[3] Briggs looked at data from studies of contemporary children's educational and social developmental theories.

[2][4][5] However, while investigating the works of various philosophers, scientists and psychologists, she was unable to identify one definitive theory of type that encompassed all aspects.

To create richer characters for her fiction writing, she attempted to understand the details of human personality and behaviors.

[8] In 1945, Katharine and Isabel, with the help of Lyman Briggs, ran the first assessment on George Washington Medical School students.

[9] Keeping in mind her mother's early work, during World War II, Isabel created a test that would help identify a person's appropriate war-related job.

Isabel took over the studies and with hers, her mother's and Jung's observations, was able to initiate the creation of a pencil-paper questionnaire to assess type.

Katharine and Isabel claimed that everyone fits into one of the 16 possible combinations of personality type, with a dominant preference in each of the four pairs.

[9] The MBTI is often criticized as pseudoscience by some who claim that Briggs developed the assessment in her home before doing any extensive scientific research, instead of the other way around.