It isn’t a pleasant emotion, but it’s one that comes naturally to many of us. We feel jealous when we feel threatened: specifically, when something we want seems like it might be taken away. This is slightly different to envy, which is how we feel when someone else has something we want for ourselves. But both can eat away at our happiness and our relationships, so we’ve set out to learn more about them.

Even though we typically date people who are pretty similar to ourselves, very few couples have exactly the same personality, hobbies, interests, and (importantly) level of appeal to the opposite sex. When asked, most people agree that they’re either more or less appealing than their love interests.

This is where things get interesting, because our behaviour changes radically depending on how we estimate our own value in a relationship.

If we think we’re more of a catch than our partner, we’re considerably less likely to worry about things like the risk of infidelity. We’re also less likely to forgive our partners if they do betray us – perhaps because we know we have other options available, some of whom will treat us with more care.

If, on the other hand, we think our partner is more attractive than we are, our perception changes. We are suddenly susceptible to jealousy, and we become more likely to suspect that our partner is being (or will become) unfaithful in some way – whether or not they really are. This is doubly true if we’re emotionally, physically, or financially dependent on our partner, and need their reassurance to feel good about ourselves.

But in this position, we're also more likely to forgive our partners for infidelity. The reasoning is something like this: we might not be the most attractive person in the relationship, but we can offer things that others can’t, including tolerance of our partner’s misdeeds (imagined or real). Perhaps they can find someone more interesting, more intelligent, or better-looking, but they can’t find someone as patient or forgiving as us! And so insecurity in a relationship is associated with lower levels of trust, but higher levels of forgiveness.

There are also sex differences at play here. When they suspect that their partner is unfaithful in some way, men are likely to search through their partner’s belongings, or inspect their clothing for evidence. But women are more likely to internalise their jealousy and cry when alone or, in some cases, try to win back their partner’s love by making a big effort to be attractive. They’re also more likely to pretend that they’re indifferent to the wound, whereas men are more likely to display anger or violence.

In fact, when it comes to partner violence, jealousy is the single largest motivator by some way.

“In every study in which a respondent has had a chance to check or list jealousy as a cause, it is the most frequently mentioned reason.”

We know now that men and women behave differently when they experience jealousy. But the reasons for this are a subject of quite some controversy within the behavioural science community: while some argue fiercely that this is a result of fundamentally different feelings, and the byproduct of millions of years of evolution, others argue with equal determination that the proposed evolutionary explanations are confused, and that in fact men and women feel pretty much the same way when confronted with infidelity.

Here’s the evolutionary argument: for most of human history, men competed over the best women to mate with. But because conception is hidden in humans, men had no way of knowing for sure that their sperm was the one that had produced any given child. This meant that it was a competitive advantage for them to display jealous behaviour (including violence) in order to dissuade their female partners from mating with any other prospects, and vice versa.

This explains why men are more likely to be jealous when they perceive sexual infidelity, whereas women are more likely to be jealous when they perceive emotional betrayals. In fact, female sexual infidelity is the primary cause of divorce, despite the fact that men are more likely to be unfaithful. Men also act violently out of sexual jealousy far more than women, particularly when their partners are young – although it’s worth noting that men are just more likely to be violent in general.

The conflicting argument goes like this. The evidence that male and female reactions differ with regard to sexual vs emotional infidelity is flimsy, and a few studies have shown that it’s flawed: when women are under pressure (being forced to think hard about other things at the same time), they say – as men do – that physical cheating would be worse than emotional cheating. We also know that when it comes to romance, men are considerably more romantic than women: they’re more likely to believe in “the one,” whereas women tend to be practical and realistic. This isn’t the outcome you’d expect if men were motivated exclusively by the need for sex and relational power.

Our own verdict is that more evidence is needed, on both sides. But in the meantime, if you're feeling jealousy about your partner, the old advice is the best: work on your self-confidence. If it turns out that your suspicions of their behaviour were warranted, you'll be in a better place to break free from the relationship. If they weren't, better self-confidence will help you become able to trust your partner as they deserve and begin to rebuild your relationship.

Finally, we recommend being honest: evidence shows that whilst negative expressions of romantic jealousy (such as aggression or refusal to trust your partner) do considerable harm to relationships, positive expressions of jealousy (such as open dialogue about your feelings) can make relationships stronger than they would have been without it.