Understanding the effects during childhood of influences stemming from the environment is especially important for the development of intellectual abilities. Indeed, intellectual abilities as measured by intellectual quotient (IQ) have been shown to be one of the most stable human features and to be associated with a wide ränge of important social outcomes such as employment, marriage, income, life-expectancy, and also behaviors such as antisocial behavior (Herrnstein & Murray, 1994; Gottfredson, 1997). Thus, it is not surprising that experts are still investigating when and how to manipulate the environment to create significant permanent IQ gains. The focus of this chapter is to examine the effect of the environment on intellectual abilities by investigating the impact of a total environmental change on IQ in childhood, using a longitudinal adoption design.
Environmental Influences on Intellectual Abilities
Human characteristics are better understood when they are examined via the interplay between genetic background and psychosocial environments in the context of developmental theories (Hill, 2002). Intellectual abilities are no exception. Authors using various research methods (family, twin, and adoption studies) converge in concluding that intellectual abilities in childhood are accounted for by not only genetic factors, but also by environmental influences (Plomin, DeFries, McClean, & McGuffin, 2001). Early experiences such as parent-child interaction, maternal depression, or child maltreatment have been shown to affect brain biological organization and maturation (DeBellis, 2001). Even in later years, the process of learning has an impact on brain structural organization (see Rutter, 2002). But very few studies have examined environmental influences on childhood intellectual abilities Controlling for potential confounds of genetic factors, in order to disentangle these two types of influences. Genetic and environmental risk factors do not operate independently (Rutter, Pickles, Murray, & Eaves, 2001) and their respective effects have to be pulied apart to fully appreciate their contribution to the development of intellectual abilities.
One exception is a recent twin study using cross-sectional data that showed that adult domestic violence accounted for 4% of the Variation, on average, in children's IQ independently of genetic influences (Koenen, Moffitt, Caspi, Taylor, & Purcell, 2003). This study is the first to show that exposure to violence is linked to an environmental effect on young children's IQ.