How would you feel if you suspected your partner had enjoyed a one-night stand while away on holiday without you? What if, instead of having sex on the trip, you believed she or he had fallen in love with someone? In either case, if your partner will probably never see the other person again, would that make the situation any easier to cope with?
Faced with either scenario, most of us would feel intensely jealous: it's a very basic, normal reaction. But does the universality of jealousy indicate that it might be genetically programmed?
The first study to investigate the genetic influence on jealousy was recently published. Researchers put the questions at the top of this blog to more than 3,000 pairs of Swedish twins. Fraternal twins share about 50% of their genes; identical twins share exactly the same genetic make-up. By comparing the answers given by each group of twins, the researchers were able to show that around one third of the differences in levels of jealousy across the population are likely to be genetic in origin.
In both scenarios – fears about a partner sleeping with or falling in love with a stranger – women reported more jealousy than men. But the researchers also found a gender difference between relative reactions to the idea of sexual or emotional betrayal. Men were far more troubled by the thought that a partner had been sexually unfaithful than by potential emotional infidelity. Women tended to respond to each scenario with more equal levels of jealousy.
Why is this? The answer, according to some scientists, may lie in evolutionary pressures. For both men and women, reproduction is key. But men, unlike women, cannot be certain that they are the biological parent of their child, and so they are naturally more perturbed at the thought of sexual infidelity than they are about emotional infidelity – because it jeopardises the successful transmission of their genes. Women, though relatively less perturbed by the idea that their partner may have been sleeping around, are nevertheless dependent on their mate for their survival and that of their offspring.
That's the theory. Given that we can't zip back in a time machine to human prehistory, it's an explanation that seems impossible to prove or disprove.
Though genes appear to play a part in jealousy, the Swedish results also show that the kinds of things that happen to us in our lives – the way we're brought up, the people we're around, the events we experience – are far more important. Only one third of the variation in jealousy seemed to have a genetic origin, so the rest must have been down to environmental differences.
But whether genetic or environmental, hard-wired or learned, there's no doubting the ubiquity of jealousy. It's an emotion that almost everyone feels at some point, and a major cause of relationship problems. Although much of this jealousy is illusory, we all know that the eye (if nothing else) can wander. In Britain, the National Survey of Sexual Attitudes and Lifestyles found that 82% of men and 76% of women reported more than one partner during their lifetime, with more than a third of men and almost a fifth of women clocking up 10 or more. Some 31% of men and 21% of women said they had started a new relationship in the previous year, with 15% of men and 9% of women seeing more than one person at the same time.
Occasionally, then, we have grounds to be worried: jealousy alerts us to a looming problem in our relationship. If your partner has been unfaithful in the past, naturally you'll worry that they might stray again in future. Much of the time, though, jealousy is pointlessly corrosive, making both partners miserable for no good reason. In these cases, how can we get the better of our jealousy? How can the "green-eyed monster" be tamed?
Consider the evidence for your jealousy. What about the evidence that might contradict our fears? What would we tell someone if they came to us with the same worries? Have a chat with a trusted friend to get an independent perspective on how likely it is that your partner is deceiving you.
Talk to your partner. When two people hold differing views of what's acceptable in the relationship – how much time to spend together, how frequently to keep in touch, whether it's okay to stay in contact with ex-partners and so on – misunderstanding and jealousy are always a risk. If you haven't agreed the ground rules for your relationship, make it a priority.
Weigh up the pros and cons. People often believe that their jealousy – for all the pain it brings – actually helps them. So it's a good idea to draw up a list of the pros and cons, both of being jealous and of trusting your partner. On balance, which one seems the best option?
Get to the bottom of your fears. What is it, do you think, that lies at the root of your jealousy? Do you dread being alone? Do you fear humiliation? When you've identified the fears fuelling your jealousy, think constructively about how you'd handle the situation.
Set yourself some ground rules. We can find ourselves trapped in a vicious cycle: jealous behaviour feeds jealous thoughts, which in turn trigger more jealous behaviour. And so on. To break this cycle, it helps to set ourselves some ground rules. When you find yourself worrying about your partner's faithfulness, save those thoughts for a daily "worry period". Set aside 15 minutes each day, and postpone all your worrying until then.
Concentrate on the good stuff. Jealousy skews our perspective. To counter it, we need to make a deliberate effort to view things more positively. That means focusing on the good parts of our relationship: the things about our partner and our life together that we like, the things that keep us coming back for more. Focus on the positive by doing more positive things together. And remember to have your own interests and activities that boost your self-esteem.
Daniel and Jason Freeman are the authors of How to Keep Calm and Carry On: Inspiring Ways to Worry Less and Live a Happier Life. On Twitter they are @ProfDFreeman and @JasonFreeman100 . This blog first appeared in The Guardian: http://www.theguardian.com/science/blog/2013/nov/01/jealousy-genes.