The Evolution of a Stereotype

"Only men should be president because they are smarter than girls."

"So how does a 7-year-old develop a stereotypical attitude that boys are smarter than girls? Bigler has identified four circumstances that lead to stereotyping and prejudice.
The first component is a child's ability to visibly detect differences. This explains why children tend to form stereotypes based on race and gender, but not religion or political affiliation.
"Children focus on what they can see," Bigler explains. "They're more likely to judge a person on their hair color or age, rather than their personal beliefs and opinions."
The second component is minority status. In one study, Bigler randomly separated a class into two groups. She gave the larger group blue T-shirts. She gave the second group, consisting of only two or three children, red T-shirts. The children in red T-shirts were far more likely to want to change groups than their peers who wore blue T-shirts.
"Children are acutely aware of, and uncomfortable about, being outnumbered, even if they are separated by something as meaningless as a T-shirt color," Bigler says. "And children in the minority group sought each other’s company, becoming close friends in almost every classroom."
The third component involves explicit labeling of different groups. For instance, a teacher's greeting of "Good morning, boys and girls," or an instruction to sit "boy, girl, boy, girl" reinforces group differences. According to Bigler, children attach particular importance to something when adults label it.
Finally, implicit segregation often leads to stereotypes and bias. In the T-shirt experiment, children in classes with only red or blue T-shirts demonstrated more stereotypical opinions about their peers than did children in integrated classrooms with both red and blue T-shirts."
Dr. Rebecca Bigler

http://www.utexas.edu/features/2008/stereotypes/