How is it possible that humans can be unaware of their prejudices?

Prejudice is a negative feeling or attitude towards members of a group. Prejudice can have a strong impact on how people behave and interact with others even unconsciously or without the person realizing that they are under the influence of their internalized prejudices. The common features of prejudice include negative feelings, hatred, and discrimination against members of a group based on their race, sex, caste, culture, minority status, nationalism, socio-economic status, religion, etc. Prejudice results in stereotyping, discrimination, and bullying. According to psychologist, Gordon Allport, stereotypes and prejudices emerge in part as a result of normal human thinking.