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"Every single participant in the study was happier when he or she acted extroverted than when he or she acted introverted," Fleeson said. "Even introverts can act extroverted and become happier by changing their behavior."

Regardless of whether a person is shy or outgoing, being more talkative, adventurous, bold or assertive has a positive effect. Nearly any extroverted behavior seems to have a positive impact on mood, he said. Singing out loud to a song on the radio, walking over to an attractive girl to talk to her, asking a question in class or voicing an opinion all seem to work.

People who tracked their moods for two weeks reported feeling happier when they acted outgoing and less so when acting quiet or reserved. When participating in group discussions in the lab, people instructed to act assertive and energetic indicated afterward how much fun they had. When asked to act passive and shy, the same participants indicated they were unhappy in the situation and did not have fun. Self-evaluations in this exercise matched up with how the participants evaluated each other's behavior and mood.

Fleeson, who co-authored the study with two Wake Forest undergraduate students, also wanted to find out if the short-term positive effect of acting extroverted carried over for longer periods of time. When people were asked to rate their levels of extroverted behavior and their mood at the end of each week for 10 weeks, the results also suggested a strong connection between acting extroverted and greater happiness.

"As a society, we tend to think of happiness as something that comes from outside us. It's kind of a radical idea that we have some control of happiness, that personality is a factor in happiness and that, to some extent, we have control over our personalities," Fleeson said.

If people want to be happier and choose to act more outgoing, adventurous or assertive, then they have the power to directly improve their own well-being, he said. "The research demonstrates that extroversion can actually cause happiness."

Editor's note: Fleeson's study-"An Intraindividual Process Approach to the Relationship Between Extraversion and Positive Affect: Is Acting Extraverted as 'Good' as Being Extraverted?"- appeared in the December issue of the Journal of Personality and Social Psychology.