It's one of the biggest questions in romance. What is it that makes two people work well together? What determines whether or not a date will end in love, apathy, or even dislike? And how can we predict that in advance?

It's often said that you're a combination of the five people with whom you spend the most time. There's some truth in it: people that spend a lot of time together do tend to become similar, both in language and behaviour (ever catch yourself picking up a phrase or gesture from a friend?) and in tastes and outlook. The effect is strongest in well-established couples: if your parents are still together, they're probably a good example. The longer a couple has been around, the more similar to each other they're likely to be.

This is where it's very easy to mix up cause and effect, and think: well, if all the most enduring couples we see are made up of two people with a lot in common, it must be that having lots in common makes people compatible!

In fact, similarity is often the result of a long-term relationship between two compatible people; not the cause. The pair could have been very different before spending all those years together. There is rarely a reliable way to tell. And that's to say nothing of couples we can all think of whose differences persist over time: perhaps one is very sociable, the other very introverted, and they work well together because each has a moderating effect on the other.

Fortunately, however, we're not the first to wonder about what makes two people work well together. Whilst most literature on compatibility focuses on existing couples, there's a wealth of research on 'couple formation' (read: dating), and below we'll discuss some of the more interesting bits.

It's not in your DNA

To start with, it's worth saying what doesn't guarantee compatibility, and one of the biggest red herrings here is a fairly recent fad. Genetic matchmaking, in which you're paired with someone else based on parts of your genome, is increasingly popular. But it's probably not a predictor of long-term happiness.

That's not to say there are no benefits to coupling up with someone whose genome (and specifically immune system) is very different from yours – there are several, particularly if you plan to have children – but a small increase in biological suitability does not a happy marriage make. Unfortunately, it doesn't even guarantee you'll like each other.

All in the mind

A much better predictor of compatibility is personality. We've known for a long time that certain personality traits correlate with relationship satisfaction: if you're generally positive, outgoing, conscientious, and easy to get on with, you're likely to have a better relationship and happier partner than if you're anxious, shy, scatterbrained, and prickly. Your personality and your partner's personality shape how the two of you respond to challenges and changes in circumstance. Over the course of your lifetime, those will probably include births, deaths, and marriage(s). On a date, we generally hope there's nothing more serious than running late or being served the wrong meal – but those are nonetheless likely to be illuminating. Can your date handle other people's mistakes with grace, or are they easily aggravated? Do they see the funny side of things, or does stress make them self-conscious? The answer to both of these questions can help indicate what kind of partner they'll be.

More broadly, people with similar personalities are more compatible than those with dissimilar personalities. Even having similar flaws can be a good thing, because it facilitates greater empathy. Perhaps most interestingly of all, just believing that you're similar to a potential partner makes you more inclined to like them. Back in the days when OkCupid research was still Googleable, the business ran an experiment on its users to see what would happen if they misled people about how compatible they were with each other.

The results were fascinating: telling 'incompatible' people that they were a good fit made them behave as if it were true. In fact, telling people they were well-matched was even more effective than matching up people who really were compatible (but who were told otherwise). It's a little like the placebo effect for dating: optimism leads to good outcomes, and pessimism to bad ones.

Opposites (sometimes) attract

But not all compatibility is about similarity. Above, we mentioned couples whose members are very different from each other, but who still seem to work well together. This is the opposite of the 'similarity' match type we've discussed so far. It's sometimes referred to as a complementary match: a partnership in which each person's traits complement the others.

As a general rule, matches between people who are very different from each other are unlikely to work. In the general population, it's rare to see, say, an 80-year-old dating a 20-year-old. It's rare for equally devout people of different religious faiths to pair up long-term; it's rare for people from working class backgrounds to end up with people from extremely privileged backgrounds.

The differences we perceive in couples around us tend to be far smaller, and related to personality or outlook rather than demographic factors. In fact, outlook and attitude are two of the few areas in which couples can differ without affecting their relationship satisfaction. This is probably because attitudes are easily identified during dating, so partners can factor each other's attitudes into their decision to continue with the relationship. Personality traits can be more elusive, particularly early on in relationships (is she naturally quiet, or just bored? Is he really enthusiastic or just trying to impress you?). But overall, couples with similar personalities are happier.

Many people in relationships consider themselves very different to their partners, but if the relationship has lasted, it's likely that these differences are smaller than imagined. Some confounding variables do exist, such as the fact that couples are happier when one person is more outgoing than the other. But we'll save that for another blog...

Save me from what I want

One particularly tricky component of compatibility is our own preferences, i.e. what we want in a partner. And teasing out the difference between what we think we want and what we actually do want is no mean feat. We know, for example, that people generally report wanting partners quite different from them – yet in reality, they select for very similar matches. Theories from popular culture can easily affect our surface-level beliefs about love without having a meaningful impact on our deeper, more ingrained preferences. Often, those only emerge in our behaviour.

We're particularly bad at choosing our own partners online. Dating apps encourage us to view prospective partners as lists of traits and attributes, rather than complete human beings, and it's easy to end up making 'shopping lists' of what we want instead of seeing people as individuals.

This means we end up matching with partners who tick a lot of boxes on paper, but who underwhelm us in person or fail to hold our interest over time.

There's also the fact that sometimes, human as we are, we choose partners whose initial appeal (charm, charisma, physical attractiveness) can distract us from an absence of more enduring, important attributes like empathy, altruism, and emotional stability.

But none of that is to say our preferences aren't important in romantic matching. There are several ways to draw a complete picture of someone's ideal partner. And what each of us looks for in a partner reveals things about us, too: desired partner traits often correlate with personality traits in ourselves. People who want a creative partner are less likely to be conscientious; people who want a very attractive partner are more likely to be extroverted. Many of these connections exist.

Confidence is key

Despite not always being great at picking our own partners, one thing we are good at is working out our own level of eligibility as a partner. Put simply, self-esteem tends to be a good predictor of how attractive we are. There are lots of hypotheses as to why this might be the case; most of are them based on the idea that self-esteem in healthy humans is a measure of relationship success (both romantic and otherwise), and of positive feedback from others.

This is important because people who consider themselves about as eligible as each other tend to be more compatible than those whose self-ratings are very different. A couple made up of two people who think they're about average is likely to be more enduring than a couple in which one person thinks they're in the top 10% of people, and one person thinks they're in the top 20%.

Of course, how appealing we consider ourselves is made up of factors that differ between individuals; most notably, for women self-esteem is influenced more by their physical appearance than for men. There's also some evidence that men have higher self-esteem (and so are likely to rate themselves as more eligible) than women in general. This is somewhat counter-intuitive when it comes to romance, given that men outnumber women more than 8 to 1 on most dating apps, and eligibility is largely determined by mate value. It's also not an effect we've seen at Swan (whose gender ratio approaches 50/50): female users typically rate themselves as very slightly more eligible than male users.

But however eligibility scores vary between groups, they have an important bearing on compatibility. People tend to seek out others with similar traits and in similar social positions to themselves, whether those positions are good or bad. That means self-rated eligibility – when taken into account alongside the other factors identified in this article – is a useful predictor of whether or not there'll be sparks.

The final word

Compatibility is so nuanced, and can consist of so many different variables, that it's almost impossible to distil to a simple verbal formula. Our Character Assessment has 150 questions, and we collect feedback from our members over time to get an even richer and more complete sense of their personalities. We even ask people to assess their dates' characters, and use the resulting averages as a kind of third-party verification of personality traits.

Nevertheless, we've put together our best effort. Here is the recipe for romantic compatibility:

1. You should be demographically similar (e.g. of a broadly similar age, education level, social background)

2. You should be similar in personality (but not identical)

3. You should consider yourselves about as eligible as each other

4. You should believe that you're compatible

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It goes without saying that other factors help. Being rich or successful enough to weather money problems is likely to increase your chances of long-term partnership, as is being emotionally stable and having a very supportive family or friendship network – although remember that facing some degree of social opposition can bind a couple together. There is an argument that meeting online indicates that statistically speaking, you're not a good match, so try to avoid that. Genetic compatibility is also not completely made up, so make sure to give your partner a good sniff. And don't forget to up-weight your self-esteem slightly if you're a woman, but only if you're not part of Swan's audience.

Or, of course, you could click here.