The science of soulmates: Is there someone out there exactly right for you?

On Valentine's Day, there's the temptation to believe that somewhere out there is "The One": a soulmate, a perfect match, the person you were meant to be with.

In the Middle Ages, troubadours and Arthurian tales recast that longing as "courtly love", a fierce, often forbidden devotion like Lancelot's for Guinevere, in which a knight proved his worth through self-sacrifice for a beloved he might never openly declare.

By the Renaissance, writers such as Shakespeare were talking of "star-crossed lovers", couples bound together by an overwhelming connection yet pulled apart by family, fortune or fate, as if the universe itself both wrote their love story and barred them from a happy ending.

Viren Swami, Professor of Social Psychology at Anglia Ruskin University (ARU), in Cambridge, has traced our contemporary European understanding of romantic love back to medieval Europe and those stories of Camelot, Lancelot, Guinevere and the chivalry of the knights of the round table that swept across the continent.

"These stories first pushed the idea that you should choose one other individual as your companion and that companion is for life," he says. "Before that, in much of Europe, you could love as many people as you like, and love was fluid, and it was often not about sex."

Over time, as people are uprooted from their agricultural communities as industrialisation tears apart people's familiar attachments, individuals become "alienated", he says. "They start looking for one other individual to save them, to save them from the wretchedness of their lives."

Today's dating apps turn that story into an algorithm, which Swami calls "relation-shopping". The search for a soulmate turns into the opposite of what they are looking for: "For many people, that's a really soulless experience.

"You're shopping for a partner… going through possibly dozens of people on the dating app until you get to a point where you go… I need to stop," he says. "We are attachment-based creatures," he says. "We desire that bond."

But in his lectures, he tells students they need to leave the idea of a soulmate, without giving up their desire for The One. It sounds like a contradiction, but for Carroll, it's the difference between destiny and graft. "A soulmate is just simply found. It's already pre‑made.

But a one and only is something two people carve out together over years of adapting, apologising, and occasionally gritting their teeth," he says.

Carroll's argument draws on decades of research, which he put together in his report, The Soulmate Trap, much of which distinguishes between what psychologists call "destiny beliefs" - the idea that the right relationship should feel effortless - and "growth beliefs", which focus on what partners can do to make things work.

In a widely cited series of studies in the late 1990s and early 2000s led by Professor C. Raymond Knee at the University of Houston, researchers found that people who believed relationships were "meant to be" were far more likely to doubt their commitment after conflict.

Those with more growth-minded views tended to stay more committed, even on days when they argued. Those with growth-based views, Carroll argues, still want something special, but expect rough patches. "They ask… what can they do to make their relationship better, have improvement and have growth?"

In his view, the soulmate belief is a trap - not the romance itself, but the expectation that love should never be hard. The most "soulful" part of a long relationship, he says, is not a cinematic charge, but having "front-row seats not only for each other's strengths, but... [their] challenges and weaknesses".

For Carroll, when love is treated as fate, people become less willing to do the unshowy work that actually keeps love alive. Carroll says the soulmate trap makes it much harder when a relationship hits its first serious snag.

"The first time there's any type of struggle, the immediate thought is, 'well, I thought you were my soulmate. But maybe you're not, because soulmates aren't supposed to deal with things'," he says. "But if relationships are going to go long term, it's never just going to be a downhill run."

Vicki Pavitt, a London-based love coach, often helps people who thought they'd found their soulmate, only to discover that the fairy tale came with emotional manipulation, flakiness, and a constant sense of anxiety.

"When there is a lot of chemistry and the spark, I think that can sometimes be about opening old unhealthy patterns, like old wounds", she says.

"A person who is inconsistent or plays a bit hot and cold can make you feel 'I can't wait to see them again', but what's really happening is they're giving you so much anxiety and that it has you wanting more".

This bond can seem like love, she says, and leads to people magnetically drawn into unhealthy dynamics because they are familiar, not because they are the perfect match. One study often cited is by Canadian psychologists Donald Dutton and Susan Painter.

In research published in 1993 while they were at the University of British Columbia, they followed 75 women after they had left abusive partners. The team measured how strongly the women still felt attached to their exes and compared this with what their relationships had been like.

They found the strongest bonds were not in women who had consistently been abused, but in those whose partners alternated between charm and cruelty.

Dutton and Painter argue that this trauma bond helps explain why people can feel magnetised back to relationships that are objectively bad for them – because the mix of danger and affection is familiar, not because it is healthy.

It is that distinction Pavitt tries to surface in coaching: "It's about discerning whether the chemistry you feel is showing me this person's compatible with me or if it is a familiar sense of anxiety. "In my language, I never talk about soulmates," she says.

"I don't personally believe that there is one person for everybody... but I do believe that we become "The One" for someone." If ruling out the existence of a soulmate sounds unromantic, the biology of attraction points in the same direction.

Hormonal contraceptives may subtly reshape how partners feel about each other. Research suggests that pills which flatten the natural ebb and flow of fertility can dampen shifts in attraction that typically occur across the menstrual cycle, potentially altering initial mate choice.

One large study of 365 hetrosexual couples found that women's sexual satisfaction was higher when their current contraceptive status matched what it was when they first chose their partner, hinting that changes in pill use can change how a partner is experienced.

These effects are small but could help explain some couples' puzzling shifts in chemistry over time. If hormones and pills can tilt who feels like "The One", then it becomes harder to argue there is a single, pre‑ordained match – which is where the mathematicians come in.

Psychology and biology offer one way of thinking about "The One", but mathematics puts forward another. Dr Greg Leo, an economist at Vanderbilt University in Nashville, Tennessee, has come up with a compatibility algorithm. It finds that not only might you have a "One" you have lots of "Ones".

In his Matching Soulmates paper in the journal of Public Economic Theory, everyone is in a computer simulated dating pool, where thousands of digitally created daters rank each other. His algorithm picks "first‑order soulmates": pairs who choose each other in a stable matching.

It removes them, and runs it again with those left, and you get second‑order soulmates, and so on. In his simulations, it was extremely rare for someone to have their mutual first picks; but many people had those that were second or third picks.

In this scenario a couple counts as happy if each is near the top of the other's list and neither can find someone they and that other person would both