Monogamous marriage is not just a social custom that we are free to alter as we wish. It is older than any existing government, any laws, any language.

We have strong evidence -- in our bodies, in our populations -- that monogamy has been a significant part of the rise and triumph of the human species.

We have no evidence of any civilization that did not have it at the root of its social organization for the overwhelming majority of its citizens.

This does not mean that monogamy "comes naturally" to everybody. Civilization -- large numbers of people living together for long periods of time with a persistent identity -- depends on most or all of the citizens suppressing one or another "natural" desire for the sake of the survival of the whole community.

Many human desires that are suppressed today were once valuable traits that helped the survival of the species.

Survival As a Community

Long ago, the human species committed its survival and enhancement to the community rather than the gene.

Obviously, we still pass genes from parent to child, but the survival of the species does not depend solely on genetic traits.

By living and acting together, in ever larger numbers, all the members of each community had a better chance of their genes persisting through future generations.

It's as if we said, Instead of acting only in the interest of our own offspring, we will commit our lives to helping the whole community survive.

We are betting that our community is so strong and healthy that it will and should survive, and in so doing, it will give our children and grandchildren the best chance of carrying our own genes forward into the future.

That's why the most successful communities and civilizations are the ones whose members are so loyal that they will die for the survival of the community -- or, more to the point, train their children to be willing to die for it, and to sacrifice their personal desires, obeying laws and customs for the good of the whole.

Not everyone has equal allegiance to the community; not everyone is willing to make the sacrifice. If too many people behave disloyally or put self-interest above the good of the whole, it becomes a bad bargain for everyone. When trust in the community breaks down, the community itself no longer has the ability to protect its members' future genetic heritage.

The community must either repair its damaged fabric and rebuild trust and allegiance, or it will disappear.

Let's look at the older (and less successful) forms of organizing primate social units -- simplified for the sake of clarity.

Gorilla

The most individualistic pattern is the Alpha-centered tribe. Alpha is the biggest, strongest male -- he has beaten up and cast out any rivals, and now he and only he gets to mate with all the females. At least until he gets older, and a younger, stronger male comes along to displace him.

Thus the strongest male gorilla always gets to pass on his genes. The species thus selects for huge, strong males ... and loyal, submissive females who voluntarily stay with and mate with whichever male wins the contest.

This severely limits the size of the community, however. Only a few adolescent males and, of course, the females are available to help the male protect the tribe. The male can't be everywhere, can't see every danger. And with the amount of time it takes to gestate and bring to maturity the young gorillas, any attrition by predators risks severe damage to the survival prospects of the community.

Chimp

Let's move to a simplified version of chimpanzee culture. There is still an alpha male, but the community is larger. There are quite a few non-alpha males who continue to be loyal members of the community.

Why? Because they are not cut off from reproductive opportunity. Observers of chimp life report several mating strategies that make it worthwhile for junior males to hang around.

First, there's also opportunistic mating. With a large enough tribe, several females are likely to come into estrus at the same time. Alpha can only pay attention to one at a time. While he's busy, younger, weaker males can pounce.

Second, strong males who are not contestants for alpha position will capture and carry off females for long isolated forced mating sessions -- kidnap and rape.

These strategies provide several genetic benefits. With more males contributing to the gene pool, the genetic diversity is greater. Since a larger number of males get to reproduce, they stay around to help defend the tribe against predators and make war against rival chimpanzee tribes. The need for deception, observation, daring, and cleverness might even enhance intelligence.

But these strategies also have other consequences. A species dependent on kidnap, sequestration, and rape will tend to have much greater difference in size between males and females -- males will be markedly bigger than females, so carrying them off and subduing them is easier. At the same time, females will be more promiscuous -- instead of being utterly submissive to a single alpha, they will be receptive to quick dalliances.

Young sexually active males, whose reproduction is opportunistic and likely to be interrupted by an angry Alpha, are easily aroused and reach climax very quickly -- the quicker the better, so they're done before they get caught. It's to the advantage of the tribe, however, that mature Alpha males mate much more slowly.

And scientists report that Alphas seem to be tolerant of the dalliances of the younger males with females who are younger or not at the peak of estrus, and therefore less likely to conceive.

Since chimps are the primate species genetically most similar to humans, it is hardly a surprise that we see vestiges of these reproductive strategies in human beings today.

Even though most societies violently suppress kidnap, sequestration, and rape, there are still plenty of human males with strong impulses toward such behavior.

There are far more males -- especially young ones -- who are desperately eager for opportunistic mating, and who immediately lose interest once coition has been achieved. Most societies try to suppress this random mating among those too young to create stable families, though usually not with violence.

These are male traits, but they all have their corresponding female trait: There are plenty of females in human societies who are receptive to opportunistic mating. There are also plenty of human females who seek to be submissive to a strong, aggressive male.

We should not be puzzled at the number of abused or controlled women who are slow to leave the abusive or over-controlling mate. A genetic disposition toward cooperating with male reproductive strategy would once have enhanced the females' chances of surviving in good health to bear healthy children -- who would, if male, behave just like their fathers.

These are pre-human primate strategies, but they are what we resort to whenever our trust in monogamous marriage breaks down.

It should be obvious that from the beginning of the genetically distinct human race, different patterns must have developed.

Human Strategies

First, we humans sustain much larger communities than any other primate -- larger than chimps or baboons. That means that we must have found even better strategies to encourage non-alphas to remain part of the community.

(See Shirley Strum's fascinating Almost Human for more details about baboons, the primate group that in some ways is most similar to humans -- and almost as successful as we are in adapting to most environments -- even though it is genetically more different from us than the apes.)

Second, the size difference between human males and females is almost trivial compared to most of the simians. Not only have we abandoned and suppressed kidnap, sequestration, and rape, which demands a smaller, weaker female, we also rely on females to do work that requires nearly equal strength and size.

In other words, the human species has stopped treating females as mere possessions, to be stolen and used at the will of males, but instead is far more likely to treat them as helpmeets, able to participate significantly in defending and providing for the tribe, and the relative sizes of our bodies show our long-term commitment to marriage-as-partnership.

Third, our species invented male bonding. While there is still plenty of male rivalry, it has largely been channeled into play or work.

Male bonding is, in some ways, an adoption of a female trait. Female baboons, for instance, develop long-lasting hierarchies and friendships, cooperating in protecting and nurturing the young.

Gilgamesh

Human males also can develop remarkably intense friendships. The earliest known epic story enshrines the story of such a friendship. The unbearably harsh young king, Gilgamesh, is sent by the gods a rival, a wild man named Enkidu, who is first spotted running naked in the forest.

It is significant that Enkidu is tamed by mating with a civilized woman. He quickly becomes more civilized than Gilgamesh. Specifically, when Gilgamesh behaves like an Alpha male -- claiming, as king, his right to first mating with every young woman who reaches sexual maturity -- Enkidu blocks him.

They fight -- the standard male contest between Alpha and rival -- but when Gilgamesh finally wins, instead of killing his enemy, they become fast friends. From that time on they work together to protect the kingdom from enemies. The gods charge Enkidu with the responsibility to protect the king. When Enkidu loses his courage during their stalking of a great demon (Humbaba) and wants to turn back, Gilgamesh fights with him to make him stand with him in battle.

But when Humbaba appears, it's Gilgamesh who runs and hides. Now Enkidu goads Gilgamesh into fighting, and side by side, the two of them defeat the demon. Gilgamesh is still the king, but he knows he could not have succeeded as a hero (defender of the tribe) without Enkidu, nor could Enkidu have done it without Gilgamesh.

Male bonding allows males, despite or even as a result of rivalries, to continue to live together and work cooperatively. The bond allows them to accept that both will have reproductive opportunities; they do not try to exclude each other from mating.

Without male bonding to replace the endless rivalry of alpha-centered tribes, the single most important development in human society would have been impossible.

Monogamy

Humans didn't invent monogamy. Many nest-building bird species have practiced it (with some cheating) for, presumably, millions of years. But we're the only primates I know of that practice it.

Monogamy confers great advantages on the community as a whole and on women in particular. Where monogamy prevails, almost all males and females have a reasonable chance of reproductive success. And women have far more opportunity to direct their own lives as individuals rather than as a possession of whatever male is strongest or available, while still having the advantages of male protection and support.

This is a vast leap forward from mere male bonding, where the strongest males become friends and cooperate in sharing the females. Now even the weaker males are allowed to mate -- though the tendency even today is for higher-prestige males to get higher-prestige females (though markers of prestige vary from society to society).

One advantage to communities that adopt monogamy is that traits other than physical strength and stamina can be developed and passed on. Instead of valuing only the males that can make a physical contributions to defense or to the chase, males whose contribution is memory, creativity, inventiveness, or dexterity can also reproduce, and females who share and/or value those traits can mate with them so that their offspring can improve on them.

The disadvantage is that individuals whose traits might have been weeded out by lack of opportunity to reproduce are now able to mate and pass along their weaknesses. But the advantages from diversity of virtues apparently far outweigh the disadvantages from persistence of dysfunctions.

This means that monogamy, though it is overwhelmingly pervasive in successful human communities, is not the only reproductive strategy passed along genetically. Most human societies try to obliterate rape, but by its very nature rape is likely to lead to at least some reproduction, and so the genetic predisposition toward that strategy persists.

Likewise, most societies try to suppress, not so much the randiness of adolescent males, but their success in reproduction. But again, enough adolescents succeed in mating despite adult attempts at suppression that the trait continues in many (though by no means all) young human males.

Alpha males also persist, both in prestige-male polygyny and the serial polygyny of our culture, where certain males feel themselves entitled to mate with any female. And, of course, there continue to be females who are drawn to powerful men and accept a role as one of Alpha's many mates.

All the primate reproductive strategies still survive in the human race. But monogamy remains the leading pattern in the most successful societies.

The Rise of the Uxor

Monogamy depends on -- and rewards -- a trait called "uxoriousness": husbandliness. While there is no shortage of males pursuing the old reproductive strategies, there is also a widespread male type, the uxor, the man who wants to bond with one woman and remain loyal to her and, especially their children together.

This trait is, like male bonding, a transfer of a female pattern to the male: most females have always had a strong yearning toward knowing and nurturing their children. Human males exhibit this trait far more frequently than males of any other primate species.

Societies that reward and encourage the uxor are far more likely to succeed at monogamy than societies that honor and encourage the alpha or opportunistic strategies.

Community Success

How do we judge that one community is more successful than another? If that community is able to extend itself across time and space. But since communities consist of individuals who usually, one way or another, have the ability to opt out, communities can only continue to exist if they command the loyalty of their members.

Communities do this by providing enough food, protection, and reproductive opportunities for most members to have a reasonable expectation of the survival of their descendants -- the persistence of their genes.

Just as male bonding allows men to accept and cooperate in the reproductive success of rivals, so also allegiance is the degree to which members of a community regard the success of the whole as being the equivalent of their own success.

Most of us hunger to feel allegiance. The loyalty gene is strong and pervasive. Fanatical devotion to sports teams is one expression of this community-building trait.

Human communities survive best when their members are capable of such strong allegiance to the whole that they can even tolerate the death of their own child in defense of it. The feeling that they have succeeded when the whole group succeeds allows them to mask or overcome the grief and rage at the community's failure to protect their own child.

In short, they are willing to sacrifice or tolerate individual loss for the good of the whole.

Communities that best promote such altruism are the communities most likely to succeed. No society lasts long without instilling in its members the doctrine that it is noble to die for your country.

Altruism and monogamy are not the only traits that are dominant in every successful civilization. Another important one is the concept of property -- that ownership persists even when the owner is not present.

This extends into marriage: Peaceful community life depends on married people being able to trust that their spouses will continue to belong to them even when they are apart. Monogamy is not possible without respect for property -- including spouses governing themselves by the principle, "I belong to her"; "I belong to him."

Altruistic loyalty, monogamy, and property grew side-by-side in the human species.

But when a society throws away any or all of these principles, as ours has been doing for the past fifty years, what replaces it is the chimp or gorilla. Those strategies work -- we still have chimps and gorillas in the world -- but they can't maintain civilizations or large communities.

The community that abandons the principles that led to the rise of the human species to the top of all nature will soon find itself absorbed or overwhelmed or replaced by a community that holds firmly to them.