What are the forces that govern or drive the processes, characteristics, and behaviors that develop across childhood? Basically, these forces are a combination of nature and nurture. The term nature refers to the biological forces that govern development. To a certain extent our development is programmed by the genetic codes we inherit. This biological program unfolds throughout childhood. In some respects, child development can be compared to the blossoming of a flower: A seed sprouts, grows into a fragile seedling, and eventually becomes a mature flowering plant. Nature provides the genetic program contained in the seed—and in the child. By now you know that genes influence the color of your eyes, but did you also know they play a role in determining your height and weight, your level of intelligence, and your basic personality?

Nurture refers to the environmental conditions and supports that influence development. A plant needs sunlight, water, and the proper temperature to grow—and it helps if someone pulls the surrounding weeds and adds fertilizer. Children also need to be nurtured: they need love and support from parents, siblings, extended family, teachers, peers, and other people important in their lives. Children can be greatly affected by how these important people nurture them.

Other elements of nurture include a child's economic and sociocultural environments. Poverty, malnutrition, and a lack of adequate medical care can alter a child's developmental path. Cultural heritage and diversity can enrich a child's life, and the neighborhood where the child lives can determine the schools and peer groups that a child will have.

Throughout history, philosophers and scientists have debated the relative roles of nature and nurture. John Watson, a renowned American psychologist of the early twentieth century, was a strong proponent of the nurture school. He wrote:

[G]ive me a dozen healthy infants, well formed, and my own specified world to bring them up and I'll guarantee to take anyone at random and train him to become any type of specialist I might select—doctor, lawyer, artist, merchant-chief and, yes, even beggarman and thief, regardless of his talents, penchants, tendencies, abilities, vocations, and ancestors (1930/1924, p. 104).

Watson argued that experience and learning—nurture—determined what children would become. But other researchers have disagreed, pointing out that characteristics such as personality are determined more by genetics (nature) than by nurture. Today, however, developmental scientists understand that nature and nurture work together and it is impossible to distinguish their separate effects (Lerner, 2006; Rutter, 2002). Rather than arguing about which one is most important, we are interested in learning exactly how the two factors interact with each other. The interacting effects of nature and nurture are evident in the field of behavior genetics.

In behavior genetics, researchers study the relative roles of nature and nurture in development. By studying twins and adopted children, behavior geneticists have been able to estimate that variability in psychological traits and behaviors, including intelligence, emotionality, and basic personality variables, are approximately 40% to 60% due to variability in genetic inheritance. For example, researchers have shown that IQ scores of identical twins are much more similar than scores of fraternal twins (Bouchard & McGue, 1981). Because identical twins come from the same fertilized egg, their genes are exact copies (give or take a few errors in cell division). However, fraternal twins come from separate fertilized eggs, so they are no more alike genetically than any other sibling pairs. Given the fact that both identical and fraternal twins tend to share similar family and learning environments, scientists attribute the greater similarity in IQ between identical twins to their greater genetic similarity.

Adoption studies also show a strong genetic component in IQ. Studies of children who were adopted as young infants have shown that these children's IQs at age 18 are more similar to the IQs of their biological mothers than to the IQs of the adoptive mothers who nurtured them (Loehlin, Horn, & Willerman, 1994; Scarr, Weinberg, & Waldman, 1993).

The impact of nature, or genetic inheritance, is obvious when children are born with genetic disorders such as Down syndrome or cystic fibrosis. Research also has shown genetic links to conditions such as schizophrenia, depression, and severe obesity. The role of nurture, however, also emerges in many of these same studies. For example, inheriting genes linked to schizophrenia, depression, or obesity does not guarantee that a person will actually develop the condition. Many people who inherit these genes do not develop the condition. Nurture certainly also has an impact on traits such as IQ. IQ scores for identical twins, for example, are more similar when the twins grow up together than when they are raised in separate families (Bouchard & McGue, 1981). Both identical and fraternal twins show similar patterns of emotional attachment to their parents, indicating that shared family experiences (nurture) play a larger role than genetics (nature) in establishing attachment patterns (O'Connor & Croft, 2001). Rather than arguing about whether development is controlled completely by nature or completely by nurture, thanks to the results of behavior genetics research, we are now beginning to understand the interactive roles played by both of these forces.

Children are influenced by nature and by nurture, but children also influence their own development. To some extent, children are free to make their own choices, and these choices can have consequences for development. When parents ask children to clean their room, for example, children can choose to obey or not. Their choices can then influence how their parents respond to them. And think of the larger choices children make as they grow up: whether to fight with bullies or find peaceful resolutions, whether to try drink\king alcohol or abstain, whether to drop out of school or continue on to college. Choices such as these can be influenced to some extent by the basic personality characteristics the child inherits (nature) and by the supports and pressures in the child's life (nurture), but they are also often made by the child's own freedom to choose different options. And the child's choices can then influence how other people respond back to them resulting in a reciprocal relationship—people influencing children and their development, and children making choices and influencing other people. It goes both ways.