But new research casts that old research in a different light.

Donna McMillan, PhD and Jamie Klein, BA, of St. Olaf College in Northfield Minn. have found that when completing various measures of the kind frequently used in psychological research, such as the Likert-type scale, extroverts were more likely to choose extreme answers than . So, for example (though not necessarily one McMillan used) if asked to rate on a scale of 1 to 5 how happy they are, extroverts are more likely to choose 5! Wheeeeee! than introverts.

Maybe this is because extroverts are genuinely happier than introverts. Or maybe, suggests McMillan, it's because extroverts appear to require more cortical stimulation than introverts, and circling 5 is more stimulating than choosing a ho-hum 4.

"Also, with the way is conceptualized right now, it has a pretty heavy dose of enthusiasm and energy, what's sometimes called surgency," she says. "I thought that also could lead extroverts to be inclined to circle the extreme on a questionnaire."

These theories of extroversion contributed to McMillan's interest in doing this study. But personal observations also got her thinking.

For example, McMillan, who is a clinical psychologist, had one introverted patient who had recovered from . Nevertheless, she recalls, "He felt like he didn't live life with as much zest compared to some people around him. And I wondered if part of it was that he was seeing extroverts and thinking that's the way everyone should be."

McMillan also mentions a student in her class who filled out an online measure of character strength, things like honesty and fairness. She told McMillan she'd come up in the 30th percentile or less of most of the things. "I laughed and told her that is just not true," says McMillan. "But she's a highly introverted person and I said ‘I bet you refused to circle the extreme ratings,' and she agreed she had."

And finally, there have been incidents between McMillan and her husband, who is more introverted than she. Frequently, she says, "I will express something more extremely than he will, even though I suspect we feel the same way about it."

McMillan's first studies using the Likert-type scales are interesting enough, but follow-up research is even more compelling. In that, she used pairs of words that basically mean the same thing, but one word is more extreme than another--for example stone and boulder, hot and sweltering, attractive and beautiful--and asked people to choose the ones they were most attracted to. Again, extroverts went for the more extreme words: boulder, sweltering, beautiful.

This is fascinating and potentially important stuff, though we have a long way to go before we understand what it all means, beyond the fact that if you're doing market research, you might want to have extroverts rate your product.

The next step research must take is figuring out why extroverts go for the more extreme words.

"Is it because it fits the way they experience life?" McMillan wonders. "Are they experiencing life in a more sweltering or boulder sort of way? Or is it self-stimulation?"

How to find the answer to this is "the million dollar question," says McMillan.

But as long as this question remains unanswered, doubt is cast on all those studies that tell us that extroverts are happier. And that makes me happy.

How do we measure ? I'm continuing the discussion on my other blog Real World Research. Please join us.

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My book, The Introvert's Way: Living a Quiet Life in a Noisy World, is available for pre-order on Amazon. It will be released December 4, just in time for party/festive/family-togetherness season. You know you need it.

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Illustration by Bill Sodeman via Flickr (Creative Commons).