I first heard the expression "theory of mind" from professor Milind Watve when I was visiting IISER Pune in the middle of last year, shortly before I joined here myself. Somehow I don't recall hearing the expression before that. It is not a theory of how the mind works. Rather, it is an ability - the ability to appreciate that other people have a different mind from one's own. Wikipedia defines it as the "ability to attribute mental states - beliefs, intents, desires, pretending, knowledge, etc. - to oneself and others and to understand that others have beliefs, desires, and intentions that are different from one's own."
It's strange that such an obvious ability should even have a name. After all, there is no "theory of body" - the ability to appreciate that other people have a body different from one's own! If I go for a walk I don't assume everyone else is going to do the same, and if I punch someone in the face (true confession: I have never ever hit anyone in my entire life, yes I know it's shameful ;-) ) I don't assume this action will injure my own face.
And yet, when it comes to the mind, this sort of clarity is not universal. On one hand it appears to be present in birds, but on the other, it's strikingly absent in a whole lot of people I know. So perhaps the term "birdbrain" is a little unfair! But let me first tell you about Milind and the birds (I am recounting last year's talk from memory, so it may not be complete accurate). With the help of undergraduate students, he set up and studied the following situation. A particular species of bird lays its eggs in a nest that's designed like a tunnel dug into a hillside. The eggs hatch inside and the bird enters regularly with food for the chicks. However, the entrance of the nest is concealed and the bird would naturally like to keep it that way, to avoid predators enjoying its offspring for dinner. So it never enters the nest when someone is looking. Instead, when it returns home with tasty worm-flavoured desserts it perches at some distance and waits. Only when it judges no one is looking does it enter the nest.
Here's where the bird's "theory of mind" comes in. If a human being hangs around in a place where she can be seen by the bird and can also see the entrance, the bird will simply wait. Only after the human being goes away will it enter the nest. But now suppose that the human stands in a place where she cannot see the entrance of the nest (e.g. her line of sight is blocked by a tree). And suppose the bird can see her, but can also see that she (the human) does not have a view of the nest. Experiments appear to show that in this situation the bird is confident that there is no danger, and will therefore go ahead and enter.
Good for birds, I have to say. But are human beings equally skilled? Experiments show that children below the age of 4 tend to fail the "false belief task" or "Sally-Anne test", a standard probe for the presence of theory of mind. Once they cross that age they tend to pass the test. I don't deal much with children just over 4, but most of the adults I know would not do too well. At least, the people I interact with constantly assume that if they know something, you must know the same thing. They assume that if they have a certain intention then you must have the same intention. They expect that if they believe something, then you believe the same thing.
There's a good side to this: people who lack a sound "theory of mind" tend to give away their secrets easily. In my experience, those who constantly describe others as crooked and corrupt tend to be crooked and corrupt themselves. People who are always suspicious of others are often up to some mischief on their own part. They attribute bad motivations to others because they have bad motivations themselves, and lacking a "theory of mind" they do not realise that others may function differently. So, the next time you meet a suspicious person, ask them to do a better job of hiding their own motivations.