Why Is Sex Fun?

Though seemingly remote from human sexuality, shorebird sexuality is instructive because it illustrates the main message of this book: a species’ sexuality is molded by other aspects of the species’ biology. It’s easier for us to acknowledge this conclusion about shorebirds, to which we don’t apply moral standards, than about ourselves.

Behavior evolves through natural selection, just as anatomy does. Hence if sex is enjoyable, natural selection must have been responsible for that outcome.

Our sex is ultimately laid down by our genes.

In particular, while natural selection favors both males and females that leave many offspring, the best strategy for doing so may be different for fathers and mothers. That generates a built-in conflict between the parents, a conclusion that all too many humans don’t need scientists to reveal to them. We make jokes about the battle of the sexes, but the battle is neither a joke nor an aberrant accident of how individual fathers or mothers behave on particular occasions. It is indeed perfectly true that behavior that is in a male’s genetic interests may not necessarily be in the interests of his female co-parent, and vice versa. That cruel fact is one of the fundamental causes of human misery.

How can we explain simultaneously this general pattern of female parental care and also its numerous exceptions?

The answer comes from the realization that genes for behavior, as well as for malaria resistance and teeth, are subject to natural selection.

Perhaps our greatest distinction as a species is our capacity, unique among animals, to make counter-evolutionary choices.

Physiologists and molecular biologists regularly fall into the trap of overlooking this distinction, which is fundamental to biology, history, and human behavior. Physiology and molecular biology can do no more than identify proximate mechanisms; only evolutionary biology can provide ultimate causal explanations.