What attracted you to your spouse? His eyes? Her laugh? Does he quote Tolkien in her sleep, just like you? Did you meet at your microbiology club in college? What if one level of similarity and attraction went deeper than that, say, down to the cellular level? A new study from the University of Colorado-Boulder has shown that spouses tend to have DNA that is more similar to one another than to a different randomly selected person. The results of the study have been published in the Proceedings of the National Academy of Sciences.

The researchers utilized 825 couples who were non-Hispanic white Americans. Each participant’s genome was compared to their spouse and then to two other people who were randomly selected. Altogether, the study compared about 1.7 million single nucleotide polymorphisms (SNPs). Couples were more likely to be similar to one another than to the people with whom they were randomly paired.

“It’s well known that people marry folks who are like them,” lead author Benjamin Domingue said in a press release. “But there’s been a question about whether we mate at random with respect to genetics.”

When population genetics assumptions are made, a very common one is that mating is random. We know this isn't entirely true, as people generally have a criteria which they use when finding a mate. Further understanding mating pairs on a genetic level will help refine the models used by scientists, allowing for more accurate predictions. However, researchers are also very familiar with nonrandom, assortative mating where traits like education and economic background are concerned. In short, two custodians could be very happy together, and two lawyers could be very happy together, but it is much less likely that a custodian and a lawyer would be happy together due to the vast difference in education and income. (Please do not get offended. There’s absolutely nothing wrong with being a lawyer, if that’s how someone chooses to live his or her life.)

The researchers have noted that based on this study, this “genetic assortative mating” explains about 10% of educational assortative mating. While genetics appear to be a factor in mate choice, it is a small one.

Of course, this study doesn’t definitively prove much of anything in itself, but it does set the stage for a wealth of follow-up study, including exploring if people choose friends based on DNA as well. In 2012, it was reported that 1 in 12 marriages were of people of different ethnicities, which would likely alter the results about choosing a partner based on similar DNA. The researchers will need to expand their scope to include participants more representative of America’s diversity in the future. Analyzing the SNPs could also provide more clues about which traits humans use in choosing relationships, either romantic or plutonic.