Influence of Heredity and Environment
Few topics in the social sciences have produced more controversy than the relative influences of nature and nurture on intelligence. Is intelligence determined primarily by heredity or by one’s environment?
The issue has aroused intense debate because different views on the heritability of intelligence lead to different social and political implications. The strictest adherents of a genetic view of intelligence believe that every person is born with a fixed amount of intelligence. They argue that there is little one can do to improve intelligence, so special education programs should not be expected to produce increases in IQ. On the other hand, those who see intelligence as determined mostly by environmental factors see early intervention programs as critical to compensate for the effects of poverty and other disadvantages. In their view, these programs help to create equal opportunities for all people. Perhaps the most controversial issue surrounding intelligence has been the assertion by some people that genetic factors are responsible not only for differences in IQ between individuals, but also for differences between groups. In this view, genetic factors account for the poorer average performance of certain racial and ethnic groups on IQ tests. Others regard genetic explanations for group differences as scientifically indefensible and view as racist the implication that some racial groups are innately less intelligent than others.
Today, almost all scientists agree that intelligence arises from the influence of both genetic and environmental factors. Careful study is required in order to attribute any influence to either environment or heredity. For example, one measure commonly used to assess a child’s home environment is the number of books in the home. But having many books in the home may be related to the parents’ IQ, because highly intelligent people tend to read more. The child’s intelligence may be due to the parents’ genes or to the number of books in the home. Further, parents may buy more books in response to their child’s genetically influenced intelligence. Which of these possibilities is correct cannot be determined without thorough studies of all the factors involved.
Genetic Influences
In behavioral genetics, the heritability of a trait refers to the proportion of the trait’s variation within a population that is attributable to genetics. The heritability of intelligence is usually defined as the proportion of the variation in IQ scores that is linked to genetic factors. To estimate the heritability of intelligence, scientists compare the IQs of individuals who have differing degrees of genetic relationship. Scientists have conducted hundreds of studies, involving tens of thousands of participants, that have sought to measure the heritability of intelligence. The generally accepted conclusion from these studies is that genetic factors account for 40 to 80 percent of the variability in intelligence test scores, with most experts settling on a figure of approximately 50 percent. But heritability estimates apply only to populations and not to individuals. Therefore, one can never say what percentage of a specific individual’s intelligence is inherited based on group heritabilities alone.
Although any degree of genetic relationship can and has been studied, studies of twins are particularly informative. Identical twins develop from one egg and are genetically identical to each other. Fraternal twins develop from separate eggs and, like ordinary siblings, have only about half of their genes in common. Comparisons between identical and fraternal twins can be very useful in determining heritability. Scientists have found that the IQ scores of identical twins raised together are remarkably similar to each other, while those of fraternal twins are less similar to each other. This finding suggests a genetic influence in intelligence. Interestingly, fraternal twins’ IQ scores are more similar to each other than those of ordinary siblings, a finding that suggests environmental effects. Some researchers account for the difference by noting that fraternal twins are probably treated more alike than ordinary siblings because they are the same age.
Some of the strongest evidence for genetic influences in intelligence comes from studies of identical twins adopted into different homes early in life and thus raised in different environments. Identical twins are genetically identical, so any differences in their IQ scores must be due entirely to environmental differences and any similarities must be due to genetics. Results from these studies indicate that the IQ scores of identical twins raised apart are highly similar—nearly as similar as those of identical twins raised together. For adoption studies to be valid, placement of twin pairs must be random. If brighter twin pairs are selectively placed in the homes of adoptive parents with higher intelligence, it becomes impossible to separate genetic and environmental influences.
Another way of studying the genetic contribution to intelligence is through adoption studies, in which researchers compare adopted children to their biological and adoptive families. Adopted children have no genetic relationship to their adoptive parents or to their adoptive parents’ biological children. Thus, any similarity in IQ between the adopted children and their adoptive parents or the parents’ biological children must be due to the similarity of the environment they all live in, and not to genetics.
There are two interesting findings from studies of adopted children. First, the IQs of adopted children have only a small relationship to the IQs of their adoptive parents and the parents’ biological children. Second, after the adopted child leaves home, this small relationship becomes smaller. In general, the IQs of adopted children are always more similar to their biological parents’ IQs than to their adoptive parents’ IQs. Further, once they leave the influence of their adoptive home, they become even more similar to their biological parents. Both of these findings suggest the importance of hereditary factors in intelligence.
People sometimes assume that if intelligence is highly heritable, then it cannot be changed or improved through environmental factors. This assumption is incorrect. For example, height has very high heritability, yet average heights have increased in the 20th century among the populations of many industrialized nations, most likely because of improved nutrition and health care. Similarly, performance on IQ tests has increased with each generation, yet few scientists attribute this phenomenon to genetic changes. Thus, many experts believe that improved environments can, to some degree, increase a person’s intelligence.
Some genetic disorders, such as phenylketonuria (PKU) and Down syndrome, may result in mental retardation and low IQ. But evidence for genetic influences should not be interpreted as evidence of a direct connection between genes and intelligence. In PKU, for example, a rare combination of recessive genes sets the stage for a series of biochemical interactions that ultimately results in low IQ. These interactions only occur, however, in the presence of the amino acid phenylalanine. If the disorder is detected early and phenylalanine is withheld from the infant’s diet, then large IQ deficits do not develop.
Environmental Influences
If genetic influences account for between 40 and 80 percent of the variation in intelligence, then environmental influences account for between 20 and 60 percent of the total variation. Environmental factors comprise all the stimuli a person encounters from conception to death, including food, cultural information, education, and social experiences. Although it is known that environmental factors can be potent forces in shaping intelligence, it is not understood exactly how they contribute to intelligence. In fact, scientists have identified few specific environmental variables that have direct, unambiguous effects on intelligence. Many environmental variables have small effects and differ in their effect on each person, making them difficult to identify.
Schooling is an important factor that affects intelligence. Children who do not attend school or who attend intermittently score more poorly on IQ tests than those who attend regularly, and children who move from low-quality schools to high-quality schools tend to show improvements in IQ. Besides transmitting information to students directly, schools teach problem solving, abstract thinking, and how to sustain attention—all skills required on IQ tests.
Many researchers have investigated whether early intervention programs can prevent the lowered intelligence that may result from poverty or other disadvantaged environments. In the United States, Head Start is a federally funded preschool program for children from families whose income is below the poverty level. Head Start and similar programs in other countries attempt to provide children with activities that might enhance cognitive development, including reading books, learning the alphabet and the numbers, learning the names of colors, drawing, and other activities. These programs often have large initial effects on IQ scores. Children who participate gain as much as 15 IQ points compared to control groups of similar children not in the program. Unfortunately, these gains seem to last only as long as the intervention lasts. When children from these programs enter school, their IQ declines to the level of control groups over a period of several years. This has come to be known as the “fade-out” effect.
Even though early intervention preschool programs do not seem to produce lasting IQ gains, some studies suggest they may have other positive long-term effects. For example, the Consortium for Longitudinal Studies reported that participants are less likely to repeat grades, less likely be placed in remedial classes, and more likely to finish high school than comparable non-participants—even though both groups show about the same levels of academic achievement. Preschoolers in early intervention programs may also benefit from improved health and nutrition, and their mothers may sometimes benefit from additional education that the programs provide. Because a substantial portion of the variation in intelligence is due to environmental factors, early intervention programs should be able to produce significant and lasting IQ gains once the specific environmental variables that influence IQ have been identified. Researchers continue to search for the interventions that will increase IQ and, ultimately, academic achievement.
Source: http://encarta.msn.com/encyclopedia_761570026_6/Intelligence.html