well as their genetic diversity. Heritability estimates tend to be higher in samples with greater variability in relevant genetic influences and, conversely, lower in samples that are genetically homogeneous. Because research samples can vary in both their environmental and genetic diversity, a heritability estimate must always be understood as pertaining to observed differences between individuals in a particular sample at a particular time in a specific environment.
Heritability estimates change with development. A characteristic that is highly heritable at one age may not be particularly heritable at another (Lemery and Goldsmith, 1999). There are many reasons for this, including the changes that occur in gene activation with human growth, changes in environmental influences with increasing age, and changes in the nature of a person's engagement with the environment over time. The heritability of variations in general cognitive ability tends to increase with age, for example, as does the heritability of certain behavioral difficulties, such as those associated with antisocial behavior (Goldsmith and Gottesman, 1996; Plomin et al., 1997b). Heritability estimates are thus not consistent over the course of development.
Perhaps most important, heritability estimates describe what is in a particular population at a particular time, rather than what could be (Plomin et al., 1997b). Changes in either genetic influences or environmental influences are likely to alter the relative impact of heredity and environment on individual characteristics. Phenylketonuria is a highly heritable genetic disorder that leads to mental retardation. But with a combination of early detection and environmental interventions, retardation can be completely prevented (Birch et al., 1992). Thus contrary to the common belief that highly heritable characteristics are impervious to environmental modification, interventions that alter the relevant environment—such as educational opportunities, therapeutic support, improved nutrition—can significantly alter the development of that characteristic.
Moreover, it is important to remember that a heritability estimate describes influences on individual differences in a characteristic. Environmental influences can have a profound effect on that characteristic, however, even when heritability is high. During the past century, for example, there have been significant increases in average height owing to improved nutrition and medical care, even though individual differences in height are strongly influenced by heredity. This is because environmental changes (such as improved diet and medical care) have markedly increased average height from one generation to the next, while individual differences in height have remained highly heritable (i.e., smaller parents still have smaller children; see Figure 2-2). In a similar manner, other research (see Chapter 10) indicates that the socioeconomic status of adoptive homes has a powerful effect in elevating the IQ scores of adopted children, even though the