Who's more likely to sign up to a scientific study? An extrovert or an introvert minding their own business? I can't prove it, but it would make sense that there would be more extroverted volunteers.

The idea that I get [as an introvert] is that extroverts see introverts as depressed precisely because extroverts would likely find that lifestyle depressing. I myself find extroverts' lifestyles depressing. Given more introvert volunteers in the study you might have found that the result of that particular test may have been inverted.

The same principle may apply to all the other faces.