Four-year-olds can pay attention. They do have difficulty changing the focus of their attention in response to arbitrary commands. But recent studies show that even babies systematically direct their focus to the events and objects that will teach them the most. They pay special attention to events that contradict what they already believe. Mr. Trump refuses to pay attention to anything that clashes with his preconceptions.

Four-year-olds understand the difference between fantasy and reality. They certainly enjoy pretend play, imagining that the world is full of villains and that they are all-powerful heroes. But studies show that they know they are pretending and understand that their imaginary companions are just that: imaginary. Mr. Trump seems to have no sense of the boundary between his self-aggrandizing fantasies and reality.

Four-year-olds have a “theory of mind,” an understanding of their own minds and those of others. In my lab we have found that 4-year-olds recognize that their own past beliefs might have been wrong. Mr. Trump contradicts himself without hesitation and doesn’t seem to recognize any conflict between his past and present beliefs.

Four-year-olds, contrary to popular belief, are not egocentric or self-centered. They understand and care about how other people feel and think, and recognize that other people can feel and think differently from them. In my lab, which studies the cognitive development of children, we have found that even 1½-year-olds can understand that someone else might want something different from what they want. They understand that someone else might like broccoli, even though they themselves prefer crackers, and they will help that person get what he wants.