A broken promise can change a child’s marshmallow decision.

Waiting depends on what the child expects

A delayed reward is valuable only if it is likely to arrive. For a child in the marshmallow task, waiting means giving up something certain and available now in exchange for something larger but postponed. Whether that bargain makes sense depends not just on self-control, but on the child’s estimate of the adult making it.

If an adult has followed through before, the promised second treat may seem credible. If the adult has broken a promise, eating the marshmallow now can be a sensible response to uncertainty—not simply a failure to resist temptation.

The task contains two judgments

The child has to manage an immediate desire while also deciding how much to trust the situation. Those judgments interact: the more dependable the promised reward seems, the more worthwhile it may be to endure the wait. An adult’s earlier behavior can therefore change the difficulty of the same apparent choice without changing the treat or the child’s appetite.

That is why the task can reveal more than one thing. It can show how a child responds to temptation, but also how expectations about other people shape decisions. Waiting is not pure willpower when the reward depends on someone else keeping a promise.

What the marshmallow can—and cannot—show

A child who takes the treat quickly may be responding to the immediate temptation, to doubt that the later reward will come, or to both. The choice alone cannot tell an observer which consideration mattered most. Nor does a single moment establish a child’s character: the decision is made in a particular setting, with a particular adult and a particular promise.

The detail that changes the interpretation is simple: the future reward is not guaranteed by the child’s restraint. It also depends on whether the adult has earned the child’s confidence.