The earliest personality types were based on the ancient medical theory of the four humors, which posited that humans are filled with four different fluids: black bile, yellow bile, phlegm and blood. According to humorism, imbalances in these fluids caused diseases and other health problems.
Humorism tied physical, mental and emotional aspects together. So a person who had an excess of one type of humor would have a corresponding behavior and personality type. Ancient Greek physician Hippocrates refined the concept of the four humors into the theory of the four temperaments: melancholic, choleric, phlegmatic and sanguine. This concept remained popular until the 19th century, when the four humors theory was discredited by more accurate medical knowledge. The theory lives on in the English language, though — for instance, the word melancholy is derived from the Greek words for "black bile," the humor associated with depression.
Many modern personality classifications are derived from the theories of Carl Jung, who published the book "Psychological Types" in 1921 in German and 1923 in English. Jung created four categories of personality function: sensation, intuition, thinking and feeling. Each function was modified by a person's tendency to be extraverted (focused on and drawing energy from outside objects and people) or introverted (focused on their own thoughts and solitude). But Jung's personality type theories were based on his own ideas and interpretations of contemporary psychological theories, not on any empirical studies or research.
Teacher and aspiring fiction writer Katharine Cook Briggs had already been working on her own system of personality classification when she read Jung's book, and, as the corporate legend has it, threw her work into the fireplace and started over. She built on Jung's ideas, refined the categories and developed ways to determine which categories people fit best [source: Segovia]. She also stripped away certain aspects of his theory, like his concepts of the unconscious. Briggs worked on her personality theories with her daughter, Isabel Briggs Myers, for decades, writing out questions and categories on index cards [source: Cunningham]. Women's sudden entrance into the workplace during World War II pushed Myers and Briggs to figure out how to use personality type to determine what jobs women might do best or prefer. The test the mother-daughter duo developed eventually became the Myers-Briggs Type Indicator.