What is theory of mind, and when does it develop?

The understanding that other people have their own beliefs, knowledge and intentions, which may differ from yours and may be wrong. Its arrival transforms a child's social world, and its absence explains a great deal of otherwise baffling behaviour in young children.

The classic test. In the false-belief task, a child watches while a doll puts a marble in a basket and leaves. Another character moves it to a box. The child is asked where the doll will look for it. Children under about four reliably say "the box" — where the marble actually is — because they cannot yet separate what they know from what the doll knows. From around four to five, most say "the basket", correctly attributing a false belief.

The developmental sequence is gradual rather than sudden:

Infancy — following gaze, joint attention, checking a parent's face to judge a new situation.

Around two — understanding that others have different desires, even if not yet different beliefs.

Three to four — understanding that people know different things depending on what they have seen.

Four to five — false belief, the classic threshold.

Six and beyond — second-order belief (what one person thinks another thinks), then sarcasm, irony, white lies, and the social complexity of the school years.

What it explains practically:

Hiding badly. A toddler covering their eyes genuinely reasons that if they cannot see you, you cannot see them.

Poor storytelling, where a child omits everything you would need to follow the events, because they assume you already know what they know.

Phone calls, where a young child nods silently.

Unconvincing lies, which require modelling what another person believes — and the ability to lie effectively is therefore a developmental milestone.

Apparent selfishness in sharing, which is frequently a cognitive limitation rather than a moral one.

Variation is normal. Development is influenced by conversation about mental states, pretend play, and having siblings. Differences are also seen in autistic children, although the older framing of this as a simple deficit has been substantially revised — see the double empathy problem.