. Some of the behaviors appropriate for sex are apparently similar to those for aggression, and the two can become confused.

Another example is tropical fish, which use their colorations to show sexual receptiveness. The female display for sexual readiness can look a lot like the male display for aggression. The biologist Konrad Lorenz has documented how frequently the male fish will become angered and charge the female, or another nearby male, displaying all the behaviors appropriate to aggression rather than courtship.

This kind of link between sex and violence is seen through many different species, and we may wonder if human beings don't share a little in the confusion. If so, it's up to us to distinguish very carefully between what's passion and what's aggression.