The Science of Why “Siblings or Dating” is so Hard

A phenomenon called spousal concordance explains why you suck at this popular TikTok challenge
The Science of Why “Siblings or Dating” is so Hard
Image credit: Pavel Danilya on Pexels
Credit: Novakboii on TikTok

You squint at your TikTok feed. Surely that blonde-haired duo with identical noses are siblings … but that’s a pretty intimate hand on the lower back … so dating? 

We understand that genetics play a role in why siblings look alike, but why do couples sometimes resemble each other? This phenomenon has long intrigued scientists and romantics alike. The term used to describe the resemblance is spousal concordance. 

Although science may not be able to explain why that brother and sister are holding hands, spousal concordance helps us explain the mechanisms behind couples’ shared facial features and matching expressions. 


Scientists believe that the biological reason couples resemble each other is due to a phenomenon known as assortative mating. This is when people select partners who look similar to them.

Couples who share similar facial features have been theorised to be drawn to each other due to subconscious cues of genetic compatibility.

Even if a couple doesn’t start off looking alike, their appearances can converge as time goes on. Over the span of a couple’s relationship, they are exposed to similar environmental stressors, lifestyle factors and shared experiences.

This includes things like drinking and eating habits, smoking and leisure pursuits. These can all create similar marks and wrinkles on the couple’s faces, further emphasising their resemblance.


Aside from physical similarities, the dynamics of a relationship can cause the people in it to look more similar to outsiders. Research suggests that couples tend to unconsciously mimic the facial expressions and gestures of their partners.

This behaviour then gradually leads to a common vocabulary of facial expressions and non-verbal communication, creating further similarities that can be perceived by others.


While it might seem weird for couples to look alike, research conducted in 1987 suggests that spousal convergence can indicate how satisfied couples are with their relationship.

Couples who exhibit a greater physical resemblance tended to perceive their relationships as more cohesive and harmonious. In other words, love is apparently sweeter when it’s with someone who looks like your brother.


The phenomenon of spousal concordance shows how intricate and interesting the interactions are between biology, behaviour and the environment within a relationship.

The shared genetics and the mimicry developed throughout the relationship are factors that all contribute to the gradual convergence in the appearance of couples.

This all is to say that, if you have failed the TikTok challenge and are unable to differentiate siblings from a couple, at least there’s scientific reasons why it’s so difficult. 

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