I suspect that these studies have the limitation, that only more extroverted people agree to do such things for a $5 gift card, while the introverts just say no

I suspect that these studies have the limitation, that only more extroverted people agree to do such things for a $5 gift card, while the introverts just say no (I'm using the terms extrovert and introvert loosely here). And yeah, of course the extroverts would feel happier when talking to people, while those who wouldn't feel so rarely even participate in the study [if my suspicion is right].