The Birth of Monogamy
The tendency to establish dominance hierarchies is universal in mammalian species. Among humans, only the dominant male, or alpha, has preferential access to society’s material resources. Most males, or betas, are of intermediate rank; the bottom rung is occupied by omegas, the most despised and downtrodden segment of humanity, these men have been disempowered by Nature. A man’s likelihood of successfully passing on his genes to the next generation is determined by his status in the human dominance hierarchy. Historically, alphas sired the most offspring, indicating a pattern of differential reproductive variance. Many genetic studies confirm this (e.g. Wilder et al., 2004). The mitochondrial Time to Most Recent Common Ancestor (TMRCA) is twice the Y-chromosomal TMRCA. This male differential variance in reproductive success entails a larger effective female population size. Baumeister (2010) has summarized these genetic findings using everyday language: the two-to-one TMRCA ratio means that 80% of all women, but only 40% of men were able to successfully pass on their genes.
Throughout prehistory, it was men who had to compete for mating opportunities. This is supported by; (a) the universal female tendency towards mediocrity, and; (b) the universal male tendency towards variability. This statistically robust phenomenon is known as the greater male variability hypothesis, first investigated empirically by sexologist Havelock Ellis in 1894. Women will cluster around the midpoint of a Gaussian distribution for any given trait, but men will always be overrepresented at both extremes of the curve. Men vary far more than women because of female sexual preference for top tier men with genes for variance; on the other hand, women tend towards mediocrity because male interest in females is relatively indiscriminate. The larger effective female population size reveals that men were subject to far greater selective pressures in the ancestral evolutionary environment.
The human species is moderately polygynous. Males are polygamously inclined, but females are hypergamous. This means that they are hardwired to pursue an optimal sexual strategy of mating exclusively with top tier males. These sexual strategies are determined by innate sex differences: men produce billions of sperm over the course of a lifetime and remain fertile until their last dying breath. In addition to a 9 month pregnancy, women produce around 400 eggs over the course of their life cycles, becoming infertile with the onset of menopause. In nature, sperm is abundant and expendable, whereas eggs are rare and valuable. Women are choosier than men because their reproductive organs produce significantly less gametes, their period of maximal fertility is of brief duration and their level of parental investment is greater.
In the sexual marketplace, women are generally attracted to alpha males, but will pursue a dual mating strategy by relying on beta males for extraction of material resources. Unlike females, males are known for their bewildering diversity of sexual tastes; all women are considered sexually attractive by some fairly large segment of the male demographic. This male sexual diversity is an evolved psychological mechanism; its purpose is to ensure that no uterus goes to waste, given the substantial “costs” involved in human egg production.
During the Stone Age, which lasted for some 2 million years, alpha males with harems of 2 or 3 females served as the primary breeding unit. Only the alpha males were able to mate with the majority of females and sire offspring, but this doesn’t mean polygyny arose solely because of male power over the opposite sex; if women were easily monopolized by alpha males during the Stone Age, it was because they had evolved a natural sexual preference for alpha males that reinforced alpha male dominance in human societies. In a sense, patriarchy is an historical inevitability because of both male and female reproductive choice. Prehistoric societies were hunter-gatherer. Because of female selectivity, only the genetically superior hunter was allowed unrestricted sexual access to females. The alpha males, of course, were the best hunters; their powerful build, superior strength, great stature, chiseled facial features and high intelligence were all signs of superior ability and reproductive health. This gave them a significant genetic and biological advantage over less fortunate beta and omega males.
The sedentary agro-pastoralist and horticultural societies of the Neolithic had even higher rates of polygyny than the hunter-gatherer societies of the Stone Age. The emergence of these societies allowed for an even more unequal distribution of material resources and greater social stratification. This gave the alpha male the ability to amass even larger harems than was possible during the Stone Age. Small numbers of alpha males with exclusive sexual access to the majority of nubile females produced sizable numbers of unattached males of fighting age; they were men who had nothing to lose because they had no stake in society. Of course, alpha male control of female sexuality always produced great resentment and seething hostility among lesser, particularly beta males during the Stone Age; however, this situation deteriorated considerably during the Neolithic.
Widespread clan warfare raged across Old World Neolithic societies some 5000 to 7000 years ago. Analysis of Y-chromosome sequences reveals extensive “gene-culture hitchhiking” that affected male-specific genetic diversity. This began 1 or 2 millennia after the onset of the Neolithic transition, but ended prior to the emergence of capital-intensive agriculture during the Bronze Age. The effective male population size of the Old World was reduced to one-twentieth of its original level over the course of 60 generations or 1.5 millennia, despite no observable fluctuations in male demography during the same time period (Zeng et al., 2018); this was so drastic that effective female population size was 17 times greater than the male. How does one explain this significant reduction in Y-chromosome haplotype diversity? Why was mitochondrial genetic diversity apparently unaffected by the post-Neolithic bottleneck?
The intensification of agricultural and pastoral activities during the Neolithic facilitated the transition to patrilineal kinship-based forms of social organization; these were more effective at mobilizing individuals for intergroup competition and resource acquisition. Fierce competition for resources between genetically homogeneous patrilineal kinship groups resulted in the post-Neolithic Y-chromosome bottleneck; in other words, centuries of internecine warfare lead to the physical eradication of entire clans organized around a single patrilineage, reducing male-specific genetic diversity in the process. A primary motivating factor behind the conflict was the scarcity of young, available females, a scarcity that was exacerbated by the growing polygamy of agropastoralist and advanced horticultural societies. Women and girls were seized as the spoils of war by victorious clans and used as wives, concubines and slave-girls. Women were forced to be exogamous because of fierce competition among rival clans for sexual resources; this would ensure that effective female population size and mitochondrial diversity would remain stable throughout the mid-Holocene.
This Neolithic clan warfare subsided around 3000 AD. With the Bronze Age, came the rise of greater political complexity; the newly emerging regional polities were based on capital-intensive agriculture, which lead to even greater wealth inequality and social stratification than the agropastoralist and advanced horticultural societies of the Neolithic. Polygyny became increasingly common in these societies, but gradually declined with the development of advanced, highly socially stratified agrarian societies. In Western Europe, it is probable that the Mycenaean and Minoan civilizations were originally ruled by a polygynous elite; the Achaean warriors of Homer’s Iliad were given female captives as a reward for heroism and valor in battle. Although there is some archaeological and linguistic evidence of Proto-Indo-European familiarity with the practice of monogamy, actual legally imposed, universal monogamy is securely dated to archaic Greece and early Rome. In ancient Sparta, the 9th century lawgiver Lycurgus had passed laws criminalizing celibacy; those who persisted in an unmarried state would be punished with loss of civil rights. Similar laws against celibacy were found in many Greek city states, including Athens. In Cicero’s De Legibus, the Roman censor, a position that had been established during the early years of the Republic, was legally tasked with discouraging celibacy through a system of fines. The only exceptions to this unbroken tradition of prescriptive monogamy were the limited informal polygyny that existed because of slavery and the practices of the Hellenized Macedonian rulers of western and south Asia, who imitated the polygynous customs of Near Eastern rulers (Scheidel, 2009).
It is considered paradoxical that polygynous mating arrangements decreased in frequency, even among urban elites, wherever capital-intensive agriculture became the dominant mode of socio-economic organization. With the establishment of Greco-Roman civilization, polygynous mating arrangements, albeit those of the formal variety, virtually disappeared. The relevant literature offers two complementary explanations for this. In the econometric model, the disappearance of polygamy was caused by:
- diminishing marginal fitness returns for each wife added to the rich man’s harem, i.e. the increasing importance of “rival” wealth, inability to effectively mate guard a large number of wives and public censure of polygyny by unmarried men, and
- the greater concentration of wealth and social stratification in agricultural societies, which meant fewer wealthy men with the material resources to support large harems (Ross et al., 2018).
The cultural evolutionary approach to the origins of monogamy examines it from the perspective of group-level fitness maximization. Since the end of the Neolithic in Europe, the rich and powerful have ceased being polygynous, despite the substantial wealth inequalities that have always existed in Western societies. Research has shown that polygynous societies are less stable and more violent; the shortage of available females intensifies male-male intrasexual competition, leading to more risk-taking, less future orientation and aggressive, violent behavior among unwanted bachelors. There is a positive correlation between the number of bachelors in a society, degree of polygyny and national crime rates. A positive correlation also exists between degree of polygyny and societal stability (Henrich et al., 2012). The genomic record shows that violent intergroup competition between men increased significantly as polygyny became more widespread in the agropastoralist and horticultural societies of the Neolithic. Entire patrilineal clans were wiped out because of prolonged warfare, leading to a Y-chromosomal bottleneck. In polygynous societies, unmarried males will form raiding parties in search of plunder, which includes females of child-bearing age. During the clan warfare of the late Neolithic, warriors would seize the women and girls of defeated clans as war booty.
Because of its many disadvantages, polygyny has been replaced by monogamy in Europe. The growing popularity of monogamy around the world is explained by its societal benefits. Married men tend to be more risk-averse and future-oriented, but less crime-prone. On a national level, normative monogamy increases per capita GDP; on a familial level, it reduces intra-household conflict and improves childhood outcomes by increasing parental investment. There is even evidence that monogamy may have contributed to the rise of democratic institutions in Western nations (Henrich et al., 2012).
The “male compromise” thesis outlines the most probable mechanism for the adoption of monogamy in early historic times (Lagerlöf, 2007). As harems in Neolithic Europe expanded in size, violence among men spiralled out of control. Eventually, the safety of the king and his closest associates were threatened by the rising tide of violence. Because urban elites monopolized the kingdom’s child-bearing females among themselves, the beta male subjects, starved of female companionship and family life, became hostile and rebelled against monarchical authority. If the king was able to successfully quell the rebellion, the rewards, in terms of reproductive fitness, would exceed his wildest dreams. To take but one example, the Y chromosome of Genghis Khan is shared by 8% of Central Asian males (Zerjal et al., 2003). If the king were to fail, he risked assassination at the hands of his sex-starved male subjects. If he was the ruler of a large kingdom, he would have even more to lose; but a potential rebel would have much to gain, making rebellion against the king’s rule more likely. The king, in an effort to pacify his male subjects, embraced monogamy or, at the very least, a constrained polygyny, ensuring a more equal distribution of women in his domain. In the capital-intensive agricultural societies that emerged in Europe between 1000 BC to 1000 AD, monogamy was socially imposed from above to level the reproductive playing field and preserve societal order in an age of increasing wealth inequality and social stratification.