November 27, 2004
Genetic Causes Of Infidelity Found In Twins Study

A comparison of the sexual histories of 1600 identical and non-identical twins found that genetic variations play a large role in influencing the tendency to infidelity among women.

"We found that around 40 percent of the influence on the number of sexual partners and infidelity were due to genetic factors," Professor Tim Spector, director of the Twin Research Unit at St Thomas' Hospital in London, told a news conference.

Identical twins whose twin has strayed have twice the normal chance of straying.

The findings mean that someone with a philandering twin is far more likely to philander themselves. The average risk of female infidelity is about 22%, says Spector, but those with an unfaithful identical twin have a risk of 44%.

Women tend to stray to hook up with higher status men.

The study, which was published in the journal Twin Research, suggests that a genetic predisposition towards female infidelity may have evolved because it was important in allowing women married to "low status" men surreptitiously to become pregnant by "high status" men.

"If female infidelity and number of sexual partners are under considerable genetic influence, as this study demonstrates, the logical conclusion is that these behaviours persist because they have been evolutionary advantageous for women," the researchers write in their scientific paper.

Female straying for higher status males offers a selective advantage that could have been selected for.

From an evolutionary perspective, a woman’s best short-term strategy would be to clandestinely pursue men with better genes.

Prof Spector points out that women tend to have affairs with men of higher status than their husbands. However, the system would break down, he said, if "everyone was unfaithful, because there would be no pair-bonding".

Three chromosomes were identified in this study as likely locations for genes that influence the odds of monogamy.

It lends strong support to theories advanced by evolutionary psychologists such as Steven Pinker, of Harvard University, who argue that human sexual behaviour is at least partly determined by natural selection and our genes.


Professor Spector’s team did not identify any particular gene that contributes to a tendency to infidelity, though they did pinpoint three regions on chromosomes 3, 7 and 20 that might harbour such genes. He believes that there are between 50 and 100 genes that contribute to a tendency to infidelity.

I predict with high confidence that once DNA testing becomes cheap then before entering into more committed relationships many men and women will surreptitiously take DNA samples from each other and have the DNA tested for genetic variations that contribute to infidelity and to other behavioral tendencies as well. If your man likely to be a hard worker in the long term but is too young have a long work track record? Test him for alleles that contribute to a work ethic. Are you afraid he might have a tendency toward violence? Test for violence-promoting genetic variations. Afraid she is going to cheat? Check hundreds of genes for polymorphisms that cause cheating other undesired behaviors and decide whether the odds of undesired future behaviors are higher than you find acceptable.

Once it becomes possible to alter the brains of adults to increase or decrease the appeal of promiscuity will some men and women come under pressure from their mates to go get a treatment to remove their urge to stray? It seems plausible.

Also, once the genetic variations for monogamy and promiscuity all become identified and it becomes possible to control which genetic variations one will pass on to one's offspring will people tend to choose genetic variations that increase or decrease the incidence of infidelity in relationships? My guess is that people will tend to choose genetic variations that increase the appeal of monogamy. One reason is that people who are going to have many children will tend to favor having children who will want to stick around to take care of their own kids. But I'm not certain on that point. What do you think?

Update: One point needs to be made about hereditability of behaviors: Just because twins are, on average 44% likely to engage in infidelity of their twin has done so does not mean that those are the real odds for each twin whose twin has engaged in infidelity. Suppose, for example, that there are 100 genes that influence whether, say, married people will engage in extra-marital affairs. Well, one pair of twins might have half those genes in variations that predispose toward infidelity. Yet another pair of twins might have 75 of those 100 genes all in versions that predispose for infidelity and still another pair of twins might have all 100 of those genes in versions that predispose for infidelity.

The problem with studies of the behavior of groups twins is that such studies probably understate the extent to which genes can cause particular behaviors. There are people who have a set of genetic genes that pull them in opposing directions. But then there are people who have sets of genetic variations that very strongly pull them in particular directions. Take the twins in this study of infidelity. The twins where each twin engaged of infideilty probably, on average, have more genetic variations that predispose them for infidelity than do the twins where only one of the two twins in a pair engaged in infidelity.

You will see a lot of news reports claiming that the more sophisticated view is to see genes and environment as interacting to cause behaviors. That is true. But that argument is typically made to assure everyone that we really still each have ultiimate control over what we do and that we all have free will. But twins studies understate the extent to which genetic variations influence behavior because those studies deal with groups averaged together. Once DNA sequencing becomes cheap and each person in studies of human behavior can have their complete genome sequenced I expect to see many combinations of genetic variations found that exert extremely strong and even uncontrollably strong influences on human behavior.

Update II:

Women’s fidelity has come under fire at the University of Chicago. Some claim that women are naturally endowed with cheating hearts, and they are pointing to the size of men’s testicles as proof.

A Chicago research team, led by Dr. Bruce Lahn, found that the intensity of sexual competition in a variety of primate species is directly related to the size of males’ testicles. After obtaining DNA sequences from the SEM2 gene from 12 species of primates, including humans, the team examined the evolution of these primates’ sperm.

“We found that the rate of evolution is much higher—that is, the gene has undergone much more dynamic changes—in primate species with promiscuous females than in primate species with monogamous females,” Lahn said. “In other words, when males have to compete more in the game of fertilization, their semen protein has to make more innovations along the way.”


Human testicles fall somewhere in the middle of chimp and gorilla testicles in size. This suggests that women’s fidelity patterns likewise fall somewhere in between these two species, so that women are neither as faithful as gorillas nor as naughty as chimps.

Lahn and colleagues found that the SEMG2 gene in more promiscuous species makes semen more viscous to create a chemical chastity belt against competing sperm from other males.

Lahn's group studied semenogelin, a major protein in the seminal fluid that controls the viscosity of semen immediately following ejaculation. In some species of primates, it allows semen to remain quite liquid after ejaculation, but in others, semenogelin molecules chemically crosslink with one another, increasing the viscosity of semen. In some extreme cases, semenogelin's effects on viscosity are so strong that the semen becomes a solid plug in the vagina. According to Lahn, such plugs might serve as a sort of molecular “chastity belt” to prevent fertilization by the sperm of subsequent suitors, though they might also prevent semen backflow to increase the likelihood of fertilization.

Lahn and his colleagues compared the SEMG2 gene, which contains the blueprint for semenogelin, from a variety of primates. They began by sequencing the SEMG2 gene in humans, chimpanzees, pygmy chimpanzees, gorillas, orangutans, gibbons, macaques, colobus monkeys, and spider monkeys. These species were chosen because they represent all the major mating systems. These include those in which one female copulates with one male in a fertile period (such as gorillas and gibbons); those in which females copulate highly promiscuously (such as chimpanzees and macaques); and those in which mating practices fall somewhere in between (such as orangutans where a female will copulate with the dominant male, but may also copulate with other males opportunistically).

Update III: Over on the Gene Expression blog Godless Capitalist reports on Jared Diamond accepting the existence of variations in testes size as a function of racial and ethnic group. This is a sign that selective pressures caused by infidelity have not been uniform across the entire human race down through history since humans spread out across the planet. It seems quite possible that some or all of the genetic variations mentioned at the top of this post that influence infidelity vary their distribution along with the genetic variations that cause large testis size.

Share |      Randall Parker, 2004 November 27 04:40 PM  Human Mating

Tom said at November 28, 2004 6:02 AM:

Although I haven't seen the study, it seems plausable that someone's promiscuity might depend greatly on the environment they grew up in as well. And of course, twins grow up with extremely similar environments.

Bob Badour said at November 28, 2004 9:17 AM:


By comparing fraternal twins (who generally grow up in the same environment) with identical twins (who also generally grow up in the same environment), the cause measured really is the genetic cause.

Garson Poole said at November 28, 2004 10:16 AM:

It is a fun exercise to try and find potential sources of error in studies of behavioral genetics like this.

[Problem 1] The study tries to separate genes and environment by using a group of identical twins and a group of non-identical twins (as Bob Badour mentions). There are three assumptions in this approach (I think):

A) A pair of identical twins shares a similar environment.
B) A pair of non-identical twins shares a similar environment.
C) The similarity in environment is comparable and can be used statistically to split the effects of genes and environment.

But assumption C is probably false. I would contend that the environment shared by identical twins is much more concordant than the environment shared by non-identical twins. Identical twins may spend more time together, wear similar clothes and exchange clothes. They may exchange boyfriends. They may exchange diaries and fantasies. The bound of intimacy between identical twins might be more intense and hence they might have a more tightly shared environment than non-identical twins. This shared environment might explain the higher correlation in the decision to commit adultery. The genetic effects might be overestimated.

Garson Poole said at November 28, 2004 12:46 PM:

It is a useful exercise to try and find potential sources of error in studies of genetics like this.

[Problem 2] The study tries to separate genes and environment but the two interact in complex ways that might make them inseparable. A fundamental part of the "environment" of an individual is based on the reactions of other individuals. The perceived "ugliness" or "beauty" of an individual highly influences the reactions of others. This means that a largely "genetic" trait has a major influence in shaping the "environment". Indeed, the genetic and environmental influences are inextricably intertwined through this interaction and when a scientific study uses a crude percentage measurement it is presenting an oversimplification in my opinion.

Many physical qualities of a woman, e.g., facial contours, hair color, and eye color, are largely determined by genes, but are modifiable, e.g., by hair coloring, makeup, and cosmetic surgery. Also, the bodily measurements of a woman such as breast size, waist size, and hip size are highly correlated with genes but are modifiable, e.g., by cosmetic surgery and exercise. These measurements also depend on overall weight. For simplicity in the discussion below let us assume that physical appearance is largely genetically determined.

Returning to the topic of infidelity among women. Consider the following schematic theory that illustrates the relationship between genes and environment. Suppose that a beautiful woman receives more direct and indirect propositions to commit infidelity, and she also receives more propositions from "higher status" males. Suppose also that she is no more likely to accept a given proposition than any other women are. However, she receives so many that she is more likely to accept one or more of them than a woman who receives fewer propositions. This suggests that a more beautiful woman is more likely to commit infidelity. (I do not know if this is true. Perhaps a study trying to address this question has already been performed?) How does this relate to the results of the study? Identical twins have a very similar appearance and if infidelity is strongly linked to appearance than the genetic correlation that was found by the study is partially explained by the similarity of physical appearance.

Also consider this variant theory. There may be a set of physical traits that are attractive to men because they are thought to provide a signal of receptivity to infidelity. These traits may not be ones associated with conventional forms of beauty. Woman with the trait(s) may receive more propositions and may accept more of them. They may accept more even if they are no more likely to accept a given proposition than any other women.

Note, that these two theories suggest an interaction between the genes of physical appearance and an alteration in the environment that facilitates infidelity. The two theories above are "genetic" but they do not assume a genetic change to the brain. I think that some of the readers of the newspaper articles linked by Randall Parker will assume that there is some genetic variation in the brain that affects willingness to commit infidelity. In fact, Randall discusses altering "the brains of adults to increase or decrease the appeal of promiscuity". Such genes may exist but I suspect that the study does not yet provide definitive evidence. The main problem with the study is the artificial and oversimplified genetic/environment dichotomy.

Invisible Scientist said at November 28, 2004 1:18 PM:

Here is a company that is already selling a DNA test for infidelity, for only $49.95:

But I am woried that this test might make the society even more violent, since according
to the latest statistics, some 50 % of the men, and 40 % of the women are unfaithful during their
marriages. Additionally, in the industrialized Western countries, about 10 % of the people are
illegitimate (their father is not who they think he is.)
These statistics must be related to the theory that there is a hidden search for
better genes.

Thomas said at December 2, 2004 2:06 PM:

Well, there was a time in Western history when male infidelity was pretty much accepted.

GENEarchy said at December 3, 2004 5:55 PM:

Altering our brains? Increasing appeal? When gene/behavioral-tech gets to the point that it starts to interfere with our minds & mating/social relations, what then? I can see using gene tech to do confidential searches in the same way that "US Search" does what they do, the end of any real freedom. Will we, as quite simply gene-puppets, embrace all of this, or are we inevitably heading towards a sci/tech-induced collapse of our psyches?

Tandblekning said at March 10, 2011 6:10 AM:

Well, im a triplet so i guess that same gentects counts as twins..

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