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Researchers still have a lot to learn about the roots of introversion and extroversion, and not all of what they’ve uncovered so far has fully filtered down to the public. When it comes to extroversion, for example, they’ve realized that it can actually be broken down into two components: affiliative extroversion and agentic extroversion, which involve different motives for wanting to be around people. (When researchers talk and write about extroversion as a personality trait, they often use the term extraversion, with an a, but the distinction doesn’t really matter here.)

In a new study in Cognitive, Affective, & Behavioral Neuroscience, Erica Grodin and Tara White of Brown University provide what they say is the first evidence of a neurological footprint that can help explain the two different types of extroversion. By giving a group of 83 healthy volunteers personality assessments and putting them through an MRI machine, the researchers were able to correlate each type of extroversion with greater volume in certain regions of the brain.

White, an assistant professor of behavioral and social sciences, recently spoke with Science of Us about her research.

These concepts of agentic and affiliative extroversion were new to me, and I think they probably will be to a lot of other people as well.

In terms of popular culture, I don’t think there’s a lot of understanding about it. Agentic extraversion is about sensitivity to reward, engagement with goals and achievement, persistence, and taking a leadership position when you have an opportunity to do that. In other words, being comfortable in the limelight. It’s a social leadership dimension, really. Affiliative extraversion is also a really great trait — it’s a dimension of social warmth. People who are high on the trait, close social relationships mean a lot to them, they get a lot out of them, they’re fully engaged, they tend to have a very large group of meaningful friendships.

These are different ways that people achieve joy in their lives, different ways that they find positive engagement in the world. What’s interesting is that based on self-reports, people can be high on both, but they can also be high on one and moderate on another. Even though the traits are correlated, it doesn’t mean that you’re going to be the same on both, which is really interesting.

So someone who’s very high in affiliative extraversion, but lower in agentic extraversion, would feel a strong sense of connection when they’re with their friends or they’re at a religious service?

Absolutely.

But they wouldn’t necessarily feel a drive to seek out a leadership position in one of those areas?

That’s likely — I would say that.

How about someone in the reverse case — lower affiliative extroversion, higher agentic extroversion? It sounds like simply hanging around people they like wouldn’t be enough, that they’d almost feel like there was something missing because they aspire to some sort of leadership position?

It’s hard to impute how people feel in different sorts of situations, but it seems consistent with what you might predict, that agentic extraversion is really about going for reward. That reward can be in social context, it can be in a workplace context, it can be lots of contexts.

Is it unlikely that someone would be very high on one trait but very low on the other, as opposed to high on one and moderate on the other?

The traits are positively related, which indicates that they are not entirely independent. In addition, they also have a normal distribution, meaning that high scores and low scores are going to be naturally less common than scores in the middle. Putting these two things together, I would think that the combination of low scores on one and high scores on the other would be relatively uncommon. This is just thinking through the math, based on what is known about the distribution of scores on each trait, and the relationship between the traits.

The goal of your study was to figure out if there were independent neurological underpinnings to each of these types of extroversion, right?

Exactly. I’ve always thought that since — just based on people sharing where they get joy and engagement from — the fact that those things are consistently separable in so many different studies means that there’s got to be some biological basis for that, that it’s got to be mirrored in the brain somehow. So I thought this was a nice opportunity for us — we had a reasonably large sample size to look at, and they had all filled out a personality instrument that assesses both types of extroversion.

So, breaking down the results: All extroverts had greater volume in the medial orbitofrontal cortex. But people with high degrees of agentic extroversion also had greater volume in other areas: the left parahippocampal gyrus, left cingulate gyrus, left caudate, and left precentral gyrus in both men and women, and the right nucleus accumbens in men but not women. It’s easy to drown in all of these brain-region names, so for those of us unfamiliar with this stuff, what is it that these regions have in common? What story is being told by the data your study produced?

The right nucleus accumbens is a region involved in incentive reward and also in volitional motor approach of reward — so basically it’s part of a system that identifies rewards and helps propel you toward them. The other regions are all involved in the cognitive control of behavior and the planning and execution of voluntary movement. So it actually makes sense that this dimension of temperament and personality that’s sort of a get-yourself-out-there-in-the-world thing is related to gray matter volume in those regions. It’s actually pretty striking.

So people high in agentic extroversion had these markers on top of the normal markers all extroverts had. But there were no specific markers for affiliative extroversion?

Right.

Have researchers discovered any parallel distinctions when it comes to introverts? As in, different introverts being driven by different motivations?

Not that I know of. It’s an interesting idea, though.

What do you think is the most important takeaway from your research?

I think this is really just giving us a window. It’s interesting to learn about the neural core of everyday emotions, and differences between those emotions, and differences between people and how they are engaged with the world more generally. And one of the things that I thought was so interesting about this is that people often say, “Well, you know you can’t trust what people say about their emotion,” and so many scientists don’t trust self-reports from human subjects.

And this study, I think, is validating that you can trust what people say about their emotional experiences in the world. I think that’s probably the largest takeaway: that when people tell you how it is to be them, we should trust that and respect that, because there’s a deep reality in their brains that is surfacing when they tell you what they get joy from and how they like to engage in the world.

This interview has been edited.