"...they gave elementary school children either a blue T-shirt or a red T-shirt to wear throughout the school day for six weeks.
"Teachers treated those color groups in the same ways they would use gender. Teachers said, “Good morning, blue and red kids!,” “Let’s line up blue, red, blue, red.” Kids had their names on either a red or blue bulletin board and had either a red or blue name card on their desk. But again, teachers had to treat both groups equally and not allow them to compete with one another. They simply “used” color in the same way many teachers “use gender.”
"After only four weeks, children formed stereotypes about their color groups. They liked their own group better than the other group. Red-shirted children would say, “Those blue-shirt kids are not as smart as the red-shirt kids.” Just like they do with gender, they said that “all blue kids” act one way and “no red kids” act another way (this differed based on which group they were in). They began to segregate themselves, playing with kids from their own color group more than with those from the other group.
"They were also more willing to help kids in their own color groups. Children walked into a classroom in which we had staged two partially completed puzzles. We had surreptitiously draped a red shirt across one puzzle and a blue shirt across the other. When given the option, children were more likely to help out the child they thought was in their group.
"In all of these studies, there was always a very important control group—in addition to the group of students who wore colored T-shirts, there were classes in which the teacher who didn’t talk about the color groups. She didn’t sort by color or use the color grouping to label each child. In other words, it was like being in a class of boys and girls where the teacher doesn’t mention or sort by gender; she simply treated them like individuals. In these classes, children didn’t form stereotypes and biased attitudes about groups. If the adults ignored the groups, even when there were very visible differences, children ignored the groups too. [...]
"... it seems that children pay attention to the groups that adults treat as important. When we repeatedly say, “Look at those girls playing!” or “Who is that boy with the blue hat?,” children assume that being a boy or girl must be a really important feature about that person. In fact, it must the single most important feature of that person. Otherwise, why would we point it out all the time?
"If children see a difference, they look to experts in the world (us grown-ups) to see if the difference is important or not. Don’t forget that they see plenty of differences in people. For example, they see differences in hair color. We come in brown hair, black hair, blond hair, red hair, and gray hair. But no adult ever labels this visible category, saying “Look at that brown hair kid.” “Okay, all the brown-haired kids and black-haired kids over here. All the red- and blond-haired kids over there.” Children ultimately learn to ignore these as meaningful categories, but they still notice they exist. If I ask someone’s hair color, a child can tell me. It just isn’t a meaningful category. They don’t develop attitudes about what it means to have red hair or brown hair (even the occasional blond joke isn’t constant enough for children to notice).
"But with gender, children notice the difference and adults make it meaningful. Children see the category. We made sure of that with our pink or blue shirts. Also, the experts in the world, their parents, always label the category. We put a figurative flashing neon arrow on gender and say “Pay Attention! Important Information Here!” And guess what, they pay attention."