A few weeks back, as you'll probably recall, the media became briefly aroused by a study examining what makes people satisfied, or not, with their sex lives. It isn't just a matter of how much sex you're having, apparently; it's also a question of having more than other people. (Quality wasn't considered.) Predictably, the phrase "keeping up with the Joneses" was deployed, as was that stock photograph of four feet poking out of a bed. So far, so unsurprising. We measure our wealth in comparison to others. Why not sex, too?

You had to read the reports carefully to notice an important caveat: unless you've got very thin walls, or very candid friends, or a periscope and a lot of time on your hands, you don't have a clue how much sex other people are having. As the study's author acknowledged, researchers can only estimate the average frequency of sex in any given group using surveys in which people might exaggerate or lie. And when you conclude that you're having more or less sex than others, those "others" exist, first and foremost, in your head.

These fantasy people play a troublingly central role in our lives. In recent years, we've heard again and again that certain phenomena are "contagious": metaphorically speaking, you can "catch" happiness, or unhealthy eating habits, or certain kinds of opinions, from those around you. But the contagion metaphor distracts from the fact that in many cases it's not other people who infect you but the impression you've formed of what your peer group's like. It's as if you could catch something literally contagious – ebola, say – merely by convincing yourself your friends already had it.

This insight lies at the heart of what's known as "social norms intervention", an ingenious technique for changing behaviour examined in March in a New York Times essay by Tina Rosenberg. The classic examples of its success concern efforts by US universities to tackle binge drinking. Once upon a time, colleges used scare tactics, warning of the hellish consequences of overindulgence. Then they discovered something curious: whether students drank a lot or not, they reliably overestimated – a lot – how much other students consumed.

So the authorities tried a new strategy: ads reminding students that most of their peers, most of the time, drank moderately. "Two-thirds of Northern Illinois University students… drink five or fewer drinks when they 'party'," read one (yes, with fusty quote-marks around "party"). Heavy drinking rates began to fall steadily. Similarly positive results have been obtained with notices explaining that nine out of 10 people pay their taxes on time, or most people reuse their hotel towels.

Perhaps we need a public information campaign to tell us how much sex our friends and neighbours are actually having. In the meantime, the social norms approach serves as a reminder to keep questioning exactly who you're comparing yourself with – and, specifically, whether or not they're real. It's a pop-psychology platitude that you should set your own rules, break free from the herd, and so on. But if that's too tall an order, at least we might try to be influenced by the real herd, not one we've invented that makes us feel worse. You're a grown-up; isn't it a bit alarming to realise how much importance you might still be attaching to your imaginary friends?

oliver.burkeman@theguardian.com

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