Biology does not mean the urge to cheat is irresistible, argue the authors. It's like the Twinkie defense. Most of us really do have the power to make our own decisions. Part of growing up and part of being human is learning to say no to tendencies that are socially troubling. To the amazement of ornithologists, that seems to be the pattern with birds, long revered as the poster creatures of lifelong fidelity.
Evidence comes in part from genetic fingerprint tests of hatchling eagles, geese and swans.
Female chimpanzees, at the more libertine extreme, were known to play the field, but supposedly only with males from their own social group. Now it turns out they've been doing the jungle equivalent of sneaking out the bedroom window. Evidence comes from a DNA study of 13 mother-infant chimp pairs in the Ivory Coast of Africa. Besides, it's no secret that human behavior is sometimes less than ideal. But again, just because infidelity is natural doesn't mean it's OK, any more than war is justified because aggression is natural.