In the famous marshmallow test, a child gets one marshmallow. If the child waits, they get two. Early studies said children who waited did better at school later.

But in 2018, researchers studied about 900 children. The link was only about half as strong. When they also looked at the children's families, the link became much smaller. A child's home may matter a lot.

In the marshmallow test, a child can eat one marshmallow now or wait to get two. Studies of Stanford children tested in the 1960s and 1970s suggested that children who waited longer later had better school results.

However, those children mostly came from one university preschool. In 2018, researchers used data from about 900 children across the US. The link between waiting at age 4 and school achievement at age 15 was about half as strong as before. After considering family income and early language skills, it shrank by about two-thirds. Another study found that children waited much longer when an adult had kept a promise to them. Waiting may depend on trust, not just willpower.

A preschooler is offered a single marshmallow. She can eat it now, or wait and receive two. Follow-up studies of the Stanford children who took this test found that longer waiting predicted higher test scores and better coping in adolescence.

But those children came largely from a single university preschool, and only a limited number were tracked. In 2018, researchers ran a conceptual replication with a multi-site dataset of about 900 children, most of whose mothers had not completed college. The link between waiting at age 4 and achievement at 15 was roughly half as strong as originally reported. Once the team controlled for family background and early cognitive skills, it fell by about two-thirds, and links with teenage behavior problems were weaker still.

Some critics argue that such controls strip out part of the genuine effect. A 2013 experiment with only 28 children looked at trust. When an adult had kept an earlier promise, children waited about 12 minutes on average. After a broken promise, they waited about 3. For a child who has learned not to rely on adults, eating the marshmallow quickly may be a rational choice rather than a sign of weak willpower.