When Donald Trump shakes hands he frequently pats the other person on the top of their hand. One couldn’t help noticing this when he met Shinzo Abe, because during the 19 seconds that they were shaking hands, Trump managed to pat the Japanese prime minister’s hand no less than six times. Patting someone’s hand in this way pretends to be an affectionate gesture of approval, but its real purpose is to remind the other person who’s actually in charge. It’s what psychologists call a “status reminder”.

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Another way that Trump reminds people of his superior status is by patting them on the arm or back during or after the handshake, and if the other person is so bold as to pat him back, he trumps them by producing an additional, terminal pat. Trump instinctively understands the rules governing patting – which are that the more important person reserves the right to pat the less important person, and if mutual patting occurs, has the right to execute the final pat.

But the most peculiar thing about Trump’s handshake style is his habit of pushing people away or, more commonly, pulling them towards him during the handshake. The news clips are full of examples of Trump pumping people’s hands and then yanking them towards him. We’ve seen him do this with his colleagues, visiting dignitaries and golfing buddies. There’s even a shot of him yanking Neil Gorsuch’s arm so violently during a handshake that the poor unsuspecting judge momentarily loses his balance.

When Trump greeted Justin Trudeau this week, there was every expectation that he’d try to strong-arm the Canadian prime minister. But Trudeau had been well briefed, because as Trump extended his hand, Trudeau stepped right up to him, grabbing his right arm with free left hand. This, and the fact that Trudeau had managed to invade his personal space, made it virtually impossible for Trump to remind Trudeau who was really in charge by giving his hand a good yank.

So why does Trump feel the need to physically manoeuvre people like this, and what does it say about his character?

There are several possible explanations for Trump’s “yank-shake”. One is that Trump has an irresistible urge to bring people closer to him, to get them on side, and this expresses itself in his habit of pulling people towards him. In support of this explanation it’s worth noting that there are occasions when, having pulled someone towards him, Trump will switch to a more intimate upright handclasp, which of course requires even closer proximity.

Facebook Twitter Pinterest ‘Trump managed to pat Shinzo Abe’s hand no less than six times.’ Photograph: UPI/Barcroft Images

Another explanation for the “yank-shake” is that it enables Trump to impose himself on the situation by catching the other person unawares and doing something that they hadn’t anticipated. After all, when they’re shaking hands with the president, who would expect to have their arm almost wrenched out of its socket? It’s noticeable that Trump’s “yank-shake” has a lot in common with his political pronouncements – they’re erratic, unpredictable and self-serving, and they don’t always conform to the other goals that he’s trying to achieve.

But the real clue is to be found in the natural, underlying logic of greetings. Physical greetings are motivated by two principles: the need to express power relations or the need to express solidarity. All power greetings – whether they involve bowing, curtsying or prostration – are asymmetrical. In other words, the actions of respect performed by one person are not performed by the other. Greetings of solidarity, on the other hand, are defined by their symmetry. In the handshake, for example, the fact that both people perform the same actions makes them equal to each other – the symmetry in their actions reinforces the symmetry in their relationship.

There are of course ways that people can subvert the vital symmetry of the handshake – for example, by applying a vice-like grip, pumping the other person’s hand too vigorously, or rotating the wrist so that your own hand ends up on top and the other person’s underneath. These little attempts at oneupmanship – rather like the minor infractions that we witness on the playing field – are still acceptable, even though they lie outside the letter of the law. They don’t do any serious damage to the essential symmetry of the handshake.

But there are at least two good reasons why we can’t apply the same leniency to the “yank-shake”. First, it involves a really dramatic departure from normal practice and people’s expectations; and second, it doesn’t allow the person whose arm is being yanked to reciprocate and do the same thing to the yanker.

It’s hard to escape the conclusion that Trump’s attachment to it lies in the fact that it enables him to take control and impose himself on the person whose hand he’s shaking. However, by doing so he displays his disregard, possibly even his contempt, for the sacred symmetry of the handshake and the precious opportunity that it offers us to treat each other as equals. There’s no way that Trump can yank people’s hands and still claim to uphold the principles of egalitarianism.