This is very common across most psychology studies from what I understand.
> In the Journal of Personality and Social Psychology, the premier journal in social psychology – the subdiscipline of psychology that should (arguably) be the most attentive to questions about the subjects’ backgrounds – 67% of the American samples (and 80% of the samples from other countries) were composed solely of undergraduates in psychology courses (Arnett 2008). In other words, a randomly selected American undergraduate is more than 4,000 times more likely to be a research participant than is a randomly selected person from outside of the West.
> In the Journal of Personality and Social Psychology, the premier journal in social psychology – the subdiscipline of psychology that should (arguably) be the most attentive to questions about the subjects’ backgrounds – 67% of the American samples (and 80% of the samples from other countries) were composed solely of undergraduates in psychology courses (Arnett 2008). In other words, a randomly selected American undergraduate is more than 4,000 times more likely to be a research participant than is a randomly selected person from outside of the West.
Source: http://hci.ucsd.edu/102b/readings/WeirdestPeople.pdf