[The basic structure and ideas for this post come from a developmental seminar I'm taking with Dr. Bennett Bertenthal at Indiana University. I'm sharing these concepts more broadly because not everyone has access to a class like this, but anyone interested in child development can benefit from understanding the thinking style and assumptions of the people who research it.]
A developmental psychologist is someone who researches how people's minds change over their lifetime. Most study babies, children, or adolescents, but some focus on old age, and they could also investigate parenthood, middle age, or emerging adulthood. Developmental psychologists care about life stages, how we change as we transition from one to the next, how we change within a life stage, and conversely, what about us stays the same as we move from one stage to another.
Developmental psychologists are concerned with processes of continuity and change rather than in particular things the human mind does. In this respect, they are different from some other sorts of psychologists, who are defined by the functions of the human mind they choose to research. (I.e., cognitive psychologists study thought and perception, personality psychologists study personality, and social psychologists study group behavior and influences). Developmental psychology, as a field, is concerned with all these areas of the human mind. Even a developmental psychologist who focuses on cognitive psychology topics--as I do--will have some familiarity with personality and social development.
First, I'm going to lay out some assumptions developmental psychologists make. Then I'll list some big questions they like to ask.
1) Gene Environment Interactions
While the nature-nurture debate is at least as intense among developmental psychologists as elsewhere, they have a unique perspective on it. They argue that you cannot explain human behavior with only genes or only experiences. Instead, they come together in a complex way, with different results than you would get from genes or environment alone. They claim that the interaction between genes and environment resembles that between vinegar and baking soda. Vinegar and baking soda are each inert, but come together to make an explosive reaction. Similarly, genes and environment come together to create an outcome--like personality traits or intelligence--that neither would have produced alone.
The least controversial interaction is probably height. A large amount of variation in people's heights is genetically determined; tall people tend to have tall children, short people tend to have short children, and siblings tend to have similar heights. However, nutrition determines whether people will grow as tall as their genes permit them to be. For this reason, my grandparents were taller than my great-grandparents, and my parents were taller than my grandparents (and the same will likely be true for you, as well). However, improvements in nutrition seem to have plateaued, and so has height; my generation (millenials) is the first in some time not to exceed their own parents' height. Notice that the genetic relationships here (parent to child) are constant across the generations from your great-grandparents to yourself, but differences in environment (nutrition) produce large differences in height.
More complicated and controversial are theories like the Orchid Hypothesis, which posits that different people are differentially reactive to their environments (whether these are good or bad). As far as I know, this theory is still new and not completely accepted, but it's based on research on stress and resilience that is widely accepted. It's pretty well known that some children who have suffered abuse and neglect will have worse life outcomes than others, and that one factor affecting this is differences in specific genes.
2) Developmental Trajectory
You don't have to be a developmental psychologist to notice that different individuals develop at slightly different rates. For example, some kids are early talkers and readers and remain ahead of their peers in language skills; other children are slower than their peers in developing language and reading skills. Some kids are taller than their peers from an early age, and maintain this status over time, while others start out short and remain that way. More interesting than that, though, are children who start out behind their peers in a skill and come out ahead, or vice versa. For example, Einstein, though a late talker, developed perfectly adequate speaking, reading, and writing skills by adulthood, and some late-talking children today follow a similar pattern. Meanwhile, some children with precocious academic skills and high IQ scores in preschool, kindergarten, or first grade, may perform more like their peers by third grade (for this reason, experts on gifted children tend to recommend getting one's children IQ-tested at 6 or 7 years old). Children's rate of development of a skill can change, both relative to themselves at earlier ages, and relative to peers of the same age. Basically, when developmental psychologists think about growth, they imagine a line graph, where the steepness of the slope of the line represents the speed of development, and changes in the slope represent changes in the rate of development over time.
Developmental trajectory is especially interesting in two cases: when comparing typical with atypical development, and when comparing outcomes for different individual children.
For example, language development often follows a different trajectory in autism than in typical development. Speech is often delayed. Also, the rate of growth may seem to slow down for a while, stop entirely (what developmental psychologists call a "plateau"), or even reverse ("regression" or loss of language). On the other hand, language development may continue longer in autistic people than in neurotypical peers, with language skills sometimes improving into adulthood. And of course, since autism embraces people with a wide range of characteristics, you will find autistic people with pretty much every imaginable trajectory of language development. There have been lots of recent studies that attempt to find subgroups of autistic children with different trajectories, in the hopes of predicting who will have the best language outcomes, and why.
Developmental trajectory is also important when comparing different individuals from the same population. For example, some late talkers eventually catch up with their peers in spoken vocabulary, while others do not. Some developmental psychologists spend a lot of time trying to figure out why these children differ, and what can be done to help the persistently-delayed group catch up.
3) Developmental Cascades
While people can and do grow and change throughout their lives, earlier experiences profoundly shape our abilities and choices later on. The influence of earlier upon later development is called a "developmental cascade." A better term would probably be "developmental avalanche."
For example, let's say you're looking at vocabulary size from age 3 to age 5.
Age 3 vocabulary size has an effect on age 4 vocabulary size.
Age 4 vocabulary size has an effect on age 5 vocabulary size.
Age 3 vocabulary has an additional effect on age 5 vocabulary size.
So you have a sort of snowball where initial vocabulary has both direct influences and indirect ones, via vocabulary at intermediate ages. It's like a small snowball that hits more snow and becomes a bigger snowball, which hits more snow and becomes an even bigger snowball, and so on. Eventually, small differences between people early on can lead to big differences later on.
4) 2-Way Interaction between child and environment
Children aren't just shaped by their environment. They can act in different ways, and their behavior in turn shapes the input they get from their environment. For example, a child who is shy from infancy will be treated differently than a child who is outgoing from infancy. They may be reproached, or gently encouraged to interact, or pushed hard to interact, or shamed into interacting, depending on their parents' parenting style and values. This in turn will shape how the child behaves around other people, and whether they become painfully shy and retiring or quietly confident adults. A child who has been told from an early age that they're smart will probably think of themselves differently, and take different levels of risk in the classroom, than a child who has been told that they're just average, or even dumb. I'm sure you can think of many more everyday examples.
While the role of children in shaping their environment seems obvious when pointed out, it's very different from how your average parenting book describes children1. Too often, the paradigm seems to be "push the right button, receive the desired behavior;" there is little focus on the children's reasons for their behavior (good or bad), or on how the children might be pushing the parents' buttons and triggering their own insecurities about parenting or other issues. Not surprisingly, many of these books aren't written by developmental psychologists.
The four assumptions listed here lead cognitive psychologists to ask a certain set of questions.
Questions Developmental Psychologists Ask:
1) Are some capabilities innate? If so, which ones?
William James pointed out that at any given moment, there are so many shapes, colors, sounds, textures, smells, temperatures, and more that without any inborn means to sort them out, a baby's world would seem like a "blooming, buzzing confusion." I think most developmental psychologists accept that at the very least, babies are born with some basic learning mechanisms and an inclination to observe and learn about the world. But they differ on how much "software" babies come with. Some people think we're born with (tacit) knowledge of all the grammatical rules of human language, a basic understanding of how objects move (e.g., that objects fall), and/or a set of basic concepts about other people (e.g., that they have minds and intentions). Others think that we develop these concepts early in life, but aren't born with them. This debate has led to a lot of interesting research on what babies understand about people, things, quantities, and more, and is far from being resolved.
2) Are there developmental stages, and if so, how do people transition between them?
Piaget thought there were certain qualitatively different ways of thinking that everyone progressed through in a certain order at roughly the same age, and that was consistent across domains of knowledge. (I.e., if you are at the concrete operational stage in thinking about the movement of objects, then you must also be at the concrete operational stage in thinking about other people's behavior). This is a fairly extreme stage theory. His successors, the Neo-Piagetians, were a little more flexible, particularly regarding different domains of knowledge and individual differences. However, they still thought that development has discrete steps, like a staircase, rather than continuity throughout, like a wheelchair ramp. Whether a particular study seems to provide support for stage-like or continuous development seems to depend whether it uses continuous or discontinuous measures of the behavior in question, so this question is also far from being resolved.
3) How do individuals differ in their development?
I think this is fairly self-explanatory.
4) How do changes in the brain contribute to development?
This question is similarly easy to understand--but it's even harder to answer in kids than it is in adults.
5) What develops, and how does change occur?
Let's say that last year, Anna didn't understand conservation of matter, but this year she does, and can pass a Piagetian conservation of matter task. How exactly is she thinking differently now than she did last year? How did she get from the understanding she had last year to the one she has this year? This is a very difficult and abstract question, is probably the most central question in developmental psychology, and is also probably the hardest to resolve.
6) How does the social world contribute to development?
We are constantly observing, imitating, and listening to explicit teaching from other people. We grow up in cultures that provide us with tools for thinking like language, writing, the abacus, or the internet. Our cultures also determine how we spend our time at different ages, and whether we spend our time more with age peers or with people of all ages. We interact with various institutions either directly or indirectly, including schools, churches, and governments. We are assigned to categories of age, gender, ethnicity, religion, and more, all of which come with messages about how a person within our category "should" and "should not" behave. We also (in general) have innate desires to learn from and connect emotionally with other people, and get them to like us. All these things shape both what we experience and how we choose to behave.
So next time you talk to a developmental psychologist or read about a developmental study, know that development is all about change--and change is a complicated mass of factors that changes over time and differs between individuals. Their goal is to sort out that complex system.
1 I read parenting books from about 1995, when my brother was a fetus, to about 2008, when I got too busy with college to read them. So it's quite possible I've missed books from before or after these dates that take a better approach. (And for that matter, I'm glossing over a few exceptional parenting books that appeared during this time, like The Heart of Parenting, which is based on the concept of emotional intelligence, and involves helping kids recognize and verbalize their emotions).