Researchers say there is a common ancestry for many married couples, but it’s becoming less common because younger generations are more mobile.

The joke that married couples can appear similar may in fact be due to the tendency for couples to marry someone from a similar ancestry — whether they are aware of it or not.

That’s according to a new study published recently in PLOS Genetics.

In the first ever investigation into mating patterns across multiple generations in the United States, researchers examined genetic similarities between spouses in three generations of families.

The participants took part in the Framingham Heart Study, which has examined the heart health of residents in Framingham, Mass., since 1948.

The researchers from the University of Pennsylvania, the Boston University School of Public Health, and the University of California San Francisco found that of the 879 spouse participants, those from Northern European, Southern European, and Ashkenazi backgrounds were more likely to choose a spouse from the same ancestry.

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The researchers say choosing a partner from a similar background could be due to a number of reasons.

“Mate choice reflects a large number of factors including local geodemographics, social class, nationality, ethnicity, religion, anthropometric traits such as height and weight, as well as behavioral characteristics,” the researchers wrote.

“These patterns may also have reflected neighborhood characteristics, and the tendency for unions to occur locally.”

The study found that those from later generations were less likely to choose a spouse from the same ancestry.

“Intermixing between participants with Northwestern and Southern European ancestries was relatively uncommon in the original cohort but increased in subsequent generations,” they wrote.

This may be due to the tendency for younger generations to move around more frequently.

“While unions historically have been preferentially local, increased movements of the population over past decades are contributing to the decay of local endogamy, as seen in Framingham,” the researchers found.

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Genetic similarity within a population is of particular significance in the area of genomic studies as it can lead to false positives in identifying gene regions associated with disease.

It can also impact estimations of the degree to which a disease may be passed on genetically.

Over several generations, the tendency for people to pick a mate from a similar ancestry has resulted in a genetic structure that has potential to bias results of genetic studies.

Professor Peter Ralph from the Institute of Ecology and Evolution at the University of Oregon, says that in small populations, genetic similarity can also have health consequences.

“In very genetically similar populations there are often increased risks of some genetic diseases, because it is effectively more likely someone gets two broken copies of some particular gene. In practice, this only shows up in very small populations, or ones that have been very small in the recent past,” he told Healthline.

Ralph emphasizes that the findings of this study shouldn’t be misinterpreted.

“There is an implication in some of the news about this study that there’s some genetic cause that makes people find more similar spouses but that’s not supported by this study at all [or claimed by the authors],” he said.

Noah Rosenberg, PhD, is a professor of population genetics and society at Stanford University. He says the research could pave the way for improving genetic studies of disease.

“Many research methods in human genetics rely on an assumption that people choose mates randomly within a population. Although this of course is not how people choose mates, real populations often are close enough to random mating for the research methods to work properly,” he told Healthline.

“This study disentangles the component of nonrandom mating that is due to assortative mating by ancestry, evaluating changing preferences that people have had over time in a New England town for mates from similar ancestral backgrounds. It finds a way to improve genetic studies of disease to account for the effects of assortative mating by ancestry.”

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