Ever put your foot in it by presuming that someone’s date was actually their sibling? It’s surprisingly easy to do, and a pretty unfortunate way of kicking off a social interaction. If you were looking for a handy guide to help prevent you making these kinds of faux pas, we’re sorry that we’ll have to disappoint you. Science tells us that there are some very good reasons why the game of “siblings or dating?” can be so tricky to play – let’s explore a few of them.

Spousal concordance

When you spend as much time in someone’s company as you usually do with your spouse or significant other, it makes sense that you’d start to rub off on each other. The broad scientific term given to similarities that develop between romantic partners is spousal concordance.

While often studied in the context of health – recent evidence suggests a high prevalence of spousal concordance in high blood pressure among older heterosexual couples, for example – some researchers have looked into how this phenomenon might affect facial appearance.

An early study on this, published in 1987, compared photographs of married couples as newlyweds and after 25 years, finding that people judged the couples to look more similar after a quarter-century of marriage. “Kin resemblance, therefore,” the authors concluded, “may not be simply a matter of common genes but also a matter of prolonged social contact.”

However, later research has called this into question. A 2020 study found that while it’s true that a lot of spouses resemble each other physically, there’s no convergence effect over time – they don’t get “more similar”.

But while your actual face might not change noticeably, how you use it might. Humans are good at mimicry (not as good as some of our animal friends, but still). We don’t always know that we’re doing it, but we start to mirror the mannerisms, behaviors, and even accents of those we’re closest to. It’s thought that it helps us form tighter social bonds, so it makes sense that this would be observed in romantic partnerships, arguably among the closest bonds we experience.

A tendency to take on each other’s characteristic facial expressions and other quirks of behavior might lead others to view your partner and you as more similar over time. But this is all talking quite long term. Sure, you might start to resemble your spouse if you’ve been living together for decades – what about when you’ve been dating for a few weeks, and someone chases you down a hallway to ask whether you’re actually twins (this may or may not be a true story)?

Opposites attract – or do they?

Studies, including recent research from the University of Queensland, have shown that lots of people are physically attracted to potential partners who look like them.

“We found that participants rated partners who had geometrically average faces and faces similar to their own as more attractive,” said lead study author Amy Zhao. While most prior research on this topic had had people rating photos on a computer screen, this team had put almost 700 people through several rounds of speed-dating to see how people’s facial preferences stacked up in real life.

“Participants also received higher facial attractiveness ratings from partners of the same ethnicity, compared to those from a different ethnicity,” Zhao added.

While this preference is pretty much subconscious, it can be uncomfortable.

“It would freak me out, especially when someone would ask, ‘Are you twins?’” writer Zara Barrie recalled to Time Magazine, thinking of previous partners with a strong resemblance to herself. 

It’s certainly not universal, and clearly there are plenty of happy relationships between people who look nothing alike, but there are some scientific reasons why people might be drawn to similarity. 

One is that humans just generally like things to feel familiar, because that’s easier for our brains to handle. Another is the phenomenon of ingroup bias, wherein people may have preferences for dating those from the same ethnic group. 

There’s also the peculiar finding that people are attracted to composite photos of faces containing a percentage of their own facial features – a 2013 study explored this, and found that 22 percent of one’s own face was the ideal level. Importantly, this was again an unconscious preference – when asked, the participants weren’t aware that their own faces had been used to create the composite images.

Siblings or dating?

Whether it’s a couple about to celebrate their golden wedding anniversary or two people in the very first flush of romance, it can be extraordinarily difficult to answer the question, “Siblings or dating?” There’s a reason why videos with this hashtag have taken off on TikTok and other social platforms. 

Fortunately, when you’re meeting people in real life as opposed to through a screen there should hopefully be some behavioral cues you can use to inform your judgement. If those two people you thought were definitely twins start making out, perhaps it’s time for a rethink.

[H/T: Particle]