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There is a lot of discussion about stereotypes in the media. On the one hand, many people believe that stereotypes provide a useful way of reasoning about a member of a group if you have no other information about them to go on. On the other hand, there is also an understanding that even when a stereotype is reasonably accurate, it only applies to some members of a group.
Male and female cardinal
To pick an example that isn't at all controversial, you would probably agree that cardinals are (stereotypically) red birds. However, only the adult male cardinals are red, so at best that belief applies to only half of the cardinals.
Much less discussion about stereotypes focuses on why they work.
An important reason why stereotypes are effective is that they rely on what Doug Medin and Andrew Ortony called psychological essentialism. That is, when you hear about a category, you tend to believe that the members of that category share some inner essence that makes them what they are. In the case of cardinals, for example, you might think that they all share a common genetic structure that makes cardinals different from robins, sparrows, and starlings.
Stereotypes about groups of people also rely on some kind of essentialism. In order to believe that people from a particular racial or ethnic group have some characteristic in common, you have to believe that there is some essence that makes race or ethnicity a good basis for categorization.
When Satoshi Kanazawa wrote his inflammatory blog entry on this site about attractiveness and black women, a key part of his argument rested on the idea that race forms a good basis for classification, and that there is something essential about members of one race that causes them to differ from people of other races.
What factors could people possibly believe in that would support these stereotypes?
This question was explored in a paper by Ulrike Rangel and Johannes Keller in the June, 2011 issue of the Journal of Personality and Social Psychology. They explored two different beliefs that people have that support stereotypes: Genetic Determinism and Social Determinism.
Genetic Determinism is the idea that people are made what they are by their genes. Social Determinism is the idea that people are made what they are by the social factors that shape their development. (As an aside, genetic studies dating back 40 years suggest that the amount of genetic variability within a racial group is far larger than the genetic variation between groups suggesting that there is little support for the idea that genes are a strong determinant of differences across groups.)
Rangel and Keller compare a scale for measuring people's beliefs about Genetic Determinism (developed by Keller in 2005) to a scale for measuring people's beliefs about Social Determinism. They find that these two beliefs are independent. That is, some people believe that genes determine the differences between racial and ethnic groups in behavior. Others believe that social factors determine these behaviors. Some people believe that both genes and race affect racial and ethnic differences in behavior. Finally, there are some people who believe that neither of these factors is at work in racial and ethnic differences.
A particularly interesting finding in this paper, though, is that the belief in Social Determinism influences attitudes. In one study, participants from Germany either read a magazine article about social influences on human development or they read a neutral magazine article. Later, these participants rated how much they like people from Western Europe (who are part of their in-group) or people from Eastern Europe (who are not).
The ratings of members of the in-group were influenced by beliefs in Social Determinism. When people read an article about potential social influences on behavior (so that this concept was available), then those people with a high belief in Social Determinism rated members of their in-group as much more likeable than people with a low belief in social Determinism. Likeability ratings were not affected by a belief in Social Determinism if they had not read about social factors recently. Finally, ratings of members of an out-group (Eastern Europeans) were also not affected.
The influence works in the opposite direction as well. In another study, participants from Germany read either negative or neutral statements about an out-group (people from an Eastern European country). Those who read negative statements expressed a greater belief in Social Determinism than those who read neutral statements.
What does this all mean?
The more that you believe that social factors determine people's behavior, the more that you will tend to use information about these social factors to try to explain their behavior. When thinking about racial and ethnic groups, then, you will assume that factors like a common upbringing or schooling affect the actions of the members of that group.
At the same time, when you hear about differences in behavior among members of a racial or ethnic group, you will often seek an explanation for that difference. Social Determinism is an easily available explanation, and so hearing about a negative behavior by a group increases your belief in Social Determinism.
In the end, it is important to remember that that you will try to use your categories to help you reason about things whether you are aware of it or not. If you find yourself using racial or ethnic categories to make judgments about people, ask yourself whether that particular category makes sense. Think about whether you are assuming genetic or social differences that are likely to have caused the behavior. Finally, consider whether there are other categories that a person might belong to that might also explain why they do what they do.
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