So how do we explain development? Well, there are different and compatible approaches. So one set of explanation appeals to neural changes, changes in the child's brain. We talked about neurons in the first set of lectures. And it turns out that there is changes in the number of neurons that you have through development. Maybe surprisingly the big story of development isn't that the brain becomes more and more interconnected, it isn't more and more connectionary neurons, but rather fewer and fewer. So a lot of what goes on in early brain development is pruning, the getting rid of neural connections. And it's this that somehow makes more advanced, more mature thought possible. Another aspect of brain development is modernization. And what this means is that the myelin sheath around neurons that makes them run faster and more effectively takes time to develop. This develops all the way through adolescence. In fact, more generally parts of the brain are not fully developed until quite late in life. And in particular, the frontal lobes, the part of brain just behind your forehead, takes considerable time to develop and is not fully developed in adolescence, let alone in children or babies. And this can be telling because it could be part of a child's problem has to do with inhibition, which is what the frontal lobes are good at. So my favorite example of this is from the Charlie bit my Finger video is the very beginning of this lecture. So remember that Charlie is chomping down on the kid's finger, and then the kid's yelling and Charlie decides to get the finger out of his mouth. But look at what he does, if it were you, you'd just open up your mouth and stop biting. But it's like Charlie can't. Once he start into biting, he can't stop. And so what he does is, he takes his hand and pulls his brother's finger out of his mouth. And more generally, a lot of the problems with kids behavior might be because they can't inhibit absolute dominant response. And more generally the idea is that neurological differences in how kids cope with different problems, how they answer questions, how they deal with impulses and distractions might lead to a lot of differences between children and adults. But then there's a more general sort of theory compatible of neural development, but different and actually more Piagetian. Which is that Piaget was right, children really do think about the world differently from adults. This explains for instance why they fail a conservation ask. Why they think that when you spread out a bunch of objects, there is now more of them. And when you pour liquid into a different shape plasters now more of it. And now one criticism about Piaget is that there is vagueness as to how do you characterize this difference, what's going on, what changes occurred through development, but people know working around that. So Susan Carey who is now at Harvard, my graduate adviser many many years ago. For instance he's argued that conceptual change in children should be thought of as akin to changes in scientific theory by adult scientists. In fact, she was very influenced in her work by Thomas Kuhn's classic work, The Structure of Scientific Revolutions, which describes the difference between a Newton and an Einstein. Not that the Einstein knows more, not that the physicist in the Einsteinian tradition knows more than the physicist in the Newtonian tradition, but rather they see the world in different ways. The difference between a four year old and an adult in thinking about the physical world should be akin to the difference between a Newtonian physicist and a contemporary physicist in understanding of the world. And this is one dominant, very interesting approach. So developmental psychology is a vibrant field and there's a lot of debates here. And one of the debates is whether this sort of theory is right. So development psychologists who are interested in developmental change are likely to want to contrast to general views. One is that there are large scale changes between childhood and being an adult. This was of course Freud's view as you go through these major psycho-sexual changes. It's also Piaget's view, and it's also the view of people like Susan Carey, who I just talked about or argued that general profound changes in theories of the world. But there's an alternative. An alternative focuses on the idea that the brain contains specific modules, specific cognitive systems with their own developmental trajectory. And we'll talk more about this throughout the course. But the idea of modules is that we have special systems in the head that are specialized for different things. They're specialized for dealing with the physical world, dealing with the social world. And these systems have a lot of innate knowledge, a lot that's built in. And they do develop, but their development is constrained in special ways. Now one important case study for such a modular view is at a languid development. And that's what we'll talk about next lecture. But another one is social development or social understanding. And one of the case studies that has interested a lot of people in a modular conception of development is the case of autism. So autism, many of you are familiar with this, it is a disorder. It's prevalence is debated. There's a lot of controversy over how common it is and how one chooses to define it. But say roughly 1 and 2 out of 1,000. And for autism spectrum disorder, which includes Asperger syndrome and a lot of milder deficits, it's about 10 in 1,000. People with this disability, with this syndrome are often boys. And the marks of it are a lack of social connectedness, language impairment, you treat people like objects and so on. And this has led to the theory that autism is caused by damaged module, is caused by a damaged capacity for a social reasoning. I want to pursue this a little bit, but I should know that there's certain aspects of autism that can't be explained or at least can't be explained easily through this theory. For instance, individuals with autism are often hypersensitive to sounds. They're often obsessed with routines. They engage in stereotypical movements, like they do the same thing over and over again and they like to dance or flap their hands. Sometimes they'll do self-mutilation. If you think about autism in terms of very high functioning autistic people like Temple Grandin, or many celebrities, or scientists are viewed, he or she is autistic. That can be correct characterization, but notice that a lot of people with autism are severely autistic. They don't communicate, they don't speak, they can't make their way into the world by themselves. Anyway, the idea proposed by Simon Baron-Cohen and others, is that autism is caused by damage to a social reasoning module. And what's interesting in support of this is that individuals, even who are otherwise high functioning individuals of autism tend to fail at exactly those tasks that tab social reasoning. So just like you average four years old or your average three years old will fail at salient task, your average adult with autism even if he or she is otherwise pretty high functioning might also fail. Or take this simpler task developed by Simon Barron Cohen. You see a display like this and you're asked, which chocolate will Charlie take? Now for most of us the answer's kind of easy. You'll take the one that he's looking at, most likely. But individuals of autism don't naturally make that sort of inference. So one debate is whether a modular view can best characterize the course of development as opposed to a sort of more large scale theory like that of Piaget or Carey. Another question is if there are modules, what are they? So people talk about a module for understanding a physical world, a naive physics perhaps, and a model for understanding people, a theory of mind or a naive psychology. But people have also proposed modules for understanding physical artifacts or groups of people sort of naive sociology or the biological world. And when developmental psychologists get together and have arguments, those are the sort of things that they're likely to argue about. A third question is that extent there are profound differences between the minds of children and adults, what role does language play? So babies and young children think very differently than older children and adults. And babies and young children don't speak. They don't understand language. But older children and adults do. And one can ask the extent to which learning to speak changes your understanding of the world. And this is so interesting when I put it off until next lecture on language. So finally, I want to end by asking a question, who cares about development? So suppose these issues simply do not rock your bone. You dont care about imparecism or Ananias, you don't care about continuity or Piaget or object permanence. You just don't even care about children, you're just interested in adults. Still, it turns out that a lot of questions about adults, about adult psychology can be addressed through the study of children. And I want to give you what I think is a wonderful example by the late paleontologist, Stephen Jay Gould, about zebras. So here's a question about zebras, about adult zebras. Are zebras white animals with black stripes or they black animals with white stripes. This kind of a cool question. I have actually done the foggiest idea what the answer is, but I know people do know the answer. And the way they know the answer is by studies of development. You could see through studies of embryonic development and development when zebras are young, what the original coloring of the zebra is and whether these are black stripes or whether they're white stripes. More generally, we should end by considering the wisdom of the great biologist, Darcy Thompson, who in a plea for understanding importance of development said, everything is the way it is because it got that way.