"What does exclusivity mean to you?" Asks Amy Hart, a contestant on UK reality TV show Love Island in 2019. Her partner, Curtis Pritchard, is cornered and she knows it. He had been kissing other girls behind her back. Pritchard shrinks into his seat as Hart eloquently and calmly lists the issues with their relationship, starting with how he could possibly have romantic feelings for two people at the same time, how she needed him, and how he had let her down.

"If we think about all the people in relationships, about 5% would define as CNM," says Amy Muise, assistant professor of psychology at York University in Toronto, Canada. But including those who have tried CNM boosts the figure. "In lifetime experience, 21% of people have been non-monogamous at some point."

To put that in perspective, 21% is slightly less than the number of US households who speak a language other than English at home (21.9%). "I wouldn't be surprised if it was more common," says Amy Moors, assistant professor of psychology at Chapman University, California. "Something called social desirability explains why people give slightly conservative answers to questions.

We know from archaeological evidence that ancient humans lived in small, close extended family groups. Computer modelling of hunter-gatherer societies suggests that they needed to mate with individuals outside of their local group in order to maintain the population as a whole. There would have therefore been a large flow of mating individuals between hunter-gatherer societies. Maintaining a family whose exact genetic lineage was known would have been impossible.

Some studies, however, are starting to fill that gap. For one, CNM-curious people and people who had never considered being open were recruited for a series of questionnaires about their relationship and sexual satisfaction. In the beginning, none of them had approached their partner to discuss the idea of opening up to other people.

Joel suggests that the uplift in satisfaction among people who switched to CNM might have been the result of a dragging effect. A better quality of sex life with a secondary partner drags up satisfaction with the primary partner, because suddenly the pressure of one person having to provide all of their enjoyment is removed.

How do you deal with jealousy?

This describes a psychological concept called compersion – being able to experience pleasure by seeing another's pleasure. It might be more familiar to you outside the realms of romantic relationships. Think, for example, of watching someone open a gift. But compersion has also been applied to seeing someone else sexually gratified.

Chris Fraley from the University of Illinois has been collecting attachment data from respondents to an online questionnaire for two decades. In total, about 200,000 people have taken this test, and many other researchers rely on this wealth of data to establish norms for all sorts of behaviours.

That said, there is very little in the way of a profile that you can build about CNM people, according to Moors. She says that there is no correlation between age, income, location, education, race, ethnicity, religion or political affiliation and CNM in her research. People who identify as lesbian, gay or bisexual are more likely to be CNM, but that is the only pattern.

In one example, Joel and colleagues asked people to complete a questionnaire about themselves and what they were looking for in a partner. Some of the questions were very similar to what you might expect on any dating website, and many more went way beyond.

Second is partner desire, or, how much did people like you compared to their other dates. The reverse of actor desire, this is a measure of average attractiveness.

Now, depending on your preferences, you can imagine your perfect partner is standing somewhere near the bathroom sink, for example. There might be other people nearby, who would be nearly as attractive. There might be someone even funnier and more beautiful than them, but a little less kind, stood in another room downstairs.

Conroy-Beam's algorithm assumes that all preferences are weighted evenly, which might not be the case.

"We wanted them to have some buy in first before we told them about the deal-breakers," says Joel, "because often deal-breakers show up on the first date or the second or the fifth." You might not find out that someone is a smoker, or that they have another horrible quality, until you meet in person, or even several dates down the line.

Why might we not strictly observe our deal-breakers? Joel has her own theory: "I think that people just aren't actually very choosy. People feel like they need to be choosy because that is our culture. But realistically people are pretty open to a broad range of partners."

The data also suggests that being very, very attractive as a man offers no advantages over being fairly average.

"If [online dating sites] are going to match you with someone long term, that requires a lot of long-term data. This claim is exciting to me but to properly test it we would need to follow people for years," says Joel. "Another possible reason that we might not have found something is that people don't know what they want. I might not have a lot of insight into what I find attractive and what I am actually like."

For Lloyd, the data collected from eHarmony's users suggests that openness is a really important trait for long-term success. "The more genuine you are and confident you are, the better you tend to do," says Lloyd. "That approach to dating really works. Online dating has given us so many benefits. But it has also created a sense that we are all superficial and shallow.