Our cultural landscape doesn't help much in answering that. We praise stable, devoted relationships, yet we that are titillated and tempted by alternatives. A large percentage of marriages end in divorce, yet a far smaller percentage of married people get divorced-that is, our high divorce rate results disproportionately from serial divorcers.

Anthropology doesn't give a clear answer either. Historically, most cultures have allowed polygamy. But within such cultures, most people are monogamous. In the numerous cultures where polygamy is a function of wealth, there are no doubt many monogamous men who would be polygamous if they could buy more wives. And then, of course, there are the rare polyandrous cultures where one woman may have multiple husbands.

Other primates offer some insights into our human nature. Among the hundreds of primate species, some are polygamous (including our close relatives, chimps and baboons), and others monogamous (such as gibbons and marmosets). Each group has a different cluster of biological and behavioral traits.

Among polygamous primates, males typically spend much of their time competing for high rank in dominance hierarchies and for mating access to females. Males are far more aggressive, bigger, heavier and more muscular than females and have bigger canines-the better to slash an opponent with. Male baboons, for example, have twice the body weight and canine length of females. Such primates are often called, for good reason, 'tournament' species.

Males in these species have higher metabolic rates and shorter life spans than females, and they basically put zero effort into caring for kids. In other words, all a female gets from a mate are his genes, and females select for males with good genes. This has led to the evolution of conspicuous, costly displays in males that advertise good genes. These features-flamboyant facial coloration, big capes of hair, silver backs-are the primate equivalents of the peacockery of peacocks (a classic polygamous species).

Because fertile females will mate with multiple males, male-male competition extends to sperm competition. By primate standards, polygamous male primates have large testes (as a percentage of body weight) and high rates of sperm production. And they happily mate with anyone in the county who is ovulating.

Things are quite different among monogamous 'pair bonding' primates. Critically, males do much of the infant care. Thus, you don't see a male indiscriminately mating left and right (or fighting for the chance to do so), since he'll be doing a lot of work if there's a child. In these primates there isn't a high degree of 'sexual dimorphism'-sex differences in body size, musculature, metabolism and life span-and males don't have those garish secondary sexual characteristics of males of polygamous species. Testes are small, sperm count low, mating infrequent.

These profiles are consistent. If 10 seconds into watching a newly discovered type of primate you see that males are twice the size of females and have flashing neon noses, the issue is settled: it's a polygamous species. If you spend forever trying to tell the sexes apart, they're monogamous.

So by these various biological measures, are humans a pair-bonding or a tournament species? Neither. Across populations, men are roughly 10% taller and 20% heavier than women, need 20% more calories and live 6% shorter-more sexually dimorphic than monogamous species, less than polygamous species. Moreover, compared with, say, monogamous gibbons, human males have bigger testes and higher sperm counts...but pale in comparison to polygamous chimps. Measure after measure, it's the same.

It turns out that we aren't monogamous or polygamous by nature. As everyone from poets to divorce attorneys can attest, we are by nature a profoundly confused species-somewhere in between.