The widespread notion that "opposites attract" is wrong, not just psychologically but, as a new study shows, genetically as well.

Psychologists have long known that the "opposites attract" cliché is a gigantic myth. As a general rule, people are more attracted to those with whom they share personality traits, hobbies, and political and religious beliefs.

But, that's psychology. When it comes to biology, it has been long assumed that mating is more or less genetically random. But it turns out that's not true, either. Spouses are more genetically similar than two randomly selected people.

A new study led by Benjamin Domingue and Jason Boardman in PNAS examined 1,716 non-Hispanic white individuals in 825 heterosexual spousal pairs. (Note that some individuals had more than one spouse, and others had incomplete data.) Performing a genome-wide association study (GWAS) using more than 1.7 million single nucleotide polymorphisms ("SNPs" or individual DNA "letters"), they found that spouses shared more genetic similarities than random pairs from the same population, a phenomenon called genetic assortative mating (GAM). In fact, an increase in genetic similarity of one standard deviation was linked to a 15% increase in the probability of marriage.

What could explain these results? The authors examine three possible explanations: Birth region, ethnicity, and eduation. People born in the same geographic area are both more likely to marry and to share some genetic markers. The same reasoning applies to shared ethnic backgrounds (e.g., being of German or Irish descent). Even after controlling for those potential confounding factors, the authors still found a GAM effect.

So, that leaves education. It is well-known that people with similar educational backgrounds tend to marry each other, a phenomenon known as educational assortative mating (EAM). It is also known that educational achievement is partially linked to genetics. Indeed, the authors found that the effect from genetic assortative mating (GAM) explained up to 10% of the effect from EAM, but the direction of causation is unclear. Does GAM cause EAM, or does EAM cause GAM? Or is this a bidirectional relationship? That can only be elucidated with further study, particularly by examining other races and ethnicities.

The idea that spouses share genetic similarities is quite counterintuitive. Humans avoid marrying siblings and cousins. Immunological studies suggest that people are attracted to the scents of mates who have different versions of immune genes. One might predict, therefore, that spouses would be less genetically similar than randomly selected pairs, but the authors found the exact opposite. The researchers suggest, however, that different regions of the genome may be under different selective pressures. For example, immunological genes may face balancing selection (which helps maintain diversity), while other genes do not.

Finally, the authors raise a very important implication from their study: The extent of nonrandom mating is not properly accounted for in many epidemiological and population genetics analyses. That means previous investigations into how human populations evolve, as well as how genes influence socioeconomic outcomes, may not be entirely accurate.

Source: Benjamin W. Domingue, Jason Fletcher, Dalton Conley, and Jason D. Boardman. "Genetic and educational assortative mating among US adults." PNAS. Published online before print: 19-May-2014. doi: 10.1073/pnas.1321426111

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