The marshmallow test is a famous psychological experiment used to measure the development of deferred gratification in children.

The child is given a marshmallow, and told that they are free to eat it BUT that the adult will leave and return in a few minutes, and if they don’t eat it they will receive two marshmallows. In the original experiment, the children who resisted the marshmallow for the bigger reward went on to have much more successful lives.

Subsequently there’s been a lot of debate about the test. One criticism is that what you’re actually testing for is how much the child trusts adults - children who live with trustworthy adults can be secure in the belief that if they resist, they will get the promised reward. Children who cannot trust the adults in their lives rationally choose to eat the single marshmallow rather than risk getting nothing.