Of all theories of relationships, Sigmund Freud's oedipal complex has probably caused the most controversy. It began with the study of a boy known as Little Hans. In 1909, Freud's paper, Analysis of a Phobia in a Five-Year-Old Boy, outlined Hans' fear of horses. Freud believed the boy's terror was due to feelings of anger he had internalised that related to his parents.

Freud theorised that all small boys select their mother as their primary object of desire. They subconsciously wish to usurp their fathers and become their mothers' lover. Typically, these desires emerge between the ages of three and five, when a boy is in what Freud defined as the "phallic" stage of development. Because the child suspects that acting on these feelings would lead to danger, desires are repressed, leading to anxiety.

The oedipal complex is named after Sophocles' protagonist, who unwittingly murders his father and marries his mother. There is a female equivalent, known as the electra complex, but Freud was more concerned with what he termed female "penis envy".

Few people believe today that the oedipal complex has any real bearing on our lives.