Answer to Question 1
Culture, race, and ethnicity are hypothetical constructs because they are concepts that psychologists have constructed to help explain and understand behavior. They are hypothetical because we hypothesize that they exist and that they are going to be useful, explanatory concepts.
Sometimes these constructs are helpful because, when measured appropriately, they might be helpful in our understanding of why people in some groups act one way, whereas people in another group act differently. Not everybody in a group acts and thinks the same way, but the hypothetical constructs of culture and ethnicity can give some insights into group processes.
On the other hand, none of these concepts can be precisely defined and measured, which is why we rely on external markers (e.g., skin color) to represent them. These external markers may not really be very helpful in understanding behavior because they are only imperfectly correlated with thought and behavior. People within a given group differ from one another in many ways, so trying to figure out how a single person thinks is not feasible.
Answer to Question 2
Not all members of a given category are alike. In fact, degree of acculturation is a better predictor of some behaviors than is ethnicity. In addition, different variables may interact to generate behaviors and attitudes that are unpredictable from the variables individually. Further, the use of categories is inconsistent, so we aren't always sure what a category means or why an individual is placed in it. Finally, different behaviors and attitudes within groups are often better attributable to variables like socioeconomic status, fluency in English, nature of one's peers, etc.
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A Hypothetical Explanation of Conduction Aphasia
Cuzco, Peru, Inca culture.
Nikolai Buglaj, "Race"ing Sideways.
The sense of smell.