Heredity/Environment and Intelligence
STEP 4 Review the Knowledge You Need to Score High
A continuing theme of psychology known as the nature—nurture controversy asks to what extent intelligence is hereditary and to what extent it is learned. Intellectual disability resulting from genetic defects, such as Down syndrome (see Genetics and Behavior in Chapter 7), is primarily hereditary, whereas intellectual disability resulting from prenatal exposure to alcohol, fetal alcohol syndrome (FAS; see Physical Development in Chapter 13), is primarily environmental. Phenylketonuria (PKU) results from the interaction of nature and nurture (see Genetics and Behavior in Chapter 7). About 75 percent of all cases of intellectual disability result from nurture, from sociocultural deprivation in an impoverished environment. This illustrates that both nature and nurture contribute to intelligence. Theorists continue to argue about the relative contributions of heredity/genes and environment/experience to intelligence because of the important implications. If intelligence is inherited, then special educational programs for disadvantaged groups are unnecessary. If, on the other hand, intelligence can be affected by better education and an enriched environment, special programs are warranted. For example, the Head Start program was designed to provide economically disadvantaged children with preschool opportunities to ready them for elementary school. Research shows that, compared to matched control groups, children who had the Head Start experience did better in the first two grades, thus supporting the nurture position. The program reduced the likelihood that these students would have to repeat a grade or be placed in a special education class. Opponents of the program say that this advantage is short-lived. Continuing disadvantages experienced by these youngsters are not being addressed, according to the defenders.
Studies of Twins
Additional studies to gauge the influence of genes on intelligence include comparing the intelligence test scores of identical twins (who share all the same genes) reared together with the scores of fraternal twins (who share about half of the same genes). Identical twins have much more similar scores. Intelligence scores of adoptees are more like those of their biological parents than their adopted parents and get even more similar with age. Comparing the intelligence test scores of identical twins reared apart reveals that they are very similar and get even more similar with age. Brain scans of identical twins reveal similar brain volume and anatomy. Experiments with other animals, such as mice, indicate that genetic engineering can produce more intelligent animals.
Environmental Influences on Intelligence
On the other hand, some studies support the influence of the environment on intelligence. During childhood, siblings raised together are more similar in IQ than siblings raised apart. The IQs of children from deprived environments who have been moved into middle- and upper-class foster or adoptive families tend to increase. School attendance seems to result in increased IQ scores. Performance on IQ tests has been increasing steadily over the past three generations. This trend was noticed by James Flynn, who observed that every time tests were renormed, more questions needed to be answered correctly to earn the same score, yet the same proportion of the population was earning that score. In other words, a score of 100 on a present test is equivalent to a score of about 120 on a test from 70 years ago. This Flynn effect cannot be attributed to a change in the human gene pool because that would take hundreds of years. Theorists attribute the Flynn effect to a number of environmental factors, including better nutrition, better health care, advances in technology, smaller families, better parenting, and increased access to educational opportunities.
Heritability is the proportion of variation among individuals in a population that results from genetic causes. Heritability for intelligence estimates range from 50 to 75 percent. Heritability deals with differences on the population level, not on the individual level. According to the reaction range model, genetic makeup determines the upper limit for an individual’s IQ, which can be attained in an ideal environment, and the lower limit, which would result in an impoverished environment.