The marshmallow test offers a child one treat now or two if they wait. A famous 1990 study found that children who waited longer tended to do better as teenagers, and this became a popular story that childhood willpower predicts success.
In 2018, Tyler Watts, Greg Duncan, and Haonan Quan revisited the idea with about 900 children, a larger and more diverse group. They found the link between waiting and later achievement was about half as strong as the original. It shrank by roughly two-thirds once they accounted for family background and early thinking skills. Among children whose mothers had not finished college, an extra minute of waiting predicted only a small later gain, with little link to behavior.
The likely reason is that waiting reflects a child's home and resources as much as willpower. A child from a more secure home may trust more easily that the promised second treat will appear. The study is a clear example of why correlation is not causation: waiting and later success can rise together because a third factor, family circumstances, shapes both.