William Emet Blatz (/ˈblæts/; June 30, 1895 – November 1, 1964) was a German-Canadian developmental psychologist who was director of the University of Toronto's Institute of Child Study from 1925 until his retirement in 1960.

Blatz's theory posited that in infancy and early childhood, the child needs to create a secure base with its caregivers in order to gain the courage necessary to brave the insecurity implicit in exploring the world, and argued that a lack of psychological resilience and self-confidence in adulthood are born out of a failure to develop a secure base in childhood.

[1] His father, Leo Victor Blatz, had moved to Canada from Würzburg in 1868, while his mother, Victoria Mary Mesmer—a relation to Franz Mesmer, arrived from Mainz in 1870.

He earned his MA the following year, then attempted to join the Canadian Armed Forces but was rejected due to his German lineage and instead returned to the University of Toronto to treat shell shocked veterans along with Dr. Edward A. Bott.

In the course of a polite conversation, Blatz would press a button causing the chair to collapse and the volunteer would fall to the floor onto the base of their spine.

In 1946, he advised the Canadian Government to set standards and regulations for nurseries, and married his second wife, Annie Louisa Barnard Harris.

Thus, Blatz's security theory was aimed at emancipation and the development of responsibility; it emphasized the importance of gradually increasing the child's freedom to make decisions independently, to experience consequences—both successes and failures, and to acquire effective ways of coping with those consequences.