INSTRUCTOR; When we talk about developmental psychology, we're talking about the study of the continuity and change in an individual across that individual's lifespan. And there are four main themes, or issues, that underlie a lot of developmental psychology. The first one is the nature/nurture debate. Developmental psychologists, in general, want to know how our genes or our genetic inheritance, our nature, and our experiences, our nurture, influence who a person is and what that person does. 

And when we talk about nurture and we're talking about environment, we're actually talking about a lot of different things. We're talking about our interactions with parents and friends and family and our community. But we're also talking about our actual physical environment. For example, we know that children who grow up in old houses which were painted with lead paint-- if the child somehow ingests some of that lead paint, it can lead to some poor developmental outcomes. 

But usually, when developmental psychologists are talking about nature and nurture, they're not really talking about each of these things in isolation. We want to know how these two things work together. For example, there's this idea that nurture, our environment, can affect our nature, in the sense that the environment can sometimes influence which genes are expressed. At the same time, our genes might affect what environments we seek out, thereby influencing our nurture. 

Another way that nature and nurture are intertwined actually has nothing to do with genetics and actually has to do with the prenatal environment under which a person develops. So we know that the things that a woman comes in contact with, and the things that she eats, and the things that she does can influence the environment in the womb, and that that can have an effect on later development. 

The second major issue I want to talk about is the idea of continuity versus stages. And so this is asking the question, is development a gradual continuous process, or is it a sequence of separate stages? And this is actually a pretty big debate. 

So a lot of early theorists, like Piaget, and Freud, and Erikson, talked about developmental progression as a series of stages that a child would pass through, each one being defined by a certain skill set or behavior, or a certain way of thinking. And these were things that built on each other. So in order to pass through a later stage, one would have to pass through an earlier stage. Otherwise, they wouldn't have the skills necessary to pass that later stage. 

And just to be clear, these stages are separate from our concept of ages. No one is saying that a child magically picks up a new skill set at age five, instead, they're saying that as time passes, we gain skills that build on the skills that we already have, and that the sequence of those stages, the order that they come in, is far more important than the timing of those stages, or exactly when a child passes through them. 

And while historically, we've always really talked about development in terms of stages, I sometimes wonder how it could be anything but continuous. And maybe it only looks like things develop in stages because of the way we measure them. So for example, we know that height is continuous. We know that people don't simply go from 4 feet tall to 5 feet tall to 6 feet tall-- that there is actually, in fact, a slow progression in between. 

However, if you actually just go and get measured at the doctor's office once here, and you just only have those data points, it would, in fact, look like you hit 4 feet, and then 5 feet, and then 6 feet. And so it would appear stage-like, even though it was actually continuous. And it's entirely possible that development is like, that it only seems like things appear in stages because of the way that we study them. 

And that's not to say anything bad about stage theorists, because the truth of the matter is that it's hard to show that things are gradual scientifically. And as someone who studies kids in the lab, I can tell you that kids don't really tend to show a halfway understanding of things. They're either able to take the perspective of other people or they're not. And maybe it's some kind of limitation with our current research methods, but if there is some kind of middle stage between things like not understanding object permanence and understanding object permanence, we haven't really found it yet. 

The third major theme that I want to talk about is stability versus change. And this is asking which of our traits persists throughout our lifetime, and which ones change as we age. So as developmental psychologists, we don't just want to know how shy children are different from extroverted children. We also want to know whether or not shy children are going to grow up to become shy adults. 

Can we really look at a kid and know what kind of adult they are going to turn out to be? And in some cases, the answer seems to be yes. Some things seem like they are incredibly stable over time. 

And I think that in some way our discussion of the major themes of developmental psychology could stop here, and I think that a lot of textbooks or classes typically do. But I did want to bring up one more theme that seems to pervade most of developmental psychology. And that's maturation versus learning. 

And let me define some of these terms before I talk more about what this means, because in this case, when we're talking about maturation, we're talking about changes in our body or our behavior that result from our natural aging process-- so things that would happen regardless of any life experiences, or illnesses, or injuries. And then we also have learning. And by learning, we're talking about permanent changes in behavior or feelings or thoughts that result from our life experiences broadly defined. 

So what this theme is really focusing on is what causes us to develop? Do children start babbling because that's something that naturally happens at a certain age, or do they begin babbling because they observe mouth movements and sounds coming from individuals around them? And I think that that's a question that you could take both sides on. But then you think about something like crawling. And while babies certainly notice other people moving in their environment, most babies learn how to crawl without ever seeing another baby doing that behavior. 

And one thing that I want to point out about each of these four themes is that even though they're presented as something versus another thing-- as nature versus nurture, or maturation versus learning-- development is rarely ever that simple. And so usually, as psychologists, we're looking at, how does nature and nurture interact? Or how do both maturation and learning lead to a certain behavior? 

So don't feel like you need to, after watching this video, pick a side on certain things that you'll learn later on. Instead, keep these things in mind, and try to think about how these things can interact with each other in order to produce the developmental changes that we see in children and adults.