Developmental psychology is the scientific study of changes that occur in human beings over the course of their life. This field examines change across a broad range of topics, including: motor skills and other psycho-physiological processes; cognitive development involving areas such as problem solving, moral understanding, and conceptual understanding; language acquisition; social, personality, and emotional development; and self-concept and identity formation. This field of psychology examines issues such as the extent of development through gradual accumulation of knowledge versus stage-like development, and the extent to which children are born with innate mental structures, versus learning through experience.
Nature Versus Nurture
A significant issue in developmental psychology is the relationship between innateness of an attribute and environmental effects on that attribute. This is often referred to as "nature versus nurture", or nativism versus empiricism. A nativist ("nature") account of development would argue that the processes in question are innate and specified by an organism's genes. An empiricist ("nurture") perspective would argue that these processes are acquired through interaction with the environment. Today, developmental psychologists rarely take such polarized positions with regard to most aspects of development. Instead, they investigate the relationship between innate and environmental influences.
Natural human behavior is the result of already-present biological factors. Believers of this philosophy often use genetic code as support for the theories. Genetics contain the instructions for millions of protein commands that eventually determine our basic structure as human beings.
Nurtured human behavior is the result of interaction with one's environment, which can provoke changes in brain structure and chemistry. Situations of extreme stress can even cause problems like depression.
Interaction of Genes and the Environment
Heritability refers to the origin of differences among people. Individual development, even of highly heritable traits such as eye color, depends on a range of environmental factors, from the other genes present in the organism to physical variables such as temperature and oxygen levels during development. The variability of a trait can be due to proportions of genetic differences (nature) or environments (nurture). One example is the genetic disorder Huntington's disease, which is solely due to genetic differences. This can be compared to language acquisition, which is environmentally determined. An individual's genes and their environment work together, communicating back and forth to create traits.
Genes interact with signals from other genes, as well as signals from the environment. While there are thousands of single-gene-locus-traits, complex traits are product of the additive effects of many (often hundreds) of small gene effects. For example, height variance appears to be spread across many hundreds of loci.
Some concrete behavioral traits are dependent upon one's environment, home, or culture, such as the language one speaks, the religion one practices, and the political party one supports. However, some traits which reflect underlying talents and temperaments - such as how proficient at a language, how religious, or how liberal or conservative - can be partially heritable.
Environmental inputs can affect the expression of genes. The relationship between genes and the environment, called gene-environment interaction, indicates that individuals with certain genotypes are more likely to find themselves in certain environments. An example of this is a diet low in the amino acid phenylalanine to partially suppress the genetic disease phenylketonuria.
This chart illustrates three patterns one might see when studying the influence of genes and environment on traits in individuals. Trait A shows a high sibling correlation, but little heritability (illustrating the importance of environment); Trait B shows a high heritability, since correlation of trait rises sharply with degree of genetic similarity; and Trait C shows low heritability, but also low correlation generally, meaning that Trait C has a high non-shared environmental variance. This means that the degree to which individuals display Trait C has little to do with either genes or predictable environmental factors.