One of the arguments against gay marriage is that allowing gays to marry would eventually lead to polygamy, or even marriage between man and goat. Or between a man and his son to avoid inheritance taxes. And indeed it might. But the issue of polygamy has already been before the Supreme Court, and the opinion—which upheld monogamous (heterosexual) marriage as the only legitimate form of marriage—is of contemporary interest because of how it defended traditional monogamous marriage.
The Supreme Court, in Reynolds v. United States (1878):
Polygamy has always been odious among the northern and western nations of Europe, and, until the establishment of the Mormon Church, was almost exclusively a feature of the life of Asiatic and of African people. At common law, the second marriage was always void (2 Kent, Com. 79), and from the earliest history of England polygamy has been treated as an offence against society. After the establishment of the ecclesiastical courts, and until the time of James I., it was punished through the instrumentality of those tribunals, not merely because ecclesiastical rights had been violated, but because upon the separation of the ecclesiastical courts from the civil the ecclesiastical were supposed to be the most appropriate for the trial of matrimonial causes and offences against the rights of marriage, just as they were for testamentary causes and the settlement of the estates of deceased persons.
By the statute of 1 James I. (c. 11), the offence, if committed in England or Wales, was made punishable in the civil courts, and the penalty was death. As this statute was limited in its operation to England and Wales, it was at a very early period re-enacted, generally with some modifications, in all the colonies.
The court then points out that the same Virginia convention that recommended religious freedom also enacted James I’s death penalty for bigamy.
Notice that these are not religious arguments but are based on tradition long predating Christianity. If one substituted homosexuality or homosexual marriage for polygamy in this train of thought, the results would not bode well for homosexual marriage.
It also asserts the uniqueness of European civilization—that its culture and traditions are quite separate from those of Asia and Africa and that monogamy is a defining feature of the West. In this the court is quite right. Traditional European culture is the only civilization where monogamy is the norm (see here).
The court went beyond an argument from tradition, proposing that the social norm of monogamy benefits society because it is more compatible with individual freedom and non-despotism:
Marriage, while from its very nature a sacred obligation, is nevertheless, in most civilized nations, a civil contract, and usually regulated by law. Upon it society may be said to be built, and out of its fruits spring social relations and social obligations and duties, with which government is necessarily required to deal. In fact, according as monogamous or polygamous marriages are allowed, do we find the principles on which the government of the people, to a greater or less extent, rests. Professor Lieber says, polygamy leads to the patriarchal principle, and which, when applied to large communities, fetters the people in stationary despotism, while that principle cannot long exist in connection with monogamy.
This is rather remarkable given the individualist thrust of American law. Instead of prizing the individual liberty of entering into a plural marriage, the court argued for the collective good. Similarly, one could argue, as I have, that homosexuals and the society as a whole would benefit from acknowledging heterosexual marriage as a specially protected cultural norm— its special status guaranteed because of its critical importance in creating and nurturing children. The argument for homosexual marriage now emphasizes that there are children living with homosexual couples, but it is difficult to imagine that the living arrangements of these children are superior to the era prior to the general assault on traditional marriage in the family that began with the counter-cultural revolution of the left that began in the 1960s. Since the 1960s all the markers of family functioning have declined, especially the very large increases in single parenting which is clearly linked to poor outcomes for children (e.g., here and here—not to mention, every child psychology textbook that I have ever taught from). Homosexual marriage is simply the final stage in the attack on traditional institutions surrounding marriage and the family by the cultural left, including Hollywood and the rest of the elite media. So, it is the ultimate irony that the cultural left is now pleading that homosexual marriage is good for the children. Since when have they ever cared about the children?
But to get back to my main theme, there is evidence that the Supreme Court was correct: monogamy has had socially beneficial effects for Europe. I have argued that monogamy is part of a suite of traits underlying Western individualism, including the nuclear family, exogamy and a de-emphasis on the extended kinship group. (An archeological excavation of a 4600-year old site in modern Germany found evidence for monogamy and exogamy, both strong markers of individualism; PNAS 104 , 18226-18231; the article notes that such findings do not imply the universality of monogamy and exogamy, and indeed, I have it on good authority that graves in Middle Eastern sites indicate extended families; Abraham and his wives as depicted in Genesis would be an example). Monogamy appears to have been essential for Western modernization because it resulted in a low-pressure demographic profile necessary for the accumulation of capital (see here, p. 43): When economic times are poor, there are large numbers of unmarried men and women, whereas in a polygynous society like traditional China, poor economic times simply lowered the price of concubines for wealthy males; all females mated, so there was constant pressure on resources, and strong selective pressure in favor of successful males (see also below).
Monogamy implies a leveling of reproductive opportunities, so that even wealthy males must confine their mating to a single wife and relatively poor males have the opportunity to mate. On the other hand, polygyny leads to a low-investment style of parenting in which all females mate and males tend to pursue additional mates (i.e., mating effort) rather than putting a large investment in the children of one woman (see here, p. 18). Recently Joseph Henrich, Robert Boyd, and Peter Richerson have extended this by showing that lowering competition among males has a number of beneficial effects for society as a whole, including increasing parental investment in children:
In suppressing intrasexual competition and reducing the size of the pool of unmarried men, normative monogamy reduces crime rates, including rape, murder, assault, robbery and fraud, as well as decreasing personal abuses. By assuaging the competition for younger brides, normative monogamy decreases (i) the spousal age gap, (ii) fertility, and (iii) gender inequality. By shifting male efforts from seeking wives to paternal investment, normative monogamy increases savings, child investment and economic productivity. By increasing the relatedness within households, normative monogamy reduces intra-household conflict, leading to lower rates of child neglect, abuse, accidental death and homicide. (The Puzzle of Monogamous Marriage, Philosophical Transactions of the Royal Society B, 367 (2012), 657-669.
Prof. Lieber’s idea, cited by the Supreme Court, that polygyny is linked to political despotism is not at all far-fetched. It is doubtful that a society that has completely unregulated reproductive competition for females could be governed in any other way, since it would always be necessary to suppress desperate non-elite males without access to females. The social instability caused by young unmarried males has been a chronic problem in China (Q. Jiang, “Bare Branches and Social Stability,” Frontiers in the History of China 2011, 6(4): 538-561).
So it’s reasonable to think that monogamy has had positive effects on Western societies. Of course, sexual egalitarianism doesn’t imply that there is no natural selection. As noted by Richard Lynn (Dysgenics: Genetic Deterioration of Modern Populations) and Gregory Clark (A Farewell to Alms), at least until the 19th century, fertility in the West was positively correlated with social class because higher status people had better nutrition and could marry younger than people with less means.
But it does suggest that the effects of natural selection were muted compared, say, to China as portrayed recently by Ron Unz. Unz’s article on traditional China is essentially a portrait of a high-pressure demographic regime where polygyny results in a huge reproductive advantage for males at the top of the society but where the great majority of their children, especially their male children, necessarily decline in social status:
Each generation, a few who were lucky or able might rise, but a vast multitude always fell, and those families near the bottom simply disappeared from the world. Traditional rural China was a society faced with the reality of an enormous and inexorable downward mobility: for centuries, nearly all Chinese ended their lives much poorer than had their parents. … The poorest village strata usually failed to reproduce at all, while poverty and malnourishment also tended to lower fertility and raise infant mortality as one moved downward along the economic gradient. At the same time, the wealthiest villagers sometimes could afford multiple wives or concubines and regularly produced much larger numbers of surviving offspring. Each generation, the poorest disappeared, the less affluent failed to replenish their numbers, and all those lower rungs on the economic ladder were filled by the downwardly mobile children of the fecund wealthy.
There’s no question that China was a brutal cauldron of natural selection. In fact, I first became convinced that evolutionary analyses of human societies were viable in reading about polygyny in various societies, including China in the early 1980s. I recommend the 17th-century Chinese novel Jin Ping Mei for a portrait of a polygynous society, complete with intrigues by wives of different rank competing for favor for themselves and their children, to the point of murdering a potential heir. For the women, the greatest prize was to be the mother of the principle male heir; wealthy males purchased inheritance rights by providing dowries for a principal wife, while concubines were purchased from the lower classes, their children typically with lesser prospects.
Polygyny and sexual competition are at the very heart of the Darwinian landscape of classical Chinese civilization and may in the end go some ways in accounting for Chinese high intelligence and work ethic. Natural selection was doubtless less brutal in Europe, although the relative sexual egalitarianism combined with significant natural selection for traits like intelligence (beginning with pre-historic evolution in difficult northern climates and continuing until the 19th century) made possible by monogamy also led to a number of advantages which, in my opinion, led to the great divergence between the West and the rest.
I also think that the sexual egalitarianism of European society may be partly why evolutionary analyses of European societies seem less intuitively obvious and require far more theoretical sophistication—e.g., Peter Frost, “European hair and eye color: A case of frequency-dependent sexual selection?“ (Evolution and Human Behavior 27  85–103) cogently argues for a biological basis of European monogamy, and I argue in the paper cited above that a variety of interest groups, including lower and middle status males and the Church with its ideology of sexual restraint and strong condemnation of bastardy, had a role in maintaining monogamy at various periods when European monarchs could easily have afforded large numbers of concubines and supported large numbers of downwardly mobile children as occurred in China. But with very few exceptions, they did not.
On the other hand, someone living in traditional China would have had no trouble at all understanding Darwin and the importance of male sexual competition just by looking all around him. However, it’s no accident that the theory of evolution explaining all life on earth developed in the West, not in the cauldron of natural selection that was classical Chinese civilization.