Story of Psychology - Morton Hunt [156]
For another, a number of recent, statistically sophisticated studies of fraternal twins and identical twins, particularly pairs who were separated shortly after birth and reared in dissimilar homes, have established, far better than Galton could, that mental abilities are in considerable part due to genetic makeup, and therefore that intelligence testing does indeed test innate ability as well as acquired knowledge.61
This graph clearly shows the two influences of genetics and environment on IQ.
FIGURE 1
IQ and genetic relationship
Each bar shows how closely the IQ scores of the individuals listed are related. For instance, the top bar indicates that if identical twins are reared together—genetics and environment being the same—their IQs will have a greater than 0.8 correlation (or in informal terms, are very likely to be the same or nearly so.) In contrast, the bar for siblings reared apart—with only half the same genes and different environments— shows that their IQs are likely to be rather different.
(Data of this kind are often mislabeled “heritability,” which is a quite different matter. Heritability refers to how much of the range of differences in a given trait, within a group of people, results from genetics. If the heritability of intelligence were found to be zero, no part of the range from nearly zero to 200 would be of genetic origin; if it were 100 percent, all of the variance would be of genetic origin. Recent reviews of a variety of studies of heritability of IQ conclude that roughly half of the variance in IQ scores is due to genetic makeup.62 But how genetic makeup interacts with culture and which contributes how much of the result in any individual is a complex issue, only now being studied in new and illuminating ways—an area of contemporary psychology that we will see more of later in this history.)
One reason test designers have had to keep tinkering with the IQ tests is bizarre: in Western society, IQ scores have risen about three points every decade. The average IQ today would be 115 if the tests were scored as they were 50 years ago. To correct for this, scoring methods are periodically adjusted to keep the mean at 100.63 Several explanations of the improving average scores have been offered: one, that daily life has become more challenging and thereby increased people’s coping ability; two, that nutrition is better and has increased height—and, possibly, brain functioning; and three, that perhaps there has been no real increase in IQ but only in the kind of reasoning ability that is useful in test taking. At present, the definite explanation for rising IQ scores is not known.64
A more serious challenge to the status of IQ tests is the development of more complex portrayals of intelligence. Rather than the summary entity g, a number of theories have been offered in recent years that distinguish among kinds of intelligence. The two that currently command the most attention are those of Robert Sternberg of Yale and Howard Gardner of Harvard. Sternberg distinguishes between analytical intelligence (such as one uses in solving anagrams), creative intelligence (called upon in problem solving), and practical intelligence (used in the management of everyday affairs).65 Gardner’s view is more complex: He identifies (and offers evidence for) eight intelligences, some of which are promoted by Western society, others by other societies.66 Here—and we will spare ourselves the details—are his eight:
—Logical-mathematical
—Linguistic
—Naturalist
—Musical
—Spatial
—Bodily kinesthetic
—Interpersonal
—Intrapersonal
His evidence is persuasive—and indeed nearly everyone has known people who are particularly gifted in one or more of these areas but all too average, or even deficient, in some of the others.
Yet despite these and other challenges and oppositions to standard IQ testing, it continues to be “one of psychology’s greatest successes,” writes Etienne Benson, a staff member of Monitor on Psychology, an online publication of the American Psychological Association. “It is certainly one of the