IV. Genetic and Environmental Influences on Intelligence
The origins of IQ have been in dispute since its inception. Prior to the 1960s, researchers were influenced strongly by hereditarianism, or the belief that human traits can be transmitted from parents to offspring through their genes. This perspective fell into disfavor in the 1960s and remained a politically incorrect research topic through the 1990s. Advancements in the genetic sciences at the turn of the 21st century, however, ushered in a new understanding of the origins of IQ.
No other discipline has done as much to inform us about the origins of IQ as has behavioral genetics. Behavioral genetics researchers use a variety of complex methods, including the use of large-scale twin studies, to dissect human behavior and traits into three main components: (1) the proportion of the variance in IQ associated with genes, (2) the proportion of variance in IQ attributable to environments that are similar for all family members (i.e., shared environments), and (3) the proportion of variance in IQ accounted for by environmental influences unique to individual family members (i.e., nonshared environments).
In the study of intelligence, examinations of identical (monozygotic [MZ]) and fraternal (dizygotic) twins are preferred, because they allow researchers to estimate the degree of heritability in complex traits. Heritability refers to the amount of variance in a trait or behavior—in this case, IQ—that is accounted for by genetic influences. Researchers use twin data because identical twins share approximately 100% of their DNA, whereas fraternal twins share only about 50% of their genetic makeup.
If IQ is 100% heritable, then MZ twins would be concordant on measures of IQ—that is, they would score roughly the same. IQ scores would, however, be less concordant between fraternal twins and should be uncorrelated between individuals chosen at random. Conversely, if environmental variables are responsible for IQ differences between individuals, then estimates of heritability should be reduced substantially, and they should not follow the patterns expected by genetic theory (i.e., with MZ twins correlating higher than dizygotic twins).
Numerous behavioral genetic studies have shown that, on average, genetic influences are pervasive across a range of human traits and behaviors. Virtually any human characteristic is genetically influenced. The remaining variation in human traits, however, is usually found to be associated with nonshared environmental influences, such as unique peer group associations or differential exposure to environmental toxins. Shared environmental influences, such as socioeconomic status or parental education, frequently account for little to no variance in human characteristics.
Findings from behavioral genetic research into human intelligence indicate that intelligence is heavily influenced by genetic factors. Estimates of the heritability of intelligence generally range between 60% and 80%, with some studies finding that intelligence is almost 100% heritable. Estimates derived from twins separated at birth and reared apart also have detected very high levels of genetic influence, usually above 70%. Conversely, shared environmental influences usually show little to no influence.
The relative contributions of environmental and genetic factors to intelligence, however, vary by age. In infancy and early childhood, estimates of heritability rarely exceed 40%, and test–retest reliabilities range from low to moderate. Estimates of common environmental effects range from 20% to 30%, on average. Unique environmental influences account for the rest of the variance in IQ early in life. This pattern reverses, however, by age 12, when genetic influences become dominant, environmental influences decline substantially, and test–retest reliabilities remain remarkably strong and consistent over time.
Estimates of heritability do not provide any information regarding which genes are associated with IQ. Recent research, however, has helped to fill in this void. Neuroscientific findings, usually based on complex brain imaging scans, have shown that IQ is moderately associated with brain size, is strongly associated with the overall number of cortical neurons, is strongly associated with the volume of grey matter in the frontal cortex of the brain, and is associated with neuronal conduction velocity (i.e., the efficiency of the neurons in transporting messages; see Ellis &Walsh, 2003). These biological functions are primarily under genetic control. Because of this, many scholars now argue that the reason IQ is highly heritable is because genes are inherited that control these basic neurological functions.
On the other hand, environmental influences on IQ are notoriously difficult to detect, because the genes associated with cognition are also associated with social behaviors. Parents who read regularly, for example, are likely to have more books in their home and to have children with above-average IQs (Ellis &Walsh, 2003). This correlation has led many social scientists to erroneously conclude that the number of books in a home positively influences a child’s IQ. This conclusion is erroneous, because the correlations among parental reading, the number of books in the home, and the IQ of the child involve both genetic and environmental influences. High-IQ parents are more likely to read and hence to have more books in their home than are low-IQ parents. Once shared genetic influences are taken into account, scientists find frequently that socialization influences, such as parenting, appear unrelated to individual IQ. Indeed, planned interventions designed to permanently increase IQ, such as Head Start, have typically failed to produce lasting results (Ellis & Walsh, 2003).
Although it is fair to say that IQ likely cannot be increased, it is equally fair to say that IQ can be reduced. Evidence shows that the behavior of pregnant women can negatively influence the development of the fetus. Insults to the developing central nervous system from maternal drug and alcohol use, smoking, and high levels of stress hormones are associated with compromised neurological development and reduced IQ. Birth complications, such as oxygen deprivation and toxemia, have been found to reduce IQ. Moreover, environmental insults after birth can also occur when young children ingest lead and other heavy metals, when they sustain brain damage due to accidents or abuse, or when they are severely neglected.