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Genetic Similarity Theory states that we seek out similar others in order to give our genes the best chance to produce copies of themselves. As Richard Dawkins says in The Selfish Gene, it is genes that survive to the next generation with more copies being found in siblings and related co-ethnics. Therefore, the theory goes, by benefitting genetically similar others, we are benefitting copies of our genes. Speed daters match on genotype, which shows evidence for ability to detect genetically similar others. On a subconscious level, we have the ability to detect genetically similar others.

Assortative mating is a form of sexual selection in which those with similar genotypes and phenotypes mate with each other more often than in would be expected under a random breeding model. One of the numerous ways we match by genetic similarity is phenotype. If the phenotype is similar, more often than not, the genotype is as well. This is what drives friendships and marriages, as well as being the cause for ethnocentrism.

Rushton (1987) showed that humans are able to detect degrees of genetic similarity in others, and prefer those most similar to themselves for friends and spouses than less genetically similar individuals, which is the basis for ethnocentrism. A husband and wife are, on average, as close as fourth cousins. Due to matching by GST, spouses should also match on heritable traits such as IQ, body measurements and personality traits. As McCrae et al (2008) write:

Altruism, Modesty, and Tender-Mindedness are characteristics that most people desire in a spouse (cf. Buss, 1986), but people are most likely to find a mate with these characteristics if they have them themselves. This is an instance of the principle that people with desirable qualities have more options in seeking a desirable mate. At the same time, it seems likely that there is a sense in which disagreeable people may actually prefer the company of their own kind, like the haughty Duke in Robert Browning’s “My Last Duchess,” who disposed of his wife because she was too indiscriminately nice.

Everyone has the perfect spouse in their head that they dream of. However, the type of spouse we end up with will, more often than not, be genetically similar to ourselves. Even spouses who are not of the same race or ethnicity match up on heritable traits such as The Big Five, IQ and physiological measurements.

Divorce is also influenced by genetic factors. Jockin, Mcgue and Lykken (1996) found that 40 percent of the variability in the heritability of divorce comes from genetic factors that affect the personality of one spouse. Traditionalism, extraversion and neuroticism (2 of the Big 5 Personality Traits) are causes for divorce. A few reasons I can think of for neuroticism and extraversion being personality traits correlated with divorce is highly neurotic people are more likely to be stressed, anxious, have hypochondria (the worry of contracting an illness) and obsessive behavior. This can put extra strain on a marriage, leading to both of the spouses not being happy in their marriage, leading to divorce. With extraversion, more extraverted people are more open to meeting others and are more social and talkative. This will lead to feelings of jealousy, causing a strain on the marriage.

The genetic and environmental influences responsible for marriage are different from those that are responsible for divorce. Evidence exists that after mate selection, there may be some protective factors for the couple, such as religion. While other factors that place couples at risk for divorce, such as alcoholism, are also genetic in nature.

Trumbetta and Gottesman (2000) suggested endophenotypes with one being oriented to pair bonding and the other to mate diversification. Pair bonding, obviously, leads to a happier marriage as both spouses are monogamous, whereas mate diversification is associated with multiple marriages. It sounds to me like those who pair bond are more introverted whereas those who have diversity in marriage partners are more extraverted, leading to high divorce rates due to jealousy and cheating. The conclude that there are significant genetic influences on both endophenotypes with unique environmental factors accounting for the rest of the variance,

Spouses, as well as friends, sort on characterisitics such as race, socioeconomics, physical attractiveness, level of education, family size and structure, IQ and longevity. This is the Selfish Gene in action. By seeking out copies of itself (which would be in co-ethnics in higher frequencies), the gene is able to ensure its survival onto the next generation.

Even in couples who are not the same race or ethnicity match on other heritable characteristics. Rushton and Nicholson (1988), tested predictions from genetic similarity theory and found that spouses select each other on the basis of more genetically influenced cognitive tests. It’s known since The Bell Curve came out in 1994 that spouses select each other based on IQ. What Rushton and Nicholson noted in the study was that estimates of genetic influence calculated on Koreans and Canadians predicted assortative mating in European Americans in Hawaii and California. Americans of mixed ancestry made up for ethnic dissimilarity  by matching up on the more heritable traits, whereas the correlation is lower for those traits that are more influenced by the environment. The observations on genetic selection were weaker but still had a positive correlation, when the g factor was taken out of the equation. This suggests that we choose mates based on the general intelligence factor. This effect is seen in, for instance, white women who date black men. They, more often than not, have lower average IQs than the mean (100).

Pan and Wang (2011) showed that spouses are similar in academic achievement as well as IQ. 6 out of the 8 traits tested (reading, spelling, arithmetic, vocabulary, verbal and full-scale IQ) showing evidence of spousal correlations.

Humans have a natural instinct to marry genetically similar others. Whether the traits are environmentally or genetically influenced, spouses will match on traits with the highest correlations (BMI, waist size, arm size). Genetic Similarity Theory proposes that these phenomena is not by chance, but was how we evolved. Sexual selection, which is natural selection arising through preference by one sex for certain traits in individuals of the other sex, is the driving factor here. Through sexual selection, we humans were able to gain higer intelligence (for men) and gain higher verbal abilties that allowed to care for children (women). These differences remained even when controlling for geographic location. Spouses and friends being as similar as 4th cousins is no accident, in fact, it is evolution in action.