Are people who stay single less extraverted than people who marry? That's a question that's been posed in the comments section of this blog a number of times. A relevant study suggests that the answer is: yes, probably.
Participants in the study were 6,876 Wisconsin adults, ages 53-54 and all non-Hispanic Whites. When they were first recruited, they were still in high school, but their extraversion was not measured until they were in their early 50s. So you can see the plusses and minuses of the study already - it is a big sample, but not a diverse one. It is a longitudinal study, but the key personality variable was only measured once, so we can't know how people's extraversion may have changed as they got married or unmarried or stayed single.
So here's what we do know: The 50-something year-olds who had stayed single were, on the average, less extraverted than their high school age mates who were married at the time of the testing. That was true of both the men and the women.
Two points to keep in mind: First, the introversion-extraversion dimension is different from social anxiety or shyness. The socially anxious are fearful about social interactions; introverts just prefer more time to themselves. Second, the author did not report the mean levels of extraversion for the different groups, so we can't know whether the singles were on the introverted end of the scale or just less extraverted than the marrieds.