The other answers are good and describe the overall idea, but I wanted to focus on the fragment's meaning within the entire sentence. When you restrict a studies' participants to only people within (circumscribed by) a certain culture, one should be extremely cautious against extrapolating the findings of that study to other groups. While the sentence specifically mentions culture, it applies whenever you try to use a non-representative group to say something about a larger population.
This can be a major issue with some university studies (particularly in psychology and sociology), because you will see studies that use university students as the study participants and then try to use the results to say something about the population at large. A well-known example of this is the infamous Stanford Prison Experiment, which was comprised of participants who were almost entirely young white male students. You will also hear about similar issues in medical studies, where they don't have enough participants in all gender/ethnic groups to form a statistically representative sample. In these sorts of cases you should 'take with a grain of salt' when the researchers attempt to extrapolate the results to a larger population.