

The romcom movie genre can hardly be described as fundamental to understanding human behaviour. But there is one exception: the Rob Reiner film When Harry Met Sally contains probably the most important scene in film history (in terms of behavioural science). And yes, it is the scene in which Meg Ryan fakes an orgasm in Katz's Deli. But it is the response of the older lady at the next table that matters. When Ryan is finished, this character turns to the waiter and says: "I'll have what she's having."

The phrase gets to the core of the essential human talent for "social learning" -- learning from the example of those around us. We do it earlier than infants of our fellow primates (as young as 42 minutes after birth, according to one study); we do it better and more accurately; and we keep on doing it for fun, long after our primate cousins have given up. This is the missing piece in our understanding of human behaviour: whereas cognitive neuroscientists and behavioural economists have shown the primary role of emotion as opposed to rationality in cognition, the importance of I'll Have What She's Having (IHWSH) as a thinking short cut is clearer than ever. Evolutionary psychologists might post-rationalise modern human quirks in terms of the supposed lifestyle of our ancestors, but IHWSH shows us our lives today.

Take last summer's riots: however much politicians and judges stress the moral or psychological pathologies of individuals, most participants emulated what they saw going on unpunished around them -- helped by media effectively showing rioting as an exciting pastime.

"Culture" markets such as music, books and films are also shaped by social influence (as Duncan Watts of Yahoo! and Matt Salganik of Princeton University have shown). We mostly choose what those around us choose, whatever we tell ourselves. We do the same with the names we give our children, the dog breeds we select, the neighbourhoods we live in. Nick Christakis and James Fowler's study of the longitudinal health study in Framingham, Massachusetts, revealed the primary influence of our nearest and dearest on individuals' weight gain (you're 63 percent more likely to become obese if someone in your circle is). It's the people around us who shape our behaviour.

This simple insight builds on much of what psychologists such as Nick Humphrey and anthropologists such as Robin Dunbar have shown about the social nature of our brains -- they are social-monitoring devices much more than calculators. Think about the "mirror ­neurons" discovered by Giacomo Rizzolatti and team that entangle our individual brains with those of the people around us.

All of which runs contrary to many of the stories we each tell ourselves. We are not the authors of our own lives. We use other people's bodies and brains to navigate through life, to work out what to do, what to choose and where to go next. Now, what is that guy over there drinking?

Mark Earls, with Alex Bentley and Mike O'Brien, is author of ' I'll Have What She's Having -- Mapping Social Behavior' (MIT Press)

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