A study, published this week in PLOS Genetics, finds that people tend to choose partners who share similar ancestry. They also note that this tendency is steadily declining.

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Looking similar to your spouse might be more than skin deep.

If a husband and wife look similar to each other, it might not be simple coincidence or just a preference for particular traits.

A recent study demonstrates that spouses might often look alike because we are naturally predisposed to marry someone of a similar ancestry.

The findings have important implications for researchers investigating the genetics of specific populations.

For the majority of humanity’s history, people picked their spouse from the local area. This is referred to as endogamy. People who lived in close proximity would, more than likely, have similar ancestry.

Because of this, over generations, the affinity for genetically similar mates has created a genetic structure within the population.

Being attracted to people of similar ancestry is different from so-called phenotype based assortative mating, in which individuals tend to choose partners based on various phenotypic characteristics, such as height, hair color, or skin pigmentation.

Phenotype based assortative mating changes the frequency of alleles in the population for the specific trait that is being selected for – skin color, for instance. However, with ancestry-related assortative mating – that is, picking a mate of similar ancestry – the genetic imprint throughout a population is slightly different. Although certain phenotypic alleles are still favored – those coding for eye color or height, for instance – other genes that are not related to the phenotype are also amplified.

So, if a population particularly favors green eyes, the rate of green eye-coding alleles will increase in the population, but most other genes will remain untouched. Conversely, if a population favors marrying people with a similar ancestry, a large number of genes related to that lineage will be boosted.

It is possible to observe the differences in a population created by these two similar but separate forms of mate choice.

Researcher Ronnie Sebro, from the University of Pennsylvania, along with senior authors Josée Dupuis from the Boston University School of Public Health in Massachusetts and Neil Risch from the University of California in San Francisco, set out to study this effect.

For the first time, they examined mating patterns across multiple generations of an American population.

The investigators used data from the Framingham Heart Study, a long-term project that began in 1948 following the heart health of the residents of Framingham, MA. In all, they characterized the ancestry of 879 of these participants using genomic data.

People of Northern European, Southern European, and Ashkenazi ancestry were found to preferentially choose spouses of the same background. However, with every generation, individuals became less likely to marry someone with a similar ancestry.

This pattern meant that husbands and wives were found to be more genetically similar than would be expected. They also noted that the genetic structure created by choosing genetically similar mates has decreased over time – in other words, the population’s propensity to choose mates of similar ancestry appears to be reducing through the generations.

Understanding the genetic structure of a population is important when approaching genomic studies. Genetic similarities in a population can lead to false positives when pinpointing gene regions associated with disease. It has the potential to influence estimates of the degree to which a disease is passed on genetically. As the authors write:

“Characterizing the genetic structure of a study population is important because ignoring it can lead to undetected biases, including false positive findings in genetic association studies and inaccurate estimation of kinship and heritability.”

Although the results are intriguing, the authors are keen to extend their findings. They would like to understand “the degree to which our observations in Framingham generalize to other populations, both within the U.S. and elsewhere.”

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