The relative contributions of nature (genetic and biological inheritance) and nurture (environmental factors) in developmental processes has been and continues to be debated. Psychologists investigate, by using both longitudinal and cross‐sectional studies of subjects in each developmental period, how both nature and nurture influence behavior. When put together, the study data provide information about changes over the entire life span.
Identical twins are often studied in nature‐nurture investigations (because identical twins develop from one egg, but fraternal twins develop from separate eggs and are genetically no more similar than are any brothers or sisters). Studies have shown that identical twins are more similar in personalities, abilities, and interests than are other siblings, even those identical twins separated at birth and reared apart, a fact that supports the contention that nature (heredity) may be more developmentally important than nurture.