After Elliot broke up with his girlfriend Kelly, they stayed friends. Why wouldn’t they? The breakup had been amicable; after dating for a year and moving in together, they started squabbling over silly things and figured they were better off apart. They were on a night out in April last year with their friendship group when Elliot, a 27-year-old childcare worker from Lancashire, bumped into Tom, an old work colleague. They made small talk for a bit, before going their separate ways. But Tom had caught Kelly’s eye, and the next day she called Elliot with an unusual request: could he set them up on a date?

Elliot wasn’t opposed to the idea, but there was a problem. “I said: ‘I have no idea who you’re on about!’” he remembers. “We’d had such a brief conversation that it didn’t stick in my memory.” A few days later at work, Elliot’s team leader mentioned Tom’s name. “I thought: ‘That’s who she meant!’ I texted her saying: ‘It’s this guy.’” Kelly and Tom started messaging; Elliot even helped her craft the initial, flirtatious texts. It worked: Kelly and Tom now live together.

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There are countless articles online about how to find love after a breakup – or win your ex back, if that’s what you want. Helping an ex-partner find love with someone new is an entirely different and much rarer phenomenon. But it happens. This month, New Zealand couple Damien and Whakarongotai Nikora’s Facebook statuses went viral. They had posted announcing their split, and both had included a personal ad on behalf of the other. The pair, who were married for 21 years, titled their posts: “Why you should date my ex.” “If you get the chance to date her, lucky man,” wrote Damien of Whakarongotai.

Most people – at least those whose relationships were not abusive or vitriolic – want their exes to be happy. But actively working to help your ex find love? It is unusual, particularly in monogamous circles. Some of Elliot’s co-workers struggled to understand his actions. “They said: ‘I’d hate to see my ex getting with someone else.’ But I thought, why? You’re not together any more. Why should that concern you?”

An advantage of setting up your ex is that you know their quirks and foibles better than almost anyone else, and can vet prospective partners for compatibility. “You know your exes, and you know your friends, so you can basically tell what sort of people they will like in relationships and what they do like and don’t like,” says Andrea Campos, 25, a lawyer from London. Two years ago, she was on holiday with a female friend in Dubai when Campos’s ex-boyfriend messaged her. He had seen her friend in the photos Campos had been sharing, and found her attractive.

“I said: ‘Do you want me to play Cupid?” Campos says. He agreed. They dated for a short while, but nothing long-term came of it. She doesn’t view her actions as remarkable. “I’m not a jealous person. If I think people will get on, I’ll introduce them. You want the people you care about to be happy, and if you think they’d be happy together, you might as well help.”

Although it might seem tempting to launch into your address book and begin setting up your former flames and friends, a word of caution: it is not a risk-free exercise. Perhaps you will discover that you are not over them or, if the resulting relationship turns sour, you may get caught up in the fallout. But if you are thinking of setting up your ex with someone new, the first thing to do is check they are happy with it before you do. “It’s healthy to hope an ex-partner goes on to form other healthy relationships,” says Ammanda Major, a therapist with the relationship charity Relate. “But it depends on whether your ex wants you to do that. Otherwise, you’re treating them like a commodity, and selling them on.”

Ask yourself why you’re trying to set up your ex. Are you genuinely motivated by feelings of altruism, or are you doing it to assuage guilt, or to prove a point? “If you ended the relationship and don’t want them to feel bad about it, you might think that fixing them up with someone new will take their mind off it,” Major says. “But it can be disrespectful to what you had, if it can quickly be overcome by being set up with someone new.” Above all, respect a firm no. “We all think we know best and so-and-so might be perfect for them. But doing things without someone’s permission is never a good idea.”

Setting up your ex with someone new is an acid test for whether you are truly over that person. If the thought fills you with horror, you may still have feelings for them. “You have to be honest with yourself,” says Christina Gale, 27, who lives in London and works in social media. “If you’re funny about an ex, you’ve obviously still got feelings. If you’ve moved on, why wouldn’t you want to see them in a new relationship? I’m not a jealous person. I think everyone should be able to find love.”

Setting up her friends with her exes has become Gale’s signature move. “I’ve done this so many times!” she laughs. “I date people and think, actually, you’d be better suited to someone else I know.” One ex sticks out: they had been together for six years, but ended things amicably. He had started hanging out with another woman – as friends – but Gale was convinced it could be something more. “I went to her and said: ‘Do you think he’s good-looking? She said: ‘Yeah!’” Gale let him know – and they started dating.

Not everyone can understand a decision to set up an ex. “I have straight friends that say I’m crazy,” says Leo Camanho, 23, a student from Rio de Janeiro. “They say: ‘You’re so mature; how can you do this?’” He set up his ex-boyfriend Lucas with his best friend, Fabio, after he and Lucas broke up. He thinks setting up your exes is more common in queer circles. “In Rio, if you’re not friends with your ex, you don’t have any exes, because everyone dates everyone here.” After Fabio and Lucas started dating, they came to Camanho for his blessing. “I said: ‘Totally, you are both friends of mine, I love you both, go and be happy.’”

In New York’s queer scene, 28-year-old Remy Duran is a legend. The reality-TV star became known across the city for his little black book – in reality, a mobile phone full of contacts, with subfolders categorising sexual preference – that he uses to connect ex-partners looking for love (or sex). Duran didn’t set out to be New York’s answer to Cilla Black: his matchmaking was an incidental byproduct of his dating life. As he dated his way around New York, his social circle grew, and former partners would hit him up for advice if there was someone they had their eye on.

“People would be like: ‘Do you know that person? I’ve wanted to hook up with them for ever. And I’d be like: ‘Sure, I don’t own them, you can totally see them!’” His matchmaking is primarily altruistic – he wants to ensure people are having a good time. But there is also a deeper motivation. “It feels liberating,” Duran says. “You don’t own anyone, and it gives you a sense of closure.” He has set up at least 20 former partners, ranging from hookups to ex-boyfriends. So far, so good. “They say thank you,” says Duran. “I don’t think I’ve had any complaints.”

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Polyamorous people may be more relaxed than monogamous people about helping their exes date someone new. One polyamorous person, who prefers to remain anonymous, says: “I don’t see it as ‘setting people up’. I mean, they’re poly, they bang whoever; I just introduced them.” But outside of polyamory, setting up your ex with someone new is the ultimate affront to a monogamous dating culture that views ex-partners as possessions, rather than autonomous beings with free will. “We live in a possession-based culture where we see human beings as objects,” Elliot says. “You – you’re mine. Monogamous relationships can be like that. Even after you break up, people can’t let go of that sense of ownership. But I don’t think relationships should be possessive in that way.”

Sometimes, setting up your ex with a friend happens so organically, it feels like an act of God. When Joanna Delooze was 20, she dated a man named Aaron. On one of their dates, Aaron mentioned that he had a list of qualities he looked for in a potential spouse. Delooze, who is now 55, made Aaron show her his list. “I read it and said: ‘I’m not joking, but with the exception of one thing, my roommate Linda ticks every single box on your list.’” She laughs. “It was completely her! If I was a guy, and this was my list, I’d date my roommate.”

Delooze was absolutely right: Linda and Aaron have been together for 35 years and have six children. She is delighted to have helped engineer their happiness. “I see pictures of their family and all their kids and how they’re still together and I think it’s just great.”

Some names have been changed