I've found that a handy lay-psychological tool to use when living in Japan is to divide the brain into two parts: the rational and the ideological. The rational part makes logical decisions based on available input, while the ideological makes socially prudent rules of thumb which may contradict the available facts but serve some sociological purpose.
Take my preschoolers for example. Their ideological minds are just starting to pick up prejudices, both useful and archaic, stereotypes on behavior and appearance, and what passes for common sense. Some of them have been around me for almost 3 years now, and have heard me speaking Japanese to their parents with at least some degree of skill. Rationality demands that they recognize the fact that I speak some level of Japanese. Yet at some point, they hear from their grandparents, teachers or some other well-meaning passer-on of the received wisdom that Japanese and Foreigners are different, one of those differences being that Japanese people speak Japanese and Foreigners speak English. From that point on their ideological mind slaps a submission hold on their rational mind and convinces them that I, being Foreign, don't speak Japanese. The next time they see me they'll be acting the part of the Japanese Person Surprised To Find A Foreigner Conversant In Japanese as best they can at a mere 4 or 5 years old.
And here also I tend to really wish the Japanese language had plurals, which would allow for a bit more qualification in such broad statements as “Foreigner(s) don't speak Japanese”, “Japanese person/people eat insects”, and “Foreign country(s) don't have persimmons”. I've convinced myself that the lazy way Japanese treats amounts of things has contributed more than a little to the amount of prejudicial conflation people can to get away with.
I've even had more than one adult speak to me for some time in Japanese, only to have them loudly applaud my reading aloud of a short word or two in Katakana a few months later, as if I were a toddler displaying object-word association for the first time. I refer to this type of phenomenon as the ideological brain killing and eating the rational brain, and it happens so often you stop seeing a point in correcting people whose ideology will re-erase what you've just told them as soon as they see any more mass media involving Foreigners.