One reason why we are so often puzzled by human behaviour, including our own, is that most of us have grown up in the grip of the Western myth that humans are essentially rational creatures. Yet our own folklore has got it right: we humans are by nature more emotional than rational; ruled more by the heart than the head.

That’s why you sometimes hear the irritated complaint: ‘Why are you being so rational about this?’, as if to say ‘being so rational isn’t normal’. Of course, you might also hear the equally irritated question: ‘Why can’t you be more rational?’ To which the correct reply is: ‘Because I’m human.’

Consider the evidence. From falling in and out of love to waging war; from our superstitions to our greed; from our unbridled ambition to our capacity for violence; from our anger to our pride; from our fragile egos to the inconsistency of our attitudes and the baseless passion of many of our arguments; from the irrationality of our beliefs to the absurdity of many of our most powerful convictions – that one football team is morally superior to all others, for instance, or that one brand is preferable to an apparently identical brand in the same product category, or that a particular politician can ‘save’ us – does any of this strike you as the behaviour of a rational species?

In fact, the more you examine us, posturing and swaggering around the planet, destroying natural resources and creating all kinds of chaos, the more you are driven to the conclusion that what is remarkable about our species is that we are capableof rational thought and behaviour.

It takes years of education, training, discipline and restraint, but we can do it! Motor mechanics can identify the source of the problem in your car’s transmission and repair it with consummate skill. Doctors can diagnose and treat their patients with evidence-based accuracy. Airline pilots can land their planes right on the spot. Carpenters can put up a frame that’s stable and square. Students can learn how to study for exams and pass them. People can get to work on time and perform all sorts of tasks that require careful, logical thought.

But when those mechanics, doctors, carpenters and students go home from their rational day’s work, we know they’ll be as irrational as the rest of us: irritable when hungry, impatient with exuberant children (who will themselves be confused by their discovery that the very same behaviour can sometimes be regarded by their parents as charming and sometimes as naughty), neglectful of phone calls to elderly parents, angry with the wi-fi and forgetful of garbage night.

What a big mistake we make – and how insensitive we are – when we try to explain each other’s behaviour as ifwe are rational beings who’ve merely lapsed. If your car keys are where you are supposed to have left them, be amazed by that; don’t be amazed when they’re not.

Some of our biggest, most significant and apparently rational decisions – what job to do, whether to have children, where to live – often turn out to be more like accidents than the result of careful consideration. It’s quite hard to find people – even highly successful people – who are doing a job they had always intended to do. Many people describe their career paths as unplanned: ‘I stumbled into journalism by accident’; ‘My mate was doing dentistry, so I did it, too’; ‘It was a matter of what was available at the time.’

In his book, Thinking, Fast and Slow, the Nobel Prize-winning psychologist Daniel Kahneman reminded us that we are very good at misinterpreting or misremembering the past, misunderstanding what is actually happening in the present, overestimating our own skill and underestimating the influence of chance on the trajectory of our lives. That’s not a bad summary of what it means to be irrational, is it?

Next time you’re trying to understand someone’s behaviour – including your own – don’t begin with the rational model; begin with this question: What motives, what desires, lie behind this behaviour?