A recent study summarised here, offers an interesting update on the classic Marshmallow Test.
"Coauthor Richard Aslin said that their findings remind us about how complex human behavior is. "This study is an example of both nature and nurture playing a role," he says. "We know that to some extent, temperament is clearly inherited, because infants differ in their behaviors from birth. But this experiment provides robust evidence that young children's actions are also based on rational decisions about their environment."
The Marshmallow Study Revisited from University of Rochester on YouTube.