Many psychology* studies study volunteers who have agreed to be studied. Those studies do not study anyone else. How universal, you might wonder, are the findings those studies report?
This Research Digest essay, about a study about studies of people who agreed to be studied, points out that people who are agreeable are more agreeable to being studied than people who are not so agreeable to being studied:
The study about people who agreed to be studied is:
Lonnqvist, J-E., Paunonen, S., Verkasalo, M., Leikas, S., Annamari, T-H. &Lonnqvist, J. (2007). “Personality characteristics of research volunteers,” European Journal of Personality, 21, 1017-1030.
*The same is true of many studies in fields other than psychology.