Opposites attract. It's a basic principle of magnetism that has long been applied to romance. But the problem is that research on relationships shows that this notion, which sounds all well and good in song form, doesn't ring true in practice.
Instead, we tend to be attracted to people who are similar to us in one way or another. It's a phenomenon known as assortative mating. Assortative mating can be seen across traits like race, age, facial characteristics and body type, all of which have a genetic basis.
Individuals in fact tend to seek out someone with similar DNA, finds a joint analysis of Latino communities in Mexico, Puerto Rico and the United States by researchers at the University of California – San Francisco, Microsoft Research, Harvard, University of California – Berkeley, and Tel Aviv University.
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Couples who pair up in these communities often have similar genetic ancestry, which proves stronger in determining mate selection than other socioeconomic factors, such as education levels, which itself has shown to be a powerful predictor of attraction in previous research.
In fact, ancestry was so similar in the populations studied that the average couple was the genetic equivalent of between third and fourth cousins. This degree of genetic relatedness among mating pairs could have potential health implications, both positively and negatively, as recessive alleles are more commonly passed in partners with similar genetic profiles.
There is a limitation in this study in that the research, appearing the latest Proceedings of the National Academy of Sciences (PNAS), in that it focused on ethnically homogenous communities. Other studies have found similar results, however, when looking at different populations.
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Last year, another PNAS study by University of Colorado – Boulder researchers examined the genetic profiles of 825 non-Hispanic white couples. The researchers looked at single-nucleotide polymorphisms, parts of DNA where humans differ from one another. What the scientists found was that married individuals were likely to be more genetically similar than two individuals randomly selected from the same population.
Assortative mating is common, some researchers argue, simply because of population stratification. Individuals tend to coalesce around like individuals, according to ethnic, racial or socioeconomic factors. But there is evidence as well of disassortative mating at a genetic level as well.
A 2009 study presented before the conference of the of the European Society of Human Genetics examined the major histocompatibility complexes (MHCs), a genetic region that plays a key role in immune response and reproductive success, in 90 married couples and compared them with 152 randomly matched pairs set up as control group. Married couples were more likely to choose a mate with diverse MHCs, likely an evolutionary strategy tied to successful reproduction.
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We all have our type, a set of characteristics we seek out in a potential mate. As much as we believe we have control over who we fall for, our genes have pretty much already made the choice for us. It's not strictly a case of, "Like attracts like," or "Opposites attract," either. As with all things concerning with attraction between two people, it's complicated.