Why Your Three-Year-Old Thinks You Had the Same Dream
Ask a three-year-old where Sally will look for her marble, and they'll get it wrong. Not because they're not paying attention - because they genuinely cannot yet understand that other people know different things.
Try this with a three-year-old. Tell them a story about two girls. Sally has a basket. Anne has a box. Sally puts a marble in her basket and leaves the room. While she's gone, Anne takes the marble out and hides it in her box. Sally comes back. Now ask the child: where will Sally look for the marble?
You and I know the answer immediately. Sally will look in the basket. That's where she put it. She doesn't know Anne moved it. But most children under four will say the box. Because that's where it actually is. They know it's in the box, so obviously Sally knows it too. Everyone knows what they know. Right?
This is the Sally-Anne test, first published in 1985 by Simon Baron-Cohen and colleagues. It measures something psychologists call "theory of mind" - the ability to understand that other people can hold beliefs that are different from yours, and that those beliefs can be wrong. 85% of neurotypical children over four or five pass it. Most children under four do not.
A 2021 meta-analysis of 81 studies covering nearly 8,000 children found a clear link between theory of mind and the ability to lie. Not just to tell a fib, but to maintain one. Before about age eight, children frequently give themselves away. A five-year-old might manage to keep a straight face about having peeked at a toy, then immediately blurt out what the toy was. They can produce the lie but cannot yet manage the other person's beliefs well enough to sustain it.
There is a lovely example of this gap in action. A father described his daughter, around age three, telling him about a dream she'd had. She forgot some of the details halfway through and asked her mum to help finish the story. She could not yet grasp that her dream happened inside her own head. If she experienced it, surely everyone did.
But more recent work from Arizona State University suggests the real shift might happen later than we thought. Four-year-olds who "pass" the Sally-Anne test might be using a simpler shortcut - something like "she didn't see it, so she won't know" - rather than truly understanding that beliefs are mental pictures that can be wrong. Full representational theory of mind, the researchers argue, may not arrive until six or seven.
Either way, what is clear is that it develops gradually, not all at once. And it develops differently depending on what happens around the child.
UK children in one study outperformed peers in Hong Kong on theory of mind tasks, and the gap tracked closely with how much parents narrated thoughts and feelings. "You're feeling frustrated because the tower keeps falling." "I think she's sad because her friend went home." "He doesn't know we're hiding here." That kind of language, used casually and often, actively builds the machinery a child needs to understand other minds.
Research consistently shows that children with stronger theory of mind are rated by teachers as more socially capable. They are better at resolving conflicts, more popular with other children, and happier at school. They engage in more complex pretend play - which makes sense, because pretending requires holding two realities in your head at once.
When your child seems impossibly selfish, refusing to share or ignoring another child's distress, it is worth remembering that they may not yet have the cognitive equipment to see the situation from the other side. They are not being unkind. They are being three.
Reading stories together helps, especially if you pause to ask what a character might be thinking or feeling. Role play helps. Even simple narration of daily life helps - talking about why someone looked sad at the shop, or what grandma might be expecting when she arrives.
The Sally-Anne test is a neat experiment. But the real insight is not about marbles and baskets. It is that understanding other people's minds is not something children are born with. It is something that grows, unevenly and gradually, and the people around them shape how fast and how well it happens.
You and I know the answer immediately. Sally will look in the basket. That's where she put it. She doesn't know Anne moved it. But most children under four will say the box. Because that's where it actually is. They know it's in the box, so obviously Sally knows it too. Everyone knows what they know. Right?
This is the Sally-Anne test, first published in 1985 by Simon Baron-Cohen and colleagues. It measures something psychologists call "theory of mind" - the ability to understand that other people can hold beliefs that are different from yours, and that those beliefs can be wrong. 85% of neurotypical children over four or five pass it. Most children under four do not.
The World Before Other Minds
Think about what it means to not have this yet. If you cannot grasp that someone else might believe something different from what you believe, the world looks very different. You are not being selfish when you refuse to share - you literally cannot model that the other child wants the toy as much as you do. You are not lying when you deny eating the chocolate with chocolate on your face - you may not yet understand that your parent has different information from you.A 2021 meta-analysis of 81 studies covering nearly 8,000 children found a clear link between theory of mind and the ability to lie. Not just to tell a fib, but to maintain one. Before about age eight, children frequently give themselves away. A five-year-old might manage to keep a straight face about having peeked at a toy, then immediately blurt out what the toy was. They can produce the lie but cannot yet manage the other person's beliefs well enough to sustain it.
There is a lovely example of this gap in action. A father described his daughter, around age three, telling him about a dream she'd had. She forgot some of the details halfway through and asked her mum to help finish the story. She could not yet grasp that her dream happened inside her own head. If she experienced it, surely everyone did.
When Does It Click?
The traditional answer has been around age four or five. Research by Wellman and Liu mapped out a consistent developmental sequence. Children first understand that people can want different things (around age two). Then they grasp that people can believe different things. Then comes the understanding that someone who hasn't seen something doesn't know about it. False belief - the Sally-Anne skill - comes after that.But more recent work from Arizona State University suggests the real shift might happen later than we thought. Four-year-olds who "pass" the Sally-Anne test might be using a simpler shortcut - something like "she didn't see it, so she won't know" - rather than truly understanding that beliefs are mental pictures that can be wrong. Full representational theory of mind, the researchers argue, may not arrive until six or seven.
Either way, what is clear is that it develops gradually, not all at once. And it develops differently depending on what happens around the child.
What Parents Can Actually Do
This is the bit that matters most. Cross-cultural research has found that the pace of theory of mind development varies between countries, and the difference is largely explained by one thing: how much parents talk about mental states. Researchers call it "mind-mindedness" - the habit of commenting on what a child might be thinking or feeling rather than just what they're doing.UK children in one study outperformed peers in Hong Kong on theory of mind tasks, and the gap tracked closely with how much parents narrated thoughts and feelings. "You're feeling frustrated because the tower keeps falling." "I think she's sad because her friend went home." "He doesn't know we're hiding here." That kind of language, used casually and often, actively builds the machinery a child needs to understand other minds.
Research consistently shows that children with stronger theory of mind are rated by teachers as more socially capable. They are better at resolving conflicts, more popular with other children, and happier at school. They engage in more complex pretend play - which makes sense, because pretending requires holding two realities in your head at once.
Why This Changes How You React
Knowing about theory of mind changes the way everyday moments land. When your three-year-old tells a blatant lie, the instinct is to correct the dishonesty. But what is actually happening is more interesting than naughty behaviour. They are either not yet able to model your knowledge, or they are making their first clumsy attempts to do so. Both are developmental milestones, not moral failures.When your child seems impossibly selfish, refusing to share or ignoring another child's distress, it is worth remembering that they may not yet have the cognitive equipment to see the situation from the other side. They are not being unkind. They are being three.
Reading stories together helps, especially if you pause to ask what a character might be thinking or feeling. Role play helps. Even simple narration of daily life helps - talking about why someone looked sad at the shop, or what grandma might be expecting when she arrives.
The Sally-Anne test is a neat experiment. But the real insight is not about marbles and baskets. It is that understanding other people's minds is not something children are born with. It is something that grows, unevenly and gradually, and the people around them shape how fast and how well it happens.
Sources & Further Reading