Your friends know more about your life than you do, including when you might kick the bucket.

There’s a troubling truth about self-knowledge and its limits: alarmingly often, when you’re faced with a big decision, you’re better off trusting the judgment of a friend – or even a stranger – than your own. To put it bluntly, you may be especially badly suited to figuring out whom you should marry, where you should live or what job you should do – precisely because you’re you.

For instance, according to a new study entitled Your Friends Know When You’re Going To Die, published in Psychological Science, if you want to predict the date of your death, you could do a lot worse than ask your friends. I don’t mean you should literally ask them to name a date. (That could cause some awkwardness at the restaurant table, I imagine.) But the things they know about you could still be the basis of the best estimate available.

The reasoning behind this goes as follows: we’ve long known that certain personality traits are correlated with a long life. Conscientious people, for example, are likely to eat a healthier diet, exercise more, and avoid stupidly dangerous risks like drunk-driving. And your friends, it turns out, are often better judges of how far you possess those traits than you are. Which, as Nathan Collins explains at Pacific Standard, makes their judgments of your character a better predictor of your lifespan than your own.

That’s partly a matter of numbers: as in all sorts of other contexts, you’re better off asking the views of several people and then averaging the results, rather than relying on just one. (The researchers concluded that this was indeed the main explanation for their results.)

But these findings are far from the only indication of how little we really know about ourselves or what’s best for us.

One vivid experimental example involved speed dating. A group of female students was asked to predict how much they’d enjoy a five-minute one-on-one meeting with a man they’d never met. Some women received basic information about their date in advance: his age and what he looked like, his hometown, his favourite books and movies, and so on. Other women just received a number: a rating from one to 100 that some other woman – a stranger – had given the man when she’d met him. Everyone assumes that biographical information is the best predictor of a possible match, especially if they’ve done a lot of internet dating: that way, if his favorite film was Hot Tub Time Machine, while yours was Koyaanisqatsi, you’d be in a position to reach … certain conclusions. Most women involved in the experiment thought that biographical information would be the most useful, too. But the rating from another woman proved a vastly better predictor of how the date actually went. The best way to know if you’ll like a person, place or experience may not actually be to gather all the detail you can, then compare it against all your hard-won experience – but instead to find someone broadly similar to you and ask if he or she enjoyed it.

Not that you’re seriously about to start making life’s biggest decisions by asking random friends or strangers. So the next best thing, strange as it sounds, may be to try to think about yourself in the third person, as if assessing your options from the outside. Instead of “Is this the sort of job I’d enjoy?” or “Is this a house I’d like to live in?”, ask yourself “Would [your name] find this fulfilling?” Just don’t do it out loud, or eyebrows may be raised.

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Or ask yourself what your friends would advise you to do. After all, they know you better than you know yourself – and, by the same logic, you know them better than they do. And you’re all a lot more average than you probably like to think.

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