Five hundred years of betrayal: what geneticists have told about the weak points of monogamy

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Five hundred years of betrayal: what geneticists have told about the weak points of monogamy
Five hundred years of betrayal: what geneticists have told about the weak points of monogamy

Women not have a child from a husband the more often, the lower his social value. Let's try to figure out how biology pushes them to cheating and why monogamy has every chance of revenge.


Belgian researchers have determined the likelihood of a violation of genetic monogamy in Holland and Belgium over the past 500 years. It turned out that the low income of men several times increases the likelihood that their wife, without divorce, will have a child from another. This points to the possible causes - and limitations - of such a mysterious phenomenon as monogamy in humans. Let's try to show why this is so important.

For most birds - and, apparently, dinosaurs, the only living representatives of which today are birds - monogamy is natural. Both the male and the female can warm eggs in a clutch or carry food in their beak. Technically, the male is even better: he is often larger and is almost always better at foraging. Monogamous species in such conditions will often outplay non-monogamous ones: the probability of the survival of offspring with two brooding hens is much higher, which means that reproduction will go faster.

An important caveat must be made here. Monogamy, having one partner, is social and genetic. Social is when partners, say, hatch eggs together, or even live their whole lives. Genetic - when the female has children from the male with whom she lives. It is not hard to guess that the second is less common than the first.

For females, it seems evolutionarily beneficial to take advantage of social monogamy, but at the same time have offspring from an extremely attractive outside male. Of course, the average female bird does not think in evolutionary categories, but she feels that for some reason she is strongly drawn to this male, although in most cases the other one seems to her better.

For example, female zebra finches in laboratory experiments mated almost always with one male and once every few days with another (trying to hide this from the main one). Meanwhile, genetic tests showed that the percentage of offspring from the second male in such a situation was significantly higher than one would expect from the number of mating. Obviously, some kind of genetic mechanism is helping to disrupt genetic monogamy.


This also makes sense. A male who brings up offspring well under normal conditions is great. But a male who shows great aggressiveness, perseverance or the ability to do something (for example, reproduce) secretly is a carrier of skills that can also be useful to offspring. Evolutionarily, it is advisable to preserve different behavioral strategies in a species. Who knows what challenges the future will bring? Here are the females of a number of birds and lay out, figuratively speaking, eggs in different baskets.

Mammals: Rare Dislikes of Monogamy

Mammals are a completely different story. Their mother bears offspring, she feeds him at a very early age. The meaning of monogamy is mysterious for them. Monogamy is especially rare in primates. For example, gibbons seem to have it (although field studies are still not enough to speak about this with confidence) and a number of South American monkeys.

But why do they need it? He gets the opportunity to pass his set of genes to the offspring from only one female. She can easily carry problem genes that will reduce the likelihood of the survival of the offspring, and then the genes of this particular male will disappear in the future.

From the perspective of a male mammal, genetic monogamy is extremely disadvantageous.

A similar story with people.Nuclear DNA (which is in the center of our cells, the "main") is transmitted through the male line, mitochondrial (into the mitochondria, an important organelle of the cell) - through the female. So, the variability of the "male" nuclear DNA in humans is much higher than that of mitochondrial. But geneticists know a large number of ancient variants of nuclear DNA that have become extinct over time.

If we turn to other large great apes, we will not see such a problem there. For gorillas, for example, a harem of many females, their contacts with the outside world are sharply limited by the head of the harem. The likelihood that he will pass on his nuclear DNA to a "defective" female and on her will suppress his genus is minimal: he transfers DNA to many females. Orangutans do the same, although no formal harem is formed. Common chimpanzees lead a difficult sex life, but even there, the alpha male mates with many females.


So why do people need monogamy? To understand this, it is desirable to find out: what kind of monogamy is this? Social or Genetic? And under what conditions is it violated?

Unfortunately, modern society is poorly suited as a source of information on human monogamy. Evidently, the evolutionary strategies of breeding in the developed countries of our time are a dead end. Residents of such states spend less and less of their lives in marriage and relationships, and the number of children born is falling. These people do not reproduce themselves and, even worse, those with fewer genes correlated with a propensity for education and good health are better reproduced.

In theory, this should lead to a noticeable decrease in the mental abilities of their offspring within a few hundred years. If such a model exists for at least a thousand years, then representatives of modern developed peoples will naturally die out. Therefore, we need data on non-modern societies - from the era when the reproduction of people proceeded more smoothly.

Five hundred years of violations

Scientists from the Catholic University of Leuven (Belgium) took the genealogical data of 513 pairs of modern men from Belgium and Holland. According to their documented genealogies (written genealogical records have long been developed there among non-aristocratic individuals), each of this pair must have descended from one male ancestor. The ancestors of all 513 couples studied lived at some known time span between 1315 and 1974. Dates are important here: at this time, most of the life of men and women was spent in marriage.

The average level of situations where the genealogy was not genetically confirmed was small. Only 1.6% of men had their ancestor not the same man that was indicated in their genealogy and that was married to their female ancestor when she gave birth. Thus, in Belgium and Holland, monogamy in 1315-1974 was mainly genetic, not just social.


At the same time, for peasants the probability of such an event was 1.1% (in rural areas, the hidden connection may be more difficult), and for skilled artisans and traders, the richest part of the region's population, it was only 1.0%. Among unskilled laborers and weavers, the average rate of “failure” in genealogy was 4.1%.

That is, among their ancestors, the probability of having a child not from a husband was four times higher than among peasants or wealthy artisans and merchants who lived at the same time. The average probability of having a child not from a husband for cities with a population of more than 10 thousand people was 2.3%, which is noticeably higher than the norm. It is clear that in large cities there is more low-skilled labor and there are no peasants at all.

It is interesting that for residents of small towns - both peasants and well-to-do skilled artisans and traders - the probability of having a child "not from that" was only 0.4-0.5%. Among the poorest strata of the population in cities over 10 thousand people, it was 5, 9% The gap between the probabilities of such events, as we see, can reach tenfold in the same society.

At the same time, it turned out that the likelihood of having a child not from a husband in marriage as a whole changed little over time. Only at the end of the 19th century, during the period of an upsurge in urbanization and the number of low-skilled workers in the region, were the numbers of “wrong” births higher than the norm. But the reasons here are clearly not in the change in the nature of people, but in the fact that the percentage of low-income urban residents has become higher.

A rather interesting conclusion suggests itself. Human monogamy in normal living conditions is mainly genetic.

If the material security of the family is low, and there is a noticeable number of other men nearby, then the probability of having a child not from a husband increases up to ten times.

So, there is a feeling that in people monogamy is provided by the fact that the man plays the main role in the maintenance of the family. The better he copes with her, the lower the likelihood that his wife will risk having a child from another.

Of course, unpleasant questions arise. Until 1974, men were paid much better than women. It is obvious that today this difference is very moderate. In this regard, it is unclear: what is the point for women now in genetic monogamy? By the way, modern Holland and Belgium definitely do not belong to countries where divorces are rare, and most of the life of people of fertile age takes place within the same family.

The human race: how women completely changed the behavior of men

The situation with monogamy in humans has always been mysterious. For male humans, strictly speaking, there are not many evolutionary advantages to monogamy. It is much easier for each of them to technically conceive dozens of children from different women and not take care of anyone than to conceive several from one and take care of them all their lives.


Therefore, the very emergence of monogamy in primates should, in theory, be based on the steps of the female sex. Let's take the same monogamous gibbons: they mate in any month of the year. The meaning here is simple: in normal animals, mating is carried out for the conception of offspring.

When a female, say, an ordinary chimpanzee is ready to mate, she clearly demonstrates this to the male, and then mates until she becomes pregnant. There is little room for monogamy here: it is objectively unclear how a male can tolerate the difficult (and we have said it mildly) character of a female chimpanzee in the rest of the year.

However, in a minority of primate species, mating proceeds as in humans: all year round. It is also called "incentive mating". The idea behind this reward is simple: it makes the male permanently attached to the female, which forces him to take care of the children.

In other words, although men are normally sure that they are the ones who make the most important decisions in a couple, in real life the very existence of permanent couples is determined by the behavior of women. Men rarely think about the extreme unusualness of this behavior - species with constant mating and a menstrual cycle of less than two dozen.

It might seem that it was year-round mating that led to the foundations for human monogamy. Even scientists sometimes, to simplify, say that due to reward mating and the menstrual cycle in humans, women do not show readiness to mate - like ordinary chimpanzees during estrus, for example. In the latter, as in many primates, the genitals turn red during estrus, which can be seen from afar and prompts the males to contact.

At first glance, humans do not have this, which simplifies monogamy, excluding attempts by third-party males to interact with a woman from an already established pair. In general, it seems logical that the absence of enhanced "advertising" of her readiness to mate suggests that a woman is prone to monogamy and does not seek to exacerbate it by giving signals to anyone other than her regular partner.

In fact, things are not quite as they seem. The most attentive men themselves know: on the days of ovulation, women barely noticeably turn pink in the face, especially its middle part, sometimes the timbre of the voice, and often behavior, also changes. Of course, most of the men are not attentive enough to notice this on a conscious level.

However, a series of experiments showed that a man who is next to a woman at the time of ovulation produces more testosterone, the main male sex hormone.His consciousness may be inattentive, but the body as a whole is far from being so bad and registers the fact that a woman is nearby in such a state. Accordingly, having received a portion of testosterone, he somewhat changes his behavior in such a way as to increase the chances of having sexual intercourse with her. By the way, such a change in behavior at first may not be noticed by the man himself.

Changes in the behavior of women during ovulation are not only pinking of the face or changes in the voice. A number of studies have shown that during this period their sexual motivation also grows. The smell of male sweat, which they usually do not like at all, is rated as more attractive during this period.

All this made it possible for two scientists from the United States to assume as early as 2008 that women of our species have a kind of “matryoshka” of two types of nested models of sexual behavior.

First, the outer part of the matryoshka is “extended sexuality,” the standard behavior of a woman on most days of her life. Willingness to reward mating with a regular partner, average motivation for sex. In order to have sexual intercourse these days, a woman needs enough social attraction from her partner. For example, the availability of skills highly valued by society, a social rank at least slightly higher than the typical woman's environment, wealth, a desire to spend time raising children.

Researchers call the second type of sexuality estrus sexuality. Estrus is "estrus", so we will refrain from direct translation of this expression, due to its dissonance. It is observed at the peak of ovulation, several days a month. On such days, according to a number of studies, women are more attracted to the "highly masculine" ("masculine") faces and bodies of men.

In the course of experiments, when looking at photos of men, women on these days give higher (than on other days) ratings to those men whose bodies show a greater degree of symmetry. The symmetry of the body and face is a simple but rather reliable indicator of the “genetic quality”, the absence of noticeable genetic defects and abnormalities in the partner.

Interestingly, in experiments where women during ovulation, instead of showing photographs, were allowed to see the smell of men, they also gave higher ratings to those with a high degree of symmetry of the face and body.

It turns out that a woman can choose a more physically attractive man for her (in a state of ovulation), even without ever seeing him. In addition, on ovulation days, they rated "masculine" voices (that is, a lower, usually low voice of a man is associated with increased levels of sex hormones during puberty) higher than on other days.

Based on this, the authors of the work believe that on the days of ovulation ("estrus", as they call it), Homo sapiens women prefer sexual intercourse in order to get better genes from men, while on other days - encouraging mating to maintain social monogamy.

In such a scheme, having a child not from a permanent partner is relatively normal. At the same time, the expectant mother will receive both high-quality genes from the offspring (from a third-party man), and high-quality care for him from a permanent partner. That is, at the biological level, a person (primarily a woman) is inclined to social monogamy, but not genetic.

The question arises: why is it impossible to combine good genetic material and a caring partner in one person? In theory, this is possible, but almost everything is more complicated. High body symmetry, a more masculine face, and a lower voice are often associated with testosterone levels. In men, it is produced mainly in the testes.

So: men with large testicles and high testosterone have shown in experiments a lower level of pleasure when looking at photographs of their children. Usually, this less activity indicates that the person is a less caring father.By the way, the questionnaires of the wives of such men show that they are indeed less than average involved in caring for children at home. By the way, men with a more "masculine" face shape are also more prone to cheating.


That is, biologically, the combination in one man and physical attractiveness for a woman during ovulation, and the qualities of a good parent, if possible, is hardly widespread.

Why Belgium and Holland have shown such low rates of monogamy violations

However, not all human behavior is determined by biology. And, in fact, not only human. For a long time, primatologists could not understand why alpha male chimpanzees periodically and seemingly for no reason beat their females (including branches) and sometimes kill their children. More careful observations made it possible to establish: the more often the male of an ordinary chimpanzee beats the females in his group, the higher the chances that the children in it are born from him, and not from other males.

Moreover, he kills children when he assumes that the female gave birth to them from another. Despite the fact that the alpha male normally tends to interfere with the mating of "his" females with other males, in practice this does not always work out, since females explain to non-alpha males with hints that they can go into the jungle and meet there.

Humans have similar adaptations to combat genetic non-monogamy. We have these cultural norms of so-called jealousy. Societies where it does not exist are well known: the Eskimos of the old days, some Indian tribes, the Moriori of the Chatham archipelago near New Zealand. In these societies, there were marriages and social monogamy exists, but there is no way to talk about any genetic monogamy.

Although in hiding (the Eskimos often do not hide), most women in such societies regularly had sex with other men. This behavior was the social norm there. That is, where cultural attitudes do not interfere with the biological nature of a person, there is no talk of any genetic monogamy at all.


But in most societies, there is jealousy. Societies without jealousy, as a rule, do not know real wars and (this may be related) are not technologically sophisticated enough. This means that they inevitably lose to those who have jealousy, that is, in the future, societies with jealousy should prevail. Of course, there was jealousy in Belgium and Holland in 1315-1974. It was also supported by social norms - we are talking about Christian (at that time) countries, and in Christianity, adultery is assessed as a grave sin.

Monogamous marriage and reproductive success: current situation

So, if we limited ourselves to the past of humanity, we could say that our species is socially monogamous, and if cultural attitudes are strong enough, then most often it is genetically monogamous. Deviations from the physical fidelity of spouses are possible only if the social rank of the male spouse - or his income - is so low that the woman does not have much sense in social monogamous union with him. So little that she is not afraid to risk an extramarital affair, believing (not always consciously) that if everything is revealed, she will not lose much.

However, whether this situation will continue in the future is a big question. According to statistics, in the United States, about half of all marriages end in divorce. Similar trends sooner or later come to all countries with at least a relatively modern economy. Therefore, it is not very clear today whether a person will remain so “conditionally monogamous” in the future.


Most likely, the answer to this question is yes. Statistics show that the number of children a woman has is directly correlated with the number of years she spent in marriage. In the United States, a woman who has been married for her entire life has an average of four children. The further she is from a monogamous lifelong marriage, the less her reproductive success is.

A divorced woman in the States usually has fewer than two children, never married only one.As you know, the threshold of reproduction starts with two children per woman. That is, those of them who live without marriage, on average, are doomed to the fact that their set of genes will eventually disappear. On the contrary, those women who can live in a monogamous marriage permanently will leave numerous offspring, that is, their set of genes will spread.

One might argue that in the United States, fertility in a normal monogamous marriage is much higher than in Europe or Russia. Of course it is: the classic American way of life objectively promotes procreation. However, in the rest of the world, women in marriage have many more children than those who are not in it.

This means that the Darwinian fitness of people who are not married for the main part of their lives is lower than that of those who are in them. "Little married" leave few children, those who have been married all or most of their lives - much more.

As a result, the genes of the former will be supplanted, the genes of the latter will be preserved. That is, deviations from social monogamy will be suppressed by natural selection itself. But so far this is only a theory. How events will develop in real life - only time will tell.

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