Subtitles section Play video Print subtitles What is going on in this baby's mind? If you'd asked people this 30 years ago, most people, including psychologists, would have said that this baby was irrational, illogical, egocentric -- that he couldn't take the perspective of another person or understand cause and effect. In the last 20 years, developmental science has completely overturned that picture. So in some ways, we think that this baby's thinking is like the thinking of the most brilliant scientists. Let me give you just one example of this. One thing that this baby could be thinking about, that could be going on in his mind, is trying to figure out what's going on in the mind of that other baby. After all, one of the things that's hardest for all of us to do is to figure out what other people are thinking and feeling. And maybe the hardest thing of all is to figure out that what other people think and feel isn't actually exactly like what we think and feel. Anyone who's followed politics can testify to how hard that is for some people to get. We wanted to know if babies and young children could understand this really profound thing about other people. Now the question is: How could we ask them? Babies, after all, can't talk, and if you ask a three year-old to tell you what he thinks, what you'll get is a beautiful stream of consciousness monologue about ponies and birthdays and things like that. So how do we actually ask them the question? Well it turns out that the secret was broccoli. What we did -- Betty Rapacholi, who was one of my students, and I -- was actually to give the babies two bowls of food: one bowl of raw broccoli and one bowl of delicious goldfish crackers. Now all of the babies, even in Berkley, like the crackers and don't like the raw broccoli. (Laughter) But then what Betty did was to take a little taste of food from each bowl. And she would act as if she liked it or she didn't. So half the time, she acted as if she liked the crackers and didn't like the broccoli -- just like a baby and any other sane person. But half the time, what she would do is take a little bit of the broccoli and go, "Mmmmm, broccoli. I tasted the broccoli. Mmmmm." And then she would take a little bit of the crackers, and she'd go, "Eww, yuck, crackers. I tasted the crackers. Eww, yuck." So she'd act as if what she wanted was just the opposite of what the babies wanted. We did this with 15 and 18 month-old babies. And then she would simply put her hand out and say, "Can you give me some?" So the question is: What would the baby give her, what they liked or what she liked? And the remarkable thing was that 18 month-old babies, just barely walking and talking, would give her the crackers if she liked the crackers, but they would give her the broccoli if she liked the broccoli. On the other hand, 15 month-olds would stare at her for a long time if she acted as if she liked the broccoli, like they couldn't figure this out. But then after they stared for a long time, they would just give her the crackers, what they thought everybody must like. So there are two really remarkable things about this. The first one is that these little 18 month-old babies have already discovered this really profound fact about human nature, that we don't always want the same thing. And what's more, they felt that they should actually do things to help other people get what they wanted. Even more remarkably though, the fact that 15 month-olds didn't do this suggests that these 18 month-olds had learned this deep, profound fact about human nature in the three months from when they were 15 months old. So children both know more and learn more than we ever would have thought. And this is just one of hundreds and hundreds of studies over the last 20 years that's actually demonstrated it. The question you might ask though is: Why do children learn so much? And how is it possible for them to learn so much in such a short time? I mean, after all, if you look at babies superficially, they seem pretty useless. And actually in many ways, they're worse than useless, because we have to put so much time and energy into just keeping them alive. But if we turn to evolution for an answer to this puzzle of why we spend so much time taking care of useless babies, it turns out that there's actually an answer. If we look across many, many different species of animals, not just us primates, but also including other mammals, birds, even marsupials like kangaroos and wombats, it turns out that there's a relationship between how long a childhood a species has and how big their brains are compared to their bodies and how smart and flexible they are. And sort of the posterbirds for this idea are the birds up there. On one side is a New Caledonian crow. And crows and other corvidae, ravens, rooks and so forth, are incredibly smart birds. They're as smart as chimpanzees in some respects. And this is a bird on the cover of science who's learned how to use a tool to get food. On the other hand, we have our friend the domestic chicken. And chickens and ducks and geese and turkeys are basically as dumb as dumps. So they're very, very good at pecking for grain, and they're not much good at doing anything else. Well it turns out that the babies, the New Caledonian crow babies, are fledglings. They depend on their moms to drop worms in their little open mouths for as long as two years, which is a really long time in the life of a bird. Whereas the chickens are actually mature within a couple of months. So childhood is the reason why the crows end up on the cover of Science and the chickens end up in the soup pot. There's something about that long childhood that seems to be connected to knowledge and learning. Well what kind of explanation could we have for this? Well some animals, like the chicken, seem to be beautifully suited to doing just one thing very well. So they seem to be beautifully suited to pecking grain in one environment. Other creatures, like the crows, aren't very good at doing anything in particular, but they're extremely good at learning about laws of different environments. And of course, we human beings are way out on the end of the distribution like the crows. We have bigger brains relative to our bodies by far than any other animal. We're smarter, we're more flexible, we can learn more, we survive in more different environments, we migrated to cover the world and even go to outer space. And our babies and children are dependent on us for much longer than the babies of any other species. My son is 23. (Laughter) And at least until they're 23, we're still popping those worms into those little open mouths. All right, why would we see this correlation? Well an idea is that that strategy, that learning strategy, is an extremely powerful, great strategy for getting on in the world, but it has one big disadvantage. And that one big disadvantage is that, until you actually do all that learning, you're going to be helpless. So you don't want to have the mastodon charging at you and be saying to yourself, "A slingshot or maybe a spear might work. Which would actually be better?" You want to know all that before the mastodons actually show up. And the way the evolutions seems to have solved that problem is with a kind of division of labor. So the idea is that we have this early period when we're completely protected. We don't have to do anything. All we have to do is learn. And then as adults, we can take all those things that we learned when we were babies and children and actually put them to work to do things out there in the world. So one way of thinking about it is that babies and young children are like the research and development division of the human species. So they're the protected blue sky guys who just have to go out and learn and have good ideas, and we're production and marketing. We have to take all those ideas that we learned when we were children and actually put them to use. Another way of thinking about it is instead of thinking of babies and children as being like defective grownups, we should think about them as being a different developmental stage of the same species -- kind of like caterpillars and butterflies -- except that they're actually the brilliant butterflies who are flitting around the garden and exploring, and we're the caterpillars who are inching along our narrow, grownup, adult path. If this is true, if these babies are designed to learn -- and this evolutionary story would say children are for learning, that's what they're for -- we might expect that they would have really powerful learning mechanisms. And in fact, the baby's brain seems to be the most powerful learning computer on the planet. But real computers are actually getting to be a lot better. And there's been a revolution in our understanding of machine learning recently. And it all depends on the ideas of this guy, the Reverend Thomas Bayes, who was a statistician and mathematician in the 18th century. And essentially what Bayes did was to provide a mathematical way using probability theory to characterize, describe, the way that scientists find out about the world. So what scientists do is they have a hypothesis that they think might be likely to start with. They go out and test it against the evidence. The evidence makes them change that hypothesis. Then they test that new hypothesis and so on and so forth. And what Bayes showed was a mathematical way that you could do that. And that mathematics is at the core of the best machine learning programs that we have now. And some 10 years ago, I suggested that babies might be doing the same thing. So if you want to know what's going on underneath those beautiful brown eyes, I think it actually looks something like this. This is Reverend Bayes's notebook. So I think those babies are actually making complicated calculations with conditional probabilities that they're revising to figure out how the world works. All right, now that might seem like an even taller order to actually demonstrate. Because after all, if you ask even grownups about statistics, they look extremely stupid. How could it be that children are doing statistics? So to test this we used a machine that we have called the Blicket Detector. This is a box that lights up and plays music when you put some things on it and not others. And using this very simple machine, my lab and others have done dozens of studies showing just how good babies are at learning about the world. Let me mention just one that we did with Tumar Kushner, my student.