According to the outgroup homogeneity bias, individuals see members of their own group as being relatively more varied than members of other groups.
This bias was found to be unrelated to the number of group and non-group members individuals knew. One might think that people thought members of their own groups were more varied and different simply because they knew them better, but this is actually not the case. The outgroup homogeneity bias was found between groups such as "men" and "women" who obviously interact frequently.
An example of this is that in science fiction universes, such as Star Trek, humans are almost always the only species with multiple cultures; alien species are almost universally depicted as having a single, species-spanning culture.
Quattrone, G. A., & Jones, E. E. (1980). The perception of variability within in-groups and out-groups: Implications for the law of small numbers. Journal of Personality and Social Psychology, 38, 141-152.
Quattrone, G. A. (1986). On the perception of a group’s variability. In S. Worchel & W. G. Austin (eds.) Psychology of intergroup relations, 2nd ed. Chicago: Nelson-Hall.