A happy, outgoing person who likes to be around people is far more likely to return a smile from someone else than an introverted person who doesn't enjoy being in a crowd.
Astonishing, you say? Probably not, because most of us already know that the life of the party is usually more responsive than the wallflower, but researchers at Stanford University have shown that how we respond to a happy face is more than skin deep. It involves the biological, genetic and psychological factors that made us who we are.
At the heart of the research are some of the most fundamental questions regarding the human personality. What makes us different from each other?
"What we are wondering is, where does personality come from?" says John Gabrieli, associate professor of psychology at Stanford.
Entering the Brain
Not too many years ago, researchers were very limited in their resources, and the study of personality — and the human brain — was largely an intuitive process that often led to major errors.
"It was all sort of like a black box," Gabrieli says. "You put something in and you get something out." But there wasn't any way of knowing what was really going on inside that "box" as the brain processed the information.
In recent years that has changed dramatically. Functional magnetic resonance imaging, for example, allows scientists to pinpoint which parts of the brain react to external stimuli, and in some cases, how much.
"Now we can hunt down the specific part of the brain that determines how we will interpret a situation as desirable or threatening," he adds. "That's pretty exciting."
The Stanford research, published in a recent issue of the journal Science, suggests that we respond to something that is potentially pleasant in very different ways because our personalities are not the same. Whether we react at all depends on whether a pea-sized bit of tissue near the center of the brain "lights up" when we see something nice, like a happy face. That can be measured with the non-invasive magnetic imager.
The tissue is called the amygdala, and it nearly always lights up when we are exposed to danger, or something really unpleasant. That much was pretty well established years ago, but the Stanford researchers decided to take it a step further.
The will to survive is a universal drive, so it stands to reason we would all react (though in different ways) to a threat, regardless of our personality types.
But Gabrieli and Turhan Canli, who has since moved on to become an assistant professor of psychology at State University of New York-Stony Brook, wanted to know if something pleasurable also caused the amygdala to light up. The amygdala is crucial to emotional processing by the brain, but does it work just as hard for a dull chap as it does for a party girl?
The researchers recruited 15 students to take part in the experiment, and tested each to see which ones were outgoing and optimistic (extroverted) and which ones were worried, or insecure, or a bit neurotic.
None of the participants were extreme examples of either of those personality types. They were pretty normal people, but had differing personalities.
Each was hooked up to the functional magnetic resonance imager. Then they looked at a series of photos. They were asked only to determine which of the pictures were of women, and which were of men, just to keep them focused on the photos.
The researchers got about the same response from everybody when the subjects where shown an image with an unpleasant or threatening expression. The amygdala always lit up.
But it was a different story for a happy face.
Which Comes First?
The extroverts always responded, as revealed by the amygdala, but the shy folks showed little response.
Now, 15 is not a lot of participants, "but the results were extremely robust," Gabrieli says.
"We got this very clean association between amygdala reaction to two different kinds of emotional facial expressions," says Canli.
What that means, he adds, is "the more outgoing someone is, the more his or her brain is likely to react to something perceived as pleasant."
What's still debatable, however, is who's giving the orders.
Is the amygdala calling the shots, telling some of us to react and others to remain stoic, or is the personality telling that crucial part of the brain what it wants to do.
It is, as Gabrieli puts it, the old "chicken and egg thing." The study doesn't tell us where personality comes from.
We know genetics plays a role, because a shy infant tends to remain shy at least though childhood. But "why is it that some people are so different from one another in what they find enjoyable or threatening?" Gabrieli asks.
The Stanford study adds another building block to our understanding, thanks to some amazing new tools that are helping researchers close in on the biological processes that are so closely linked with personality.
But we still have a long ways to go before we will understand why each of us became what we are.
Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.