Although it is difficult to define culture and ethnicity and to spot reliable differences in behavior across them, why should we study them and their relation to behavior?

What will be an ideal response?

Sometimes people in different cultures may think and act differently in some domains. For instance, people who speak different languages may categorize and express ideas in very different ways, as a direct result of their linguistic community. In addition, people in different cultures have different expectations, so a given behavior needs to be understood within the context of that culture.
Further, sometimes people make a big issue of the differences in the so-called eastern and western ways of thinking. Although we should be cautious in such depictions, there seem to be differences in approaches to problem solving between Chinese and American people; people in differing cultures may also show different response sets when they behave.
Finally, even "culture-free" research does not always seem to be free of cultural biases. As such, we need to consider culture as a source of theory. Ideas that may be seen as fundamental in one culture may be foreign in another. In order to understand behavior most fully, we need to take culture into consideration.