Even people who do not want to do so tend to judge others by their appearance as part of the crucial human ability to be able to make snap judgments about people and situations. Being able to put sensory input into categories is vitally important for the human brain to function and deal with the world around it. One by-product is judging others based on appearance.
In the distant past, the ability to judge other people immediately upon seeing them was crucial for survival. People needed to assess at first glance whether an unknown person was a threat or intended harm to them. Although it is much more rare that people in the modern world need to make these same snap judgments, the human tendency to judge based on appearance remains. Studies show that appearance makes a difference in practical ways. Those who are tall, thin, blonde, fit and good-looking tend to make more money than their less visually appealing counterparts.
People not only resort to stereotypes when they meet new people, they also use those stereotypes in planning their own world view and goals. When one is presented with a discrepancy to a stereotype, however, the brain secretes chemicals that provide a startle response as well as a pleasure response, allowing people to change their minds about individuals even though they initially categorized them according to their stereotypes.