The thesis of The Development of Logic in Adulthood is expressed in the author's phrase "logic of wisdom." This quality, Jan Sinnott argues, is the result of a maturing process in an individual's thinking, the refinement of decision making and interaction with others that develops over a lifetime. Adults develop complex thinking - or "postformal thought," to use the author's more technical term - out of a strong desire for unity and stability, and in recognition of the shortness of life.

Sinnott maintains that the adult person can proactively cultivate the process of refining her or his thinking over time.

She shows how this means of systematically combating alienation is applicable to many facets of a person's life: to demanding family and couple relationships; to social roles, which are made more flexible and adaptable; to workplace environments in which individuals use work to grow personally, and where concomitant living helps to resolve interpersonal conflicts; and to the fast-growing field of adult education, where active learning flourishes under teachers who employ the logic of wisdom.

The author describes various techniques derived from her experimental studies carried out over a period of more than 20 years and explores the practical application of these methods in such areas as communication and conflict resolution, decision making, cross-cultural interaction, conscious spiritual development, ecology, and educational reform.