The theory of psychological type comes from Swiss psychiatrist Carl G. Jung (1875-1961) who wrote that what appears to be random behavior is actually the result of differences in the way people prefer to use their mental capacities. He observed that people generally engage in one of two mental functions:

• taking in information, which he called perceiving, or
• organizing information and coming to conclusions, which he called judging.

Within each of these, Jung saw people preferring to perform that function in one of two ways. These are called preferences.

He also noted that, although everyone takes in information and makes decisions, some people prefer to do more taking in information (perceiving) and others prefer to do more decision making (judging).

Finally, Jung observed, “Each person seems to be energized more by either the external world (extraversion) or the internal world (introversion).” What Jung called a person's psychological type consists of his or her preference in each category.

In 1921, Jung published Psychological Types, introducing the idea that each person has a psychological type. The academic language of the book made it hard to read and so few people could understand and use the ideas for practical purposes.

During World War II, two American women, Isabel Briggs Myers and her mother Katharine Cook Briggs, set out to find an easier way for people to use Jung's ideas in everyday life. They wanted people to be able to identify their psychological types without having to sift through Jung's academic theory.