5 Steps to a 5 AP Psychology, 2010-2011 Edition - Laura Lincoln Maitland [144]
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 of 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%. 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.
Human Diversity
Racial differences in IQ scores show African Americans, Native Americans, and Hispanic Americans typically scoring 10 to 15 points below the mean for white children. When comparing groups of people on any construct, such as intelligence, it is important to keep in mind the concept of within-group differences and between-group differences. The range of scores within a particular group, such as Hispanic Americans, is much greater than the difference between the mean scores of two different groups, such as Hispanic Americans and Asian Americans. According to Leon Kamin, even if heritability is high, differences in average IQ between groups could be caused entirely by environmental factors. Neither of these statistics tells us how any one individual will score. The difference between the mean scores could result from socioeconomic differences. Claude Steele hypothesizes that at least part of the difference in IQ scores can be attributed to stereotype threat—anxiety that influences members of a group concerned that their performance on a test will confirm