Freud believed that nearly all human behavior could be understood as a function of human's innate pleasure-seeking tendency. He posited that during different developmental phases, humans fixated on specific areas of the body in order to derive pleasure. The first developmental phase was the oral stage, lasting from birth to age 1. During this stage, a person derives pleasure from his mouth, through suckling on his mother's breast. The second stage, called the anal stage, lasts from ages 1 to 3. During this stage, people derive pleasure from learning to control and master their bodily functions. Freud posited that healthy individuals are those who pass through these stages successfully, without becoming stuck in any one of them. He believed that people who become stuck ("fixated") in one particular stage would not be able to achieve healthy social and sexual maturity.
This developmental schema is subject to a number of critiques. Cultural anthropologists point out that these stages represent a culturally relative timeline and thus can't be universalized. For example, in some cultures, the weaning stage lasts up to 4 years old. Cultural differences in child-rearing are not adequately addressed in the psychosexual theory of development.
Many psychologists believe Freud's theory to be over-simplified and reductionist. Young people and adults alike derive pleasure from a number of sources: bodily, emotional, social and cognitive. It may not be useful to attempt reducing all pleasure-seeking activity to a function of latent sexuality.
Social psychologists also point to Freud's normalization of heterosexual male sexuality as a limiting factor. According to the psychosexual theory of development, those who do not adopt heterosxual, male-oriented sexuality are mentally ill. Thus, Freud's theory limits our ability to recognize the validity of different sexual identities and expressions.