That's not a useful avenue -- you have no control group. We can't compare people from India who eat a lot of curry to people from the US who don't: there are a million differences in diet, lifestyle, genetics, and environment that make it impossible to draw a conclusion about how it affects memory.
Your example is a good one actually: we know Japanese men are less prone to heart disease, but we don't conclusively know why. We can guess it's related to certain factors, e.g. fish consumption, but we can't know just by comparing the two cultures.
Your example is a good one actually: we know Japanese men are less prone to heart disease, but we don't conclusively know why. We can guess it's related to certain factors, e.g. fish consumption, but we can't know just by comparing the two cultures.