Psychologists can use dolls to reveal how we judge true and false beliefs in the world around us. Yet not everybody has this talent. as Jason G Goldman discovers.

One of the most famous lines of research in psychology began with a puppet show.

In a scientific experiment, children were invited to watch a scene featuring two dolls, then asked make a crucial decision about what they saw – see if you make the same choice as they did.

Picture a puppet-doll called Sally, who has a marble. She places it into a basket, and then leaves the room. Next, another doll, Anne, steals the marble from Sally's basket, and moves it into her own. Sally returns, none the wiser about Anne's trickery.

If you watched this charade, where do you think Sally will look for her marble? It seems obvious – her own basket – but not everybody gets the answer right. Some children, and many animals, struggle to imagine Sally’s perspective.

This talent is a signature of “theory of mind” – an ability I have been exploring in my previous two columns, on deception and distinguishing intention from happenstance. We share those two simpler forms of mindreading with other animals – but passing the Sally-Anne test requires far more sophisticated thinking.

Why? You must be able to hold two contradictory ideas at once. First, you must know the true state of the world: the marble is in Anne's basket. Second, you must understand Sally's flawed perspective of the world: Sally believes the marble is still in her own basket. In other words, you must simultaneously maintain both a true belief and a false belief in your mind without confusing the two.

Some would argue that it's a uniquely human skill, and one that is so complex that it doesn't emerge until children reach four years of age. As usual, the story is complicated.

The Sally-Anne experiment, as it has come to be known, was first introduced by autism researchers Simon-Baron Cohen, Alan Leslie, and Uta Frith in a 1985 paper in the journal Cognition. They gave the task to three groups: three- to six-year-old typically developing children, six- to 16-year-old autistic children, and six- to 17-year-old children with Down's syndrome.

All participants understood that the marble had been stolen and was in Anne's basket. But there were major group differences when asked where Sally would look when she returned. While the majority of the typically developing kids and those with Down's syndrome correctly responded that Sally would peek in her own basket, 80% of the autistic children responded incorrectly. And it wasn't that they simply pointed to a random location in space; they all pointed straight at Anne's basket, where the marble truly rested. They seemed to assume that Sally's perspective on the world mirrored their own. The researchers concluded, at least tentatively, that autism in young children involved an inability to represent the mental states of others, at least when those mental states contradict reality.

Research in the intervening decades has refined that conclusion, but the Sally-Anne task and others like it have been used extensively as a clever method for detecting the more sophisticated forms of theory of mind. Most replications of the Sally-Anne task with typically developing children have found that four-year-old children tend to pass the test, but that younger children usually fail.

The eyes have it

Another experiment using the "Smarties task" shows a similar pattern. Children are shown a sweet box labelled "Smarties" and they're asked what's inside. The children all guess there's sweets inside, but the contents are revealed to be pencils. When a puppet named Johnny appears, four-year-old children correctly answer that Johnny will think the box contains Smarties. Younger children say that Johnny thinks the box holds pencils.

Both the Sally-Anne and Smarties tasks, then, suggest that toddlers don’t understand that people can have different beliefs – but could the experiments be flawed? More recently, some researchers have suggested that the tasks may simply be failing to detect more nuanced evidence for theory-of-mind abilities.

To test this possibility, researchers have tracked the eyegaze of young kids during the Sally-Anne tests and found children as young as two years appear to understand false beliefs, at least implicitly. If Sally searches in Anne's basket, the infants stare longer in surprise than when Sally searches in her own basket. This type of response reflects what researchers call "violation of expectation." On some level, these children are aware that Sally shouldn't be looking in Anne's basket – where the marble really is – and should instead be looking in her own basket. They're just not able, for some reason, to translate that awareness into an overt verbal or motor response.

What about animals? Might simpler versions of these experiments reveal false belief abilities in non-human animals, as they did for younger children?

In 2009, Carla Krachun and colleagues conducted a version of the Sally-Anne task with 16 chimpanzees and five bonobos. Rather than using dolls, Sally and Anne were played by human actors. And in place of a marble, the researchers used a bit of food. The apes were encouraged to race Sally to the food in each instance. Yet when Sally returned and deliberately began to search in the wrong place, the apes also tried to grab it from the same location.

Does this mean that this sophisticated form of theory of mind is uniquely human? Probably not. The eyegaze of the apes suggested they knew Sally was looking in the wrong place.

When Sally reached for the wrong box, chimpanzees and bonobos often looked at the other container, where the food really was. It isn't exactly clear just what the apes understood about Sally's mental state, but that inconsistency suggests, as with two-year-old human children, that there is some sense in which chimpanzees and bonobos can represent false beliefs.

Krachun says it’s too early to conclude that understanding false beliefs is an exclusively human ability. For now, it is reasonable to suggest that explicit false belief abilities of the type exhibited by four-year-old human children are exclusive to our species, but more rudimentary forms, such as those displayed by younger kids, are not.

In other words, theory of mind comes in two tiers. The first, simpler form is what is seen in the youngest of human children, and is shared with our non-human neighbours. The more sophisticated variety of theory of mind, the one required to pass the original Sally-Anne task, emerges in humans around their fourth birthday. For now, at least, that appears to be uniquely human.

Read the first two columns in this series on reading minds, exploring deception and distinguishing intention from happenstance.

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