Have you ever seen two people walking around, out and about, and thought, ‘Isn’t it nice to see siblings with such a lovely bond?’ Until they start kissing, and you realise they aren’t actually related at all. This incestual guessing game has become its own genre of content online. TikTok is full of doppelgänger couples, like lesbian couple Carley and Mercedes, who took a DNA test to prove they weren’t related, while relationships go viral for proving “Couple Theory” (a TikTok version of our attraction to likeness).

Instagram account Siblings or Dating?, which launched in 2020 and has over one million followers, is filled with comments calling for couples to get DNA tests ‘stat’. “They met on Ancestry,” jokes one commenter about a particularly uncanny couple. The account started life as a Tumblr page in 2010, while another Tumblr account from 2014, Boyfriend Twin, features gay couples who resemble each other and asks the question, “What’s sexier than dating yourself?”

Helena Whittingham, a talent agent in her 30s, says people constantly mistake her and her partner for siblings – it’s even the reason they met in the first place. “When we met, it felt eerie but comforting, like being recognised rather than discovering something new.” They now share clothes, aesthetics and even beauty habits too. “We live together, so we have the same hair routine. Sometimes I imagine his hair could grow into mine and we’d just conjoin into one continuous mass.” Whittingham admits there’s “something slightly unsettling about it”, but says she secretly enjoys that.

Mo and Tuuli, a couple in their 20s, have received similar reactions to their relationship. “A man at Glastonbury was frightened by our resemblance to the Cullens, mistaking us for goth siblings, and bought us free drinks,” Tuuli says, with a laugh.

These couples aren’t just outliers; humans often mate with people who resemble them. “People tend to partner with those who are similar to them, a pattern sociologists describe as assortative mating,” explains Dr Jenny Van Hooff, a sociologist at Manchester Metropolitan University. “In contemporary dating cultures, similarity often functions as a shortcut for compatibility.”

But it’s not just that people initially gravitate towards a partner that resembles them. Research has found that even couples who initially bore little resemblance when first married will show increased facial resemblance over the years, particularly those who are happy in the relationship. The study was done in 1987 by Robert Zajonc, a psychologist at the University of Michigan, who suggested the increase in facial similarity resulted from decades of shared emotions, shared environments and behavioural mimicry. 

Sometimes the merging is more purposeful. Artists Genesis P-Orridge and Lady Jaye worked on their Pandrogyne Project for almost two decades, transforming their bodies via $200,000 worth of plastic surgery (including matching breast implants on Valentine’s Day 2003), hormones and styling into one singular creation. Their project framed romantic merging both as devotion and experiment, asking what love looks like when identity itself becomes porous.

In less extreme, but similar processes, couples are dying their hair matching colours, swapping wardrobes and syncing aesthetics to become a single visual unit. On social media, these synchronised fits, shared skincare routines and “get ready with us” videos can be seen as proof of romantic compatibility. ‘They work so well visually’ becomes shorthand for ‘they’re a good match’. “Visual similarity is often read as evidence of cohesion. Culturally, sameness gets interpreted as harmony, even though it tells us very little about how a relationship actually works,” says Dr Van Hooff. “In image-led and algorithmic spaces, resemblance becomes an easily recognisable sign of intimacy.”

In queer culture, this urge to merge is practically a trope. From U-Haul jokes to butch-femme mimicry to TikTok’s lesbian twins aesthetic, lovers folding into one another has long been part of the mythology. Tye, a writer in her 20s, remembers her ex shifting from high-femme long hair to a bleached pixie, the same cut Tye had at the time. “I ended up resenting her for stealing my swag,” she says. For some, mirroring can feel like flattery, until it starts to feel like theft. Olive, an artist in her late 20s, tells me she’s changed with several partners: “I don’t think they took anything from me. But sometimes I wonder if I noticed what I was giving up.” 

A lot of us slide into this without realising. Love quietly expands the borders of the self, stretching our habits towards another person. The more intimate we become, the more our sense of who we are begins to include who they are, from sharing the same beauty routine, haircut and brow shape to copying each other’s aesthetics, quirks and even the way we hold ourselves in a room. Desire doesn’t just want closeness; it wants alignment and a way of seeing and being seen that feels mutually recognisable.

Aside from a healthy dose of mirroring, an unavoidable part of being in love, merging has a shadow side. Liv, a writer in her 20s, recalls an ex who made her his “project”. “I was the Bianca to his Ye,” she says. “He made me throw out my neon thongs and my mini skirts. At 21, he had me dressing like him, a bitter Swedish architect.” And even when a relationship ends, that mirroring doesn’t always stop. For Paris-based artist Lisa Brok, her ex absorbed her aesthetic and reproduced it onto the next girl: “Same hair. Same side fringe. Same styling. Same shots”. In these cases, mirroring stops being intimacy and becomes a form of control, exposing the fine line between fusion and erasure.

“Mirroring is a normal part of intimacy, but it becomes concerning when it is no longer mutual or freely chosen. When sameness is expected, monitored, or performed for an audience, it can limit individuality and mask power imbalances within relationships,” warns Dr Van Hooff. Maybe the real story isn’t whether we become each other, but how willingly, how gently, and at what cost. In the end, for better and for worse, love doesn’t just change who we are; it leaves traces of everyone we have ever loved.