Along with Ainsworth’s study of strange situations, Walter Mischel devised a test, now a classic, to study and understand self-control in toddlers and infants, called the Marshmallow Test. It paved for deeper study into the concepts of instant vs. delayed gratification, self-control seen at a young age as a predicting factor of success in various fields, etc.
The following documentary shows Dr. Philip Zimbardo conducting the Marshmallow Test on a number of children, before discussing the implications of these (which Zimbardo delves into in his book, the Time Paradox):
Lucy J A Kongola and Balaji Niwlikar. (2023, January 6). 16) Walter Mischel: The Marshmallow Test. Careershodh. https://www.careershodh.com/careershodh-series-classical-experiments-16-walter-mischel-the-marshmallow-test/