We’ve all been there: You’re at some public venue casually people-watching and you come across a cute couple with too similar of traits. “Are they related?” you ask yourself as they pass by, eagerly anticipating some interaction that’ll offer some insight. And then they kiss, and the whole thing creeps you out. “He looks like her brother,” you mutter under your breath. Well, it turns out there’s a scientific—albiet almost incestual—reason some people end up dating their “twins,” and it has to do with location and convenience.

According to research on mating patterns from Boston University and the University of California, San Francisco, our ancestors tended to marry and mate with singles from within or near their own community. This is understandable, considering that pre-internet, proximity and convenience were pretty much the main factors in mating.

Researchers determined this by taking a close look at 879 married couples from three different generations of inhabitants in Framingham, Massachusetts. These individuals’ genetics were studied because they had all taken part in an on-going health study that dates back to 1948. Using genomic data, researchers discovered that individuals of northern European, southern European and Ashkenazi ancestry who settled in and around Framingham were more likely to choose spouses of the same background, with similar appearances.

More than explaining why blondes might be more attracted blondes, however, this insight has turned researchers on to a kink in modern genetic studies, as people who marry others with similar genes has created biased genetic structures in some populations. Because of “false positives,” it is then more difficult to identify relationships between specific genes and diseases.

“Genetic similarity within a population can be important to consider in genomic studies because it can lead to false positives when identifying gene regions that are associated with a disease, and affect estimates of the degree to which a disease is passed on through one’s genes,” the researchers reported. For a time, marrying locals became a trend, with the number of people marrying others with similar genetic histories and appearances became more commonplace. As these relationship matured, these genetically homogenous couples then had children who’d continue this cycle. And so on.

Things eventually got less weird, though there isn’t a timestamp for this. In each successive generation—i.e., along the same timeline as advancements in technology and transportation—researchers found that individuals were less likely to choose a spouse with similar ancestry. As a result, a community’s genetic structures created by past (quesionable) mating patterns have gradually become more varied over time.