Last week, we noticed an interesting habit President Trump seems to display with regularity: forcefully jerking the arms of those he's shaking hands with. In several examples, President Trump is seen shaking someone's hand, then pulling it toward him in a quick motion that sometimes seems to throw the hand-shakee off balance.

It happened with now-Vice President Pence on election night.

It happened with Judge Neil Gorsuch on the night he was nominated to the Supreme Court.

And perhaps most famously, it happened with Japanese Prime Minister Shinzo Abe in the Oval Office.

It's hard to say whether the arm-pull is a conscious decision Trump is making, or just an odd habit he picked up. But it's likely he's very aware of all of the cameras watching him, and wants to appear to give a very firm handshake — after all, we know how much he cares about hands.

Handshakes have an interesting place in society: Everyone knows the basics of shaking hands, and everyone knows how awkward and strange a bad handshake can be. To explore the psychology of handshakes a little more, I reached out to Professor William Chaplin, chair of the Psychology Department at St. John's University. Our conversation below, conducted by telephone, has been edited only for clarity.

FIX: Let's start out with the basics. Most of us shake hands with other people on a regular basis. What's the psychology that goes into that moment? Is it more of a greeting, or a way to size up the people we meet?

Chaplin: So, the handshake is a very standard, very common, almost universal form of greeting, at least in Western cultures. In addition to just being a greeting, though, it does give a person an initial impression of what the other person is like. And we’ve all experienced handshakes that are sort of weak, or limp, and that may lead us to think that this person is not a very strong person, maybe shy, or we have a firm handshake, and that makes us respond more positively to the person. Or we may have a handshake where the person is clearly trying to communicate to us that they’re the dominant one, and I think all of those communications then influence how we interpret the other behaviors we see from that person as we continue to interact with them.

FIX: You've done some pretty specific research into what people perceive as a “good handshake” when they meet someone. What are the factors that we look at, consciously or not, when evaluating someone's handshake? And do good handshakes lead to making good impressions on people?

Chaplin: We found that these characteristics all correlated positively with each other. So somebody who completely grips the hand, which is one component, tends to also grip firmly, in the sense that they’re not just putting their hand there, but actively gripping — we found that those people tended to be more likely to look you in the eye, and that those people also tended to not just put the hand out there, but also actually shake it. And together, we called that our “firm handshake” or our “good handshake composite.” So there really are components that we could identify that led a handshake to make a better impression rather than a worse impression. It was usually all of those together that were important.

FIX: Does the dynamic of a handshake change when cameras are present? For example, celebrities and public figures often have to perform this ritual in front of news cameras and photographers. Do hand-shakers approach those situations differently?

Chaplin: I think there are certainly circumstances in politics, maybe in sales situations, where the person doing the handshake really wants to make a particular impression, and they might try to modify their handshake to do that. I think that situation tells us less about the person, and more about what the situation is that they’re in. And as a result, because they’re rehearsing and thinking about it, it’s not naturally about them. Now, whether people are able to do that, and learn how to do a good handshake, is less clear. It’s not something that we studied.

But it might be very interesting to do a study where, instead of just having people come in and shake hands naturally, we deliberately say, okay, I want you to imagine that you’re about to meet the leader of another country, how would you try to make an impression on that person that you’re somebody they can trust, but also somebody who’s strong, and then see what people did? That might be interesting, and probably people are not always very accurate about thinking about what to do to convey certain impressions.

FIX: Finally, does the way someone shakes hands actually say anything about them as a person?

Chaplin: We certainly found that people who gave a weaker, less firm handshake had ratings that were much worse, and we found that it was correlated with various personality traits like shyness, and introversion and social anxiety. So it’s not just that people are thinking about somebody, it actually seems related to some more negative characteristics that the person has. People with good handshakes tended to be more outgoing, more socially at ease, less socially anxious, not as shy and so on.