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):