Researchers have studied belief congruence theory, that belief differences matter more than demographics in forming in-groups and outgroups, for decades.

This is judged as of 2014-09-30. True: the academic literature shows belief congruence theory (originating with Rokeach around 1960, holding that similarity of beliefs matters more than race or other demographics in prejudice and in-group formation) had been studied for several decades by 2014. Mostly true: research on it spans decades, but the claim's summary of the theory is slightly off (for example the theory mainly concerned prejudice or race rather than group formation in general). Mostly false: the theory exists but was studied only briefly or sporadically, or the summary seriously misstates it. False: no such theory or line of research exists.

Set before the research began.

1. E1 Belief congruence Wikipedia · 15 May 2024

   “Milton Rokeach in 1960, 'belief is more important than ethnic or racial membership as a determinant of social discrimination'”

2. E2 A TEST OF ROKEACH'S BELIEF CONGRUENCE THEORY OF ... Philippine Journal of Psychology (per archive URL; Philippine Social Science Council archive) · 1970

   “A test of the belief congruence theory of prejudice was conducted using only two”

3. E3 In-group favoritism Wikipedia · 27 July 2004

   “For example, Struch and Schwartz found support for the predictions of belief congruence theory.”

4. E4 Measuring Prejudicial Attitudes in a Situational Context: A Report on a Danish Experiment Scandinavian Political Studies (tidsskrift.dk) · 1972

   “One of the most frequently cited recent studies of prejudicial attitudes, carried out by Rokeach, Smith and Evans,4 suggests that belief congruence rather than race prejudice is largely responsible for the behavior of majority groups toward cultural and racial minorities.”

E1 says Milton Rokeach proposed belief congruence theory in 1960. E2 (1970) and E4 (1972) are independent empirical studies of it, and E3 cites later research by Struch and Schwartz that supported the theory, so the research was ongoing across many years before 2014. The sources describe the theory as being about prejudice and social discrimination based on race or ethnicity (E1, E2, E4), not group formation in general. That makes the claim's summary slightly broad, which the criteria treat as mostly true.