Answer to Question 1
Stereotypes affect the way we interpret others' behaviors. If we have certain expectations about behaviors, we might considered it unusual or abnormal when our expectations are not met. This might reflect our own stereotypes and misinformation, not the behavior of others.
In addition, when people think others have stereotypes about them, those people often conform behaviorally to the stereotypes. This pattern of behavior might lead blacks to perform lower than they could on tests when they think that people around them expect poor performance. Similarly, women have been shown to perform less well on math when in the presence of men, who might expect the women to perform poorly.
Answer to Question 2
When you use existing or intact groups, you don't have a guarantee that they are equivalent; that is, if you were to measure then on the DV, they might differ from the start. If you didn't know about this difference, you would mistakenly attribute a difference at the end of the study to the IV.
When groups are created by random assignment, there is less chance that the groups will differ systematically because differences that could be critical are spread out randomly across groups.
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