Photo: Win McNamee/Drew Angerer/Getty Images

This presidential election has been a fascinating, sometimes jarring study in personality contrasts. Hillary Clinton is, whatever else you think about her, someone who appears quite levelheaded in person, and who does not appear particularly prone to flashes of anger or lashing out at her enemies. Donald Trump is, whatever else you think about him, just about the opposite.

Never was this clearer than during the first presidential debate a couple weeks ago. Over and over, Clinton was able to get under Trump’s skin — she’d mention his privileged background or his unreleased taxes or Alicia Machado, and Trump would invariably lose his composure, coming off as somewhat less than presidential, if the subsequent polling about his performance was any indication. This is a pattern for Trump: He has extreme difficulty letting things go, trouble not responding even to minor slights and jibes. He always has to be right.

That’s part of the reason there’s so much anticipation for Sunday night’s second debate, which will be a town-hall format where, rather than being restricted to a podium, both candidates will be walking around, taking questions from individual voters, and so on. Will Trump repeat his performance? Will he attempt to get under Clinton’s skin, as he has hinted, by bringing up her marital travails?

Whatever happens Sunday, there’s an obvious question lurking here: Why does Trump seem to be so much more thin-skinned than Clinton?

If you’ve followed the various journalistic attempts to explain Trump’s unusual behavior through the lens of psychology, you might have a sense of where this is going: Yes, the answer, at least according to the personality psychologist I corresponded with, likely does have to do with Trump’s narcissism. But the full story is a bit more complicated and interesting than that, and touches upon an ongoing debate among personality psychologists.

A necessary caveat: When people say Trump is narcissistic, they’re just saying that, from the distance at which we all observe him, he seems to exhibit many of the characteristics of individuals who are high in this personality trait, or, in the case of observers making the more extreme version of such claims, those who have narcissistic personality disorder, as it’s called in the DSM 5. But no one can be formally “diagnosed” at such a distance as being high in the trait or having the disorder — not without evidence that they’ve been examined by a psychologist. Still: At this point, Trump has shown, over and over again, that he acts in a manner consistent with the hypothesis that he is, at the very least, quite high in narcissism. It’s gotten increasingly difficult to argue with this.

That out of the way, the key to understanding Trump’s thin-skinnedness might be in understanding the seemingly conflicting ways his narcissism — anyone’s narcissism — can manifest itself. Narcissism has two faces, it turns out. “There are two dimensions of narcissism a lot of us write about now — grandiose and vulnerable,” wrote Joshua Miller, a personality psychologist at the University of Georgia, in an email. Grandiose is what it sounds like: a form of narcissism in which the narcissist puffs himself up, shouting to the world that he is a great, powerful person. Which Trump certainly does a lot of. Vulnerable narcissism, on the other hand, is the narcissism of fragility: What did you say about me? As Melissa Dahl noted in her article on ‘undercover’ narcissism, George Costanza is a classic pop-culture example of how the vulnerable sort of narcissism can manifest itself: No one would confuse Costanza for a Trumpian blowhard (except on those sporadic occasions when he is, say, attempting to pass himself off as marine biologist), but he is extremely sensitive to any perceived slight or threat.

Trump seems to have both qualities. He spends a great deal of time telling everyone how fantastic he is, but then, in a manner which seems to conflict with this view, will go to great length to swat angrily at any perceived insult. There’s a reason why, up on a debate stage in front of 80 million viewers, he brought up Rosie O’Donnell, of all people.

This is where the scientific debate comes in. As Miller explained, researchers who study narcissism are in two camps. Some believe that vulnerable and grandiose narcissism effectively form two scales, and that a narcissist can be anywhere on each — “so you can be low, low; low, high; high, high, etc.,” Miller says, with regard to the two types. In this view, there can be narcissists who are extremely grandiose but not at all thin-skinned (that is, vulnerable); narcissists who are extremely thin-skinned but not particularly grandiose (Constanza-style); and narcissists who are both (Trump). “[O]ther narcissism researchers like Aaron Pincus and Aidan Wright believe that all pathologically narcissistic people experience both and fluctuate between these states,” Miller explained. Deep down, all narcissists have both grandiose and vulnerable tendencies, then, but they manifest in different ways at different times.

Miller said that he is in the dimensions rather than the fluctuations camp (my terms, not his). And, he says, “in the case of Trump, he seems to be high on both of these dimensions.” The other researchers Miller mentioned might conceptualize his behavior a bit differently: All narcissists have this fragile, vulnerable side, but perhaps the stress and the endless scrutiny of the campaign have caused Trump to tend toward his vulnerable side, at least when he’s aggressively challenged, as he was by Clinton during the first debate.

From a debate-watcher’s standpoint, of course, what matters will be whether and to what extent each candidate can bait the other Sunday night. Clinton just doesn’t really exhibit narcissistic traits the way Trump does, at least not relative to the average big-name politician. So recent history, and the now-voluminous observational evidence we have about who Trump and Clinton are (at least when they’re under the searing public spotlight), suggests Clinton will have a hard time resisting the opportunity to get Trump going — and that Trump will have trouble not rising to the bait.