Discrepancies in wealth can bring out men's worst evolved traits, including violence, poor treatment of women, and polygamy, for example by much-married South African President Jacob Zuma.

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South African President Jacob Zuma poses with his wives after giving the 2009 State of the Union speech in Cape Town. (AP)

Even for a head of state, South African President Jacob Zuma has an awfully colorful sex life. In April, the 70-year-old married his sixth wife, Bongi Ngema, in a traditional Zulu ceremony. But, unlike serial monogamists, Zuma remains married to three other wives.

His second wife, Nkosazana Dlamini-Zuma, a respected minister in Zuma's government, divorced him in 1998. And his third wife, Kate Matsho-Zuma, committed suicide in 2000. By his wives, fiancées (yes, there are likely to be more weddings), and at least four other women, Zuma has allegedly sired at least 20 children.

All that marrying doesn't slow down Zuma's extra-marital activity, either. He was acquitted of the 2005 rape of an acquaintance he knew was HIV positive. The judge accepted his defense that the sex was consensual, and evidence suggests this was no isolated affair.

I am not an expert in South African politics, or even politics in general, yet I am fascinated by the private marital (and non-marital) relations of this head-of-state. And he's not alone: the leaders of Qatar, Swaziland, Sudan, Chad, and Saudi Arabia are also polygamous. As an evolutionary biologist, I've learned that our evolutionary past interacts with our current circumstances to influence the relationships and domestic arrangements we forge. Zuma is, in many ways, the product of an environment that reinforces some of our worst evolved traits.

South Africa's immense economic inequality, crippling levels of violence, rampant sexism, rape epidemic, and 5.6 million HIV infections are not merely a list of unconnected facts. It matters that South Africa's president is a polygynous, philandering male who admitted at his rape trial that in order to reduce the risk of HIV infection, rather than wearing a condom, he showered after sex.

The crucial connection comes through an evolutionary phenomenon that began as our ancestors encountered inequality in wealth, power, and reproductive opportunities, and which disposes some men to behave in reckless, risky, and violent ways. You might call it "the problem with men," and though it's not something that manifests in all males, it is endemically masculine, and at its most extreme it can poison societies. But, much as we have dealt with obesity epidemics not by blaming obesity sufferers but by understanding the ways that evolved human physiology and psychology can create innate vulnerabilities, so too can a better understanding this evolutionary phenomenon help us to deter its damage.

A Brief History of Wealth

For most of our evolutionary history, our ancestors hunted animals and gathered plant-based foods. You and I are largely products of adaptation to this lifestyle of hunting and gathering in small groups, where everybody knows everybody else. People in these small foraging bands depended on one another and shared according to one another's needs. As a result, foraging societies can appear egalitarian to modern observers.

Throughout history, wherever hunter-gatherers began to tend gardens and herd animals of their own, they've settled around long-term bases. Groups grow bigger and the bonds of reciprocity loosen, allowing some individuals to accumulate wealth, including the land they work and the animals they keep.

Several times in the last few thousand years, human societies domesticated and mastered the farming of at least one cereal: wheat and barley in Mesopotamia, Egypt, and India; millet and sorghum in Africa; rice in southern China; corn in Mexico and Peru. Arable land, accumulated grain stores, and livestock became the first widespread forms of wealth. But wealth that can be stored and traded can also be stolen, and thus must be defended. Coalitions of neighbors or kin who could effectively repel raids from other groups, or who could raid neighboring groups themselves, thrived. Those who failed to organize often succumbed.

Effective defense required organizers and strategists. These were humanity's first elites, whose strength, intelligence, and social skills elevated them to positions from which they coordinated the group's defense. Predictably, they also used these positions to enrich themselves and entrench their power. In other words, agriculture gave rise to money, to inequitable distribution of money, and thus to all of the evils of which money is root.

Inequalities in wealth also decided, in large part, who succeeded in finding mates, raising children, and eventually sending them out into the world to find mates of their own. The increasing material inequality from gardening, herding, and then farming led to big differences in reproduction and infant survival between the wealthy and the poor. But that's really just part of the story.

When anthropologist Laura Betzig recently tracked down reproductive records for a variety of societies, including hunter-gatherers, gardeners and herders, and agriculturalists, she compared the numbers for men and women. And she found that widening inequality affects men's and women's reproductive success differently.

Because every baby has a mom and a dad (at least in genetic terms), the average number of children produced by men in a given society is similar to the average for women in that society. But a woman can only experience a relatively limited number of pregnancies before menopause, constraining her maximum number of babies to the low teens. Men suffer no such biological constraint. If the planets aligned perfectly for a man, he could theoretically sire hundreds, even thousands of children. We expect, then, the number of children produced by an individual to vary more widely among men than among women.

Betzig found that hunter-gatherer women reported only slightly less variation in offspring number than men. But with pastoralism and herding, men's variation in reproductive success grew, so that the most successful men were able to sire 20. One Kenyan Kipsigis man claimed 80 children. (This also means that there were more men who sired no children at all.) But women's reproductive success grew only slightly more variable.

Betzig also scrutinized the written records from agricultural civilizations in Mesopotamia, Egypt, India, Mexico, Peru, and China. The early emperors and kings who ruled each of these societies made themselves despots of more than just economic wealth. They also dominated the means of reproduction, gathering for themselves the most fertile women from within the society and from their military conquests as wives, concubines, and slaves. In every one of these six early civilizations, the most successful men boasted 100 or more documented children. Understandably, no woman in these societies made comparable claims.

Inequality, Fitness, and Violence

History, says the truism, is written by victors. Agriculture and civilization might sound like a good deal for men, but in evolutionary terms it favored the few who wielded great wealth and power. Biblical accounts say King Solomon kept 700 wives and 300 concubines. Wealth and influence afforded Solomon phenomenal reproductive success, but at the expense of his male subjects, who necessarily had fewer chances to reproduce.

Anthropologists have long found associations between the rate of polygynous marriage within a society and its likelihood to wage war on neighboring groups or tribes, as well as its internal violence and homicide rate. Violence can actually cause polygyny, as warrior societies lose a lot of young men in conflict, leaving many more surviving women than men. As a result, some women face the unenviable choice of either sharing a husband or not marrying at all.

Often, ancient warfare would involve raiding other villages to capture fertile women as brides. The strongest, most war-adept groups of men can defend their own villages and women, raid other villages and capture brides. The fiercest groups quickly become the most polygynous, with the fiercest warriors taking the most wives.

Polygyny can also cause violence. For every man with two wives, there is on average another man will never marry. For each ancient king with 1001 wives or concubines, perhaps a thousand other men never had a family of their own. Polygyny, an excess of males, and gross economic inequality can all raise the number of men who end up siring no children at all. These millennia of sexual selection, in which the genes of these evolutionary losers have been eliminated by natural selection, helped created the same "problem with men" that led us to Jacob Zuma.

Remember that our ancestors include those men who strove for wealth and power, and especially those men who strove hardest when the differences between winning and losing grew widest. We are far less likely to have descended from the evolutionary losers or inherited their losing genes. This long history of selection has, in other words, guided our evolution such that the greater the inequality in a society, the more frantically men will strive to come out on the top end of it. And all this striving leads to competitiveness, including the perpetuation of inequality and, at times, violence.

Analyses of North American homicide statistics by evolutionary psychologists Martin Daly and the late Margo Wilson found that being a young man presents the greatest single risk factor for either committing or suffering homicide. The same is true in every society for which they were able to find data. They wrote that most homicides in which men kill men are "rare, fatal consequences of a ubiquitous competitive struggle among men for status and respect."

At an evolutionary level, the essential dilemma of masculinity is to avoid becoming one of those unfortunate souls who either dies young or lives out his days without reproducing. Biologists call this "zero fitness," meaning they have passed on none of their genes. Inequality within a society drives men -- especially young, poor men of relatively low social status -- to act aggressively and take big risks in order to improve their prospects, first to avoid joining the zero-fitness males, and then to become one of the few men who have historically produced the larger share of descendants. Not only does inequality among men lead directly to polygyny, but both inequality and polygyny further entrench the intensely competitive conditions in which violence and misogyny thrive.

The Lot of Women

Polygyny tends to take hold in societies where the interests of wealthy and influential men win out over the interests of average men, but what about the interests of women? Irish playwright George Bernard Shaw once presciently claimed, "The maternal instinct leads a woman to prefer a tenth share in a first-rate man to the exclusive possession of a third-rate one." When differences in wealth among men are extreme, and when male-engineered societies force women to depend on men for resources, some women might gain access to more resources by sharing a very rich and powerful man with other wives than they would by choosing monogamy with a poorer man.

But sharing husbands generates new and unwelcome conflicts, both between wife and husband and among wives. A first wife and her husband start out their married life as a monogamous couple, and share a strong mutual interest in their children. The trouble begins when he starts considering a second wife. It's a good deal for him, but his first wife has little to gain and plenty to lose. She and her children will have less access to any resources that the husband controls.

This again brings us to South African President Jacob Zuma. As his wealth and power waxed, instead of allocating his rising income to taking better care of his existing offspring and wives, he added to them.

When his third wife, Kate, committed suicide in 2000, she described her 24-year marriage to Zuma as "bitter and most painful," imploring him to take care of her children: "you must not let them starve since I'll be gone, pay their school fees to enable them to further their studies ... secure guarantee [for] the apartment ... for my kids to stay without any eviction orders." It appears that Kate's agony might have stemmed in part from Zuma's failure to contribute to their children and meet his obligations as a father.

It's not clear whether Kate was alone among Zuma's wives in her suffering, but her bitterness is not surprising in light of what we know about polygynous households. Men who marry many wives, from Indonesian Muslims to American Mormons, often have trouble treating their wives equally.

Studies in sub-Saharan Africa have found that women in polygynous marriages are more likely to suffer from depression, mental illness, and physical abuse from their husbands than are monogamously married women in the same societies. And co-wives more often contract sexually transmitted diseases, including HIV, from their husbands.

Co-wives must compete for access to their husband and for material resources, and both conflict and competition can have appalling effects on the health and survival of the children. The Zuma household does not appear to have been spared these problems. Zuma's fourth wife once shoved his fifth wife, in full public view, at the opening of Parliament.

Jacob Zuma defends his polygyny, saying, "There are plenty of politicians who have mistresses and children that they hide so as to pretend they're monogamous. I prefer to be open. I love my wives and I'm proud of my children." But the question is not how Zuma feels about his wives and children, but rather how they feel about him and one another.

Women in societies where polygyny is practiced often don't view the practice favorably, and why should they? In the world's most populous nation where it is still legal, Indonesia, multiple marriages are not popular with women. Indonesian law insists that a man can only take subsequent wives if his existing wives approve and if he can treat them all equally. At least, then, the law recognizes that men and women have different interests, keeping polygyny levels modest; only around one in 20 Indonesian marriages is polygynous. Nonetheless, Indonesian women often take to the streets in protest to call for polygyny to be banned, and politicians with many wives usually lose much of the female vote.

But if polygyny is bad for women, then why do so many women become second or third wives? In countries beset by wealth inequality and where polygyny is legal, women seldom have the same employment opportunities as men and often cannot inherit or even own land or property. Under these circumstances, some women may find that their best option is to trade their fertility for a part-share in the wealth of a rich and powerful man rather than exclusive marriage to a poor one.

And, of course, it's not always fully their choice. Many women are forced into polygynous marriages by their fathers, mothers, or other relatives, sold for cash or traded for influence and status. Polygyny arises not only from inequities in wealth and power among men; it is also a symptom of great imbalances of power that favor men at the expense of women.

Genetic Democracy

Today, more than 50 countries, mostly in Africa and the Muslim world, legally allow polygyny. But almost everywhere else, polygyny has been withering for centuries. What caused the demise of polygynous marriage in so many places?

For one thing, democracy weakens the resilience of legal or socially accepted polygyny, probably because the two practices are less than compatible. More than a century ago, George Bernard Shaw observed, "Any marriage system which condemns a majority of the population to celibacy will be violently wrecked on the pretext that it outrages morality." Late in the twentieth century, biologists such as Richard Alexander suggested that wealthy polygynous leaders often struggle for the political support of lower-status men when those men cannot marry and reproduce. Likewise, because polygyny presents a net loss for women, it is deeply unpopular with women voters. So polygyny becomes, in the long term, incompatible with a well-functioning democracy because it promotes the deepest evolutionary interests of the wealthiest and most powerful men at the expense of all other men and all women.

Most countries where polygyny remains legal are countries where democratic governance, if it is present at all, has only recently superseded dictatorship or monarchy, and is still struggling. As those democracies mature, polygynists are likely to become less electable, and the state more likely to ban polygyny. Zuma's South Africa became a democracy only when it ended apartheid less than 20 years ago, and its institutions still have some growing to do. South Africa's economic inequality is among the highest in the world. According to the CIA's ranking of nations by something called the Gini coefficient, which measures income inequality, South Africa ranks third worst in the globe.

There are other reasons that democratic nations tend only to allow monogamous marriage. Democracies tend to enjoy (and promote) more equitable distribution of wealth, which inherently contradicts polygamy. And democracy increases the stake that most people feel they have in their society, improving the genuine prospects of a majority of individuals. Success then becomes more dependent on the talents and competitiveness of individuals rather than their inheritance of wealth and privilege or their gender. Obviously, such problems as inequality and misogyny still exist in democracies, but seldom on the scale seen in countries that allow polygyny.

A democratic, industrialized or post-industrial society is also exactly the sort of environment in which parents should want to invest more in each child, ensuring that they are healthy and sufficiently well prepared to succeed, rather than to have many children. As children grow dearer in modernizing nations, men and women are also forced to rely more on one another as economic and parenting collaborators. So polygyny should become less attractive to men, and being a co-wife should lose any remaining appeal for women.

In a recent review paper, Joseph Heinrich, Robert Boyd, and Peter J. Richerson argued that societies that outlaw polygamous marriage function better and outcompete neighboring societies that still permit polygamy. When a society imposes monogamy and thereby reduces all the ills that result from that particular source of frantic male striving and within-family conflict, not only do its citizens on average lead objectively happier lives, but per-capita economic productivity improves. And so other societies follow suit and outlaw polygamy. That is part of why laws prohibiting polygamy came only recently to Japan (1880), China (1953), and India (1955) as those societies sought to emulate the economic modernization of the West.

Despite a few significant holdouts, polygamy continues to dwindle. There are still Jacob Zumas in the world who use "tradition" to shield their polygamist privilege, much as the mega-rich can usually be counted on to venerate wealth and promote inequality. But millennia of evolution ensure that polygyny and inequality will always stir violent competition and risky behavior among men and the oppression of women. Fortunately, our sense of fairness also has evolutionary roots, as does our ability to cooperate for the greater good. And those better angels of our nature will consign polygyny and entrenched inequality -- and men like Jacob Zuma -- to the wrong side of history.