There is no institution of marriage in nature. Marriage is a human institution, but it is not simply an agreement between two people, it is a collective agreement between everyone in society. The presence of others as witnesses to the marriage demonstrates this. It’s the social agreement that makes it real, that creates real effects. If this were not so, then there would be no point in a marriage ceremony.
Swans and geese can live monogamously, but they are not in a state of marriage, because their relationship is based on biology, not on acceptance by feathered friends and relatives.
Chimpanzees, our closest living primate relatives, are promiscuous and ruled by an alpha male and his coalition. There is almost no ‘sexual dimorphism’ - no size difference - between male and female chimps.
Sexual dimorphism is quite pronounced in gorillas, where the huge silverback alpha male rules a harem of much smaller females. Polygyny (polygamy) in animals seems to be associated with more striking sexual dimorphism.
By examining archaeological evidence, we can surmise that some of our ancient ancestors were not monogamous and some were. Australopithecus, the first ancestor to walk on two feet had less sexual dimorphism than gorillas, but much more than humans and chimps. But, homo erectus, who evolved millions of years after australopithecus, had much less dimorphism. In fact, homo erectus had very similar sexual dimorphism to humans: more size difference between the sexes than chimpanzees, but less difference than gorillas or australopithecus.
Bernard Chapais, Anthropologist at University of Montreal, in his book, Primeval Kinship, speculates that approximately two and a half million years ago polygyny became an unstable system when homo habilis, the ancestor of homo erectus, invented stone knives and tools . Stone-age technology enabled the more nerdish homo erectus the easy means of bypassing superior muscle power by developing superior knife and spear tactics. For the first time, physical strength could be defeated by technology and brain power. This made a polygynous system, where physically stronger males monopolized females, an unstable arrangement.
Homo erectus was the first primate to walk out of Africa and the first to control fire. I believe that the collective agreement to institute monogamy is the key to these social advances perhaps the key to understanding human nature.
Monogamy in many animal species is associated with greater male participation in rearing the young, and for this and for other important reasons, I believe that becoming monogamous was the defining turning point for the human species. First, male participation helped make longer human childhoods more viable; second, monogamy greatly facilitated the sexual division of labour by making it possible for each male to provision nutritionally vital animal fat and protein for his pregnant mate and their growing offspring - something that would have been far less likely in a polygynous system; and, third, monogamy encouraged bigger and more successful human cooperative groups, by improving reliability of paternity and incorporating inlaws.
If, indeed, monogamy led to human culture, the change to monogamy did not occur because humans wanted to have culture, or because they somehow anticipated the unseen benefits of a monogamous system. Humans agreed to monogamy in order to facilitate pair-bonding. That stuff about ‘fatherhood’, ‘in-laws’, prolonged childhood, bigger brains, and language did not even exist in people’s imaginations at the time. It was all about dealing with jealousy and sexual possession. It was about desire. It was not desire to rise above nature, it was just natural desire.
Nevertheless, the effects of monogamy were revolutionary. The two million years that humans were monogamous hunter-gatherers were the crucible for human evolution. This is the time period when hominid brains grew significantly larger, and jaws and teeth grew smaller. As brains got bigger, female humans needed to give birth to babies with bigger heads, but there was only so much exit room; something had to give; that something was head size, and as a consequence, developmental readiness in human infants was significantly delayed.
Human babies are totally helpless, and their nervous systems are undeveloped compared to other animals at birth. Our period of infancy and childhood, where we require much attention and provisioning, and are incapable of surviving on our own, is significantly longer than any other animal. It was made possible by the sexual division of labour. Females gather and prepare meals. Males hunt and fight. That’s what makes a longer childhood and bigger brains possible.
But note that the division of labour, in turn, is made possible by monogamy. You can’t have a division of labour in a household if you don’t share. One of the things that monogamy does is to increase the amount shared between male and female partners. Bernard Chapais and others have pointed out how many benefits come from monogamy. Recognition of paternity becomes more plausible. An adult male has more incentive to provision his mate and offspring. I believe that monogamy set off a multiplier effect that ultimately led to human language and culture.
Here’s how it would have worked. I might want to be monogamous, but as long as someone else in the group can kill me or take my partner, I can’t realize my preferences. Suppose everyone was sick and tired of fighting and killing over females; we decided that from now on everyone gets to be paired up and anyone who tries to take more than their share is punished; for this to work, we not only need to detect cheating, we need to publicize and vigorously punish it; any group that neglects detection and punishment soon ends up with more violence and instability; whereas groups that pay attention to detecting and punishing cheaters are able to maintain a monogamous system and reap the benefits. From this it follows that every element of human culture comes from our primal ability to agree to form and follow rules of behaviour, where we expect others to do the same. To put it in general terms: the path to differentiation between humans and animals came from our ability to create and sustain a social reality by collectively regulating our behaviour, rather than solely depending on dominance.
Monogamy means the collective recognition of pair-bonding, which is, in important ways, analogous to our common notions of reciprocity and fairness, and the principle of the golden rule - “do unto others as you would have them do unto yourself.” Furthermore it requires the institution of rough equality and it unlocks the possibility of equality between the sexes.
A question the reader may be asking at this point is: if what I am saying is valid, how come we have so much "polygamy" in the world? Note that polygyny in humans is not universal, but it exists mostly in traditional agricultural societies, where landowners or animal herders are sometimes able to amass surplus wealth.
In hunting and gathering societies, which are largely nomadic, people can only keep as many possessions as they can carry on their bodies. Therefore surplus wealth is unlikely, and thus polygyny in hunter-gatherers is practiced, if at all, by a small minority.
Indeed, because polygyny means that women are monopolized by a single male, where polygyny is widespread there are going to be men who lack a mate and who may be willing to fight in order to get one. This would weaken any hunting and gathering band, making them more vulnerable to social disruption. It would make sense that groups that enforced monogamy would be more likely to survive, because they would share equitably and be more effective cooperators.
With humans, it has always been the case that individuals, and even nuclear families, cannot survive without being part of a larger group. Most hunter-gatherer bands comprise groups of thirty to ninety people. Too few and they can’t survive over generations, too many and dissension and violence split the group up.
It cannot be a coincidence that today’s hunter and gatherers all have a similar egalitarian ideology that encourages sharing and discourages boasting, inequality, greediness, selfishness, public aggression and bullying, as documented by anthropologists Boehm, Lee, and others. It is not likely that this ideology just happened to develop, since it is common to nomadic hunter gatherers no matter what part of the world they are from. It is more likely that this suppression of these public male dominance behaviours developed universally, because it was necessary for group survival.
One thing that is unique about monogamy is how effective it is as a way to channel male behaviour outside the immediate family. As the primatologist, Frans De Waal has argued, by separating sexual competition from other forms of competition, monogamy allowed a greater proportion of males to flourish and to benefit their families and societies.
Of course, we may be aware of how monogamy breaks down through divorce, abandonment, affairs, etc. The point is that it exists in all human societies, even though our natural feelings may influence us to violate it.
Every element of human culture comes from our primal ability to agree to form and follow rules of behaviour, especially when we expect others to do the same.
The anchor for human society is monogamy, because it is the first sustainable institution that incorporates collective agreement to regulate behaviour and to honour those limits through a rough equality. By deciding on monogamy, our ancestors made equality possible, and by developing social methods of control: shaming, ridicule, shunning, and banning, our ancestors created a method of maintaining monogamy in the face of centrifugal natural desires.
While some would argue that human language is the ‘ur’ institution, I believe I can make a plausible case that monogamy preceded language and actually makes language possible. If all human institutions arise from collective agreement to regulate social behaviour, then it makes perfect sense that it was the agreement to institute monogamy that formed the basic template for all succeeding human institutions, including language.
In language we have developed representations of reality called ‘words’. These representations can be created and assembled by individuals and then shared. This sharing implies a rough equality, in that in order to understand what is said, it is agreed by everyone that specific words refer to specific things or classes of things. Grammar and syntax - the structure of languages - could have developed from step by step collective agreements about how words can be combined to refer to various aspects of the world.
Before monogamy was instituted, dominance hierarchies precluded equality and equal sharing. There would have been less incentive to share information, so less incentive for a group to agree to common meanings, and, to follow rules of grammar in combining words and phrases.
On a deep level, speaking and listening to others speak requires trust. The moment I detect that someone is trying to take advantage of me is the moment that I stop trusting them. I share information with others as long as I believe that they are not going to harm me. This trust is made possible when we believe that everyone else is following rules and not taking advantage.
To sum up: two million years of human evolution equals two million years of human monogamy. Part of our evidence for this thesis is the diminished sexual dimorphism in humans and homo erectus, suggesting that erectus and sapiens eschewed chimpanzee type promiscuity and gorilla type polygyny. Then there is the fact that monogamy is prevalent in all nomadic hunter-gatherer societies and in almost all modern ones.
Monogamy is not a human instinct, nor is it a default behaviour that we can fall back on; it is a system of behaviour that requires high maintenance in order to be sustainable, and yet we have managed to make it the prevalent mode of conduct over the vast span of human existence.
By stripping away the effects of wealth and surplus on behaviour, we get, in nomadic hunter-gatherer peoples, a minimalist set of conditions, the bare bones required to sustain human society. These behaviours involve collective social controls on male domination outside the family, owing partly to the fact that our survival depends on living together in groups that include more than a single family. By maintaining a rough equality, hunter-gatherer monogamy made greater trust and social cooperation possible, and led to all the advantages of human culture, including and especially the gift of language.
