While doom-scrolling through posts on social media, you may have come across cheeky posts captioned: “siblings or dating?” The photos depict look-alike pairs and invite viewers to guess if the duo is romantically involved or a couple of family members. While an amusing game, upon reflection, you might realize that a fair number of the couples in your social circle could also pass as relatives. Is there a reason why so many people seem to find partners who look eerily similar to themselves? 

This phenomena has a possible link beginning in early development. Sexual imprinting refers to the concept of humans developing their mate preferences at a young age. When they do this, they often use their parents’ features as a guide for their inclinations. A 2012 study by researchers from the University of Turku found significant similarities when comparing facial features of heterosexual men’s partners to those of the men’s mothers. 

Additionally, a 2023 article published by the American Psychological Association argues that people internalize “self-essentialism” — a belief that one’s self, or “essence,” and values define their outward behaviors or traits. This is especially strong in places like the United States, where the culture values individualism. Conviction in one’s own identity causes individuals to over-inflate the importance of superficial level similarities, such as appearance, political stances or tastes in food. They identify these basic connections and then reason it to signify a far deeper and broader scope of agreement than they have any evidence for. Minds manufacture a tie and leads to what the authors have dubbed the similarity attraction effect.

But if we are more attracted to those who resemble us, would it not follow that we would be most attracted to our siblings, the people with whom we share the most similarities? 

Not exactly. The human proclivity to avoid inbreeding by reducing attraction to one’s siblings is known as the Westermarck effect. This disinclination is believed to be strengthened by witnessing their mother taking care of their sibling as an infant and bathing or sharing a room at a young age together. In a study where female university students were asked to imagine performing sexual activities with significant others and then with brothers, the researchers recorded increased activity of facial muscles correlated with feelings of disgust in the latter cases.

Evolutionary scientists believe that natural selection favored humans with this aversion to inbreeding in order to avoid inbreeding depression. This refers to the decrease in fitness of offspring produced from closely related parents. An organism’s genotype on average contains recessive mutations that are hidden by a dominant healthy copy. Closely related individuals, such as siblings, are more likely to have copies of these hidden mutations that, if combined, could cause severe diseases. Therefore, the offspring of humans who were not mating with close kin were more likely to be healthier and reproductively fit. 

This is not to say that you don’t love your sibling. Even if sometimes you want to strangle them. It just means that the type of love is different.

Research published in the Cerebral Cortex has found that there are differences in brain activity on functional magnetic resonance imaging scans depending on whom the feelings of love are directed toward. Love towards close relationships such as a friend, child or partner elicit far greater activation of the brain stem regions in comparison with strangers or non-human subjects such as pets or nature. Additionally, a paper titled “The neuroendocrinology of love” linked romantic love to regions of the brain responsible for motivation and drive, meaning that the brain uses this type of love as a mechanism for promoting mating and reproduction.

So while it might be awkward to look over at your significant other this Valentine’s Day season and consider that maybe they do look a little bit like your parents, at least you can thank evolution for your aversion to the thought of dating your sibling.

A version of this article appeared on p.10 of the Feb. 13, 2025 edition of the Daily Nexus.