Polygamy: An Evolutionary Guide to the Hazards of Multiple Marriage
Tim Harford writes in a recent Slate column about the economic case for polygamy (multiple spouses). His argument is excellent, but focuses primarily on the consequences for male and female prenuptial bargaining positions. I wish to take that analysis further, moving from an economic to an evolutionary perspective. From there a much darker picture emerges, one that reveals hidden dangers posed by a surplus of unmarried males. However such surpluses arise—through polygamy or some other mechanism—those hazards belong uppermost in our thinking. This analysis further provides an ultimate explanation for the moral prohibitions against multiple marriage that have arisen in many cultures, as well as a logical basis for the intuitive aversion to polygamy that most of us, both male and female, feel.
Harford’s essay is triggered by news of a Chechen leader’s suggestion that, because war has reduced the number of marriageable men in Chechnya, rather than leave large numbers of women out in the reproductive cold, men who can support more than one wife should do so. Polygyny (individual husbands having multiple wives) makes sense, of course, in cases like Chechnya where men are scarce. To see why, you need only imagine the future effects of continuing to enforce monogamous rules—large numbers of women in the current generation would fail to reproduce, the population would decline and that would further weaken the country that those missing men died to protect.
Or to put it another way, it would make no sense for marriage rules to needlessly empower a nation’s enemies by converting the death of a single soldier into a loss that perpetuates through future generations. Instead successful monogamous societies should be expected to have mechanisms for bending the rules such that birthrates can be maintained in spite of a deficit of young men.
So the real question is, are spouseless Chechen women to be forced into single motherhood, or should they instead be allowed to share other women’s husbands. From virtually every perspective polygyny is the better choice. It is certainly better for the vast majority of those kids that would otherwise have lacked fathers. And in terms of efficiency, it is clearly better for Chechnya to broadly distribute the extra burden created by those children, rather than have it fall solely on the shoulders of spouseless mothers. The only perspective—besides Russia's—from which it is arguably worse, is that of the women who would otherwise have become married monogamously. Those women would clearly be giving up a chunk of the value found in an exclusive husband.
But even from the perspective of these women it is, in the long run, probably for the best. After all, future generations will be in big trouble if Chechnya grows continually weaker due to a low birth rate, triggering a vicious cycle in which the country’s fading military prowess invites further military conflict, robbing the marriage market of ever more young men—ultimately that pattern would likely condemn most Chechen family lines to oblivion. So would-be monogamous brides are likely to accept their sacrifice as necessary and right. And those that don’t may well find themselves viewed as gluttonous. Indeed, sharing of husbands may be the historical female analog to the patriotic sacrifice that men have traditionally made on the battlefield.
Of course, one has to be very careful with any analysis that says that it makes sense for an individual to sacrifice for the good of the group. Most arguments of that form are doomed because evolution prioritizes individual interests over group interests—so, for example, from the vantage point of an already married woman, the best reproductive strategy involves other women saving Chechnya by sharing their husbands. But the present argument does not require an appeal to altruism or ‘group selection.’ Patriotism evolved for a reason, and many Chechen women will likely forgo their short term ‘best case’ in favor of a viable future for their descendents. The Chechen situation is, in fact, tailor made to trigger such types of self-sacrifice.
And sharing isn’t all bad either. There are real benefits for women who share husbands, deriving from the natural tendency to pool child rearing duties and other obligations. In fact, for many women—even ones forced to share husbands that would otherwise have been theirs alone, the net effect can be positive in the short term as well. In cases where the math is close, evolutionary costs can be reduced yet further by sharing a husband with a sister or another close relative.
Because there are real benefits to pooling resources and duties, rational women may even choose to share a husband in circumstances where there is no shortage of men. A terribly skewed distribution of wealth, for example can easily tip the scales for women, sharing a Chieftain being a better evolutionary strategy, in most cases, than marrying a popper.
Women in modern industrialized societies may feel some satisfaction and relief at having escaped such choices. But that sense is false comfort. Were alien anthropologists to study modern ‘western’ societies, they might rightly conclude that many women have strangely signed up for the costs of polygyny without demanding the benefits. Rich men routinely find ways to father the children of multiple wives, and because they tend to do it sequentially rather than under one roof, the women and children do not get the benefit of pooling their assets and effort. Or more accurately, such women have, in many cases, traded those benefits for fungible economic assets that can be used to buy cooperation from strangers in the marketplace. Is that a good deal? I don’t know that anyone has properly studied the question.
In any case, from a ‘fitness’ perspective, polygyny is, obviously, a good deal for men in a position to marry multiply. And, despite our intuition, choosing to share a husband can sometimes be a rational choice for a woman as well. So why have most modern societies come to forbid it? And why should we as individuals, deplore it? As it turns out, our gut instinct that polygyny generally benefits men is even more seriously in error than our instinct about the consequences for women. In fact, it is nearly the inverse of the truth: if there is no shortage of men in a breeding pool, then polygyny is a bad deal for all men who are not rich, a terrible deal for men who are poor, and a hazard to society at large. It has also, in modern times, come to pose a serious danger to the stability of the world.
To see where the costs come from, let’s consider an admittedly simplified hypothetical polygynous society in which all of a person’s assets and liabilities get summed to produce a desirability index. At the top of the male hierarchy, the most desirable man—no doubt wealthy and powerful—will likely get some combination of the most wives and the most desired wives. The next most desirable man will do a little worse because several of the most sought after mates have already been taken—in a monogamous society, that second man would likely have married the penultimate woman, but in the present case, he starts significantly lower on the list. And the same logic applies to all the men below the first one—when polygyny is common, the pool of brides is rapidly depleted from the top down, and everybody but the top groom marries much lower than he would otherwise have done. This decrease in mate desirability is, at an evolutionary level, likely a price worth paying for men near the top, as these slight decreases in bride desirability are more than made up for in bride quantity.
But when you get down to the guys who can afford only one wife, it is easy to see that the desirability of available mates will be dramatically reduced from the monogamous circumstance, while the number of wives they will marry is exactly what it would have been. That isn’t a good deal at all. And if we continue down the hierarchy, there will come a point at which there just aren’t any more women of marriageable age—for every woman sharing another woman’s husband at the top of society, there will be a man at the bottom with no mate and poor prospects.
Such men are very likely to be frustrated and resentful. They also have little to lose because, unless they do something to shake things up, they are likely to be reproductive dead ends. So they may be eager for a big score and willing to risk life, limb and liberty to get it. That’s a recipe for disaster. In the short term it is sure to increase crime rates. For obvious reasons, that includes rape and murder. In turn, this crime will require increasing investments in police and prisons and all of the attendant harms.
And there is another probable outcome. The rich and powerful are always reluctant to part with privilege, so instead of fixing the problem at its source, they may instead elect to kill two birds with one stone and harness all that dangerous anger by arming these lower class men and sending them on nationalistic adventures.
So, from the perspective of fostering international peace and stability, addressing polygyny should be a priority. But one must be very careful. Polygyny, itself—as the Chechen example demonstrates—is not a problem. It is the creation, by whatever means, of large quantities of surplus males that make nations dangerous, and that can even happen in monogamous societies. China, for example, is building up a surplus of men as a consequence of the ‘one child’ policy in combination with a strong traditional preference for sons.
At one level, this accumulation of Chinese males is difficult to account for. The evolutionary logic underlying the usual evenness of sex ratios involves the recognition that, as one sex becomes more common, the evolutionary rewards for producing babies of the rare sex grow, maintaining a rough balance over time. Indeed, it seems so very logical that, given the glut, Chinese families should now prefer daughters, who will be sure to marry, over sons, who may well not. And yet the expected shift in preference doesn’t emerge.
Whether this is all an accident produced by the collision of ancient Chinese desires with modern Chinese rules and medical technology, or the result of a conscious manipulation of circumstances with military objectives, one thing is increasingly clear. Given the immense size of China’s population, any significant male bias in the sex ratio of a Chinese generation is likely to become the world’s problem, because even a slight surplus of men represents a large number of men in absolute terms—an army waiting to happen. It is as if we are watching troops slowly massing along a border, except that the border exists in time rather than space. Serious concern is warranted, and yes it is the world’s business.
Of course one is tempted to look for a Chechen style solution—perhaps Chinese men could share wives, for example. But unfortunately, that idea is a non-starter. Polyandry (individual females having exclusive access to multiple mates) is an extremely rare phenomenon both among human societies and in other species. That’s because it only makes evolutionary sense in the rarest of circumstances, where conditions are so difficult that two parents are generally not sufficient to raise children successfully. In more common circumstances, the costs of polyandry to men are high and the benefits are close to nil across the board. That’s because females are the limiting reproductive resource, a woman’s maximum birthrate being not much affected by how many husbands she has. The contrast is stark—given ample wealth, a man with twenty wives has twenty times the reproductive potential of another man with one wife. But a woman with twenty husbands is, in terms of reproductive rate, not much ahead of a woman with one husband. And on top of the dubious benefits, the evolutionary costs to males that share wives are potentially huge in terms of paternal effort spent on other men’s offspring—something men have evolved to fear and avoid.
The net effect of all this evolutionary asymmetry is that women aren’t likely to be especially keen on the idea of polyandry, and men would probably rather go to war. Who knows? At war, at least you might meet somebody. Especially if the other side has experienced a sudden shortage of men.