Under the Microscope: It is not at all unusual to find educationalists who are resistant to the idea that intelligence is inherited. Their resistance is mistaken. Intelligence is significantly inherited, although the environment also plays a role in shaping intelligence. There is ongoing debate over the relative contributions of nature and nurture to intelligence.
What is meant by intelligence? The term usually refers to a general mental capacity to reason, solve problems, think in the abstract, learn and understand new material and profit from experience - all valuable in adapting to life's demands. The most widely used intelligence test, the Wechsler Adult Intelligence Scale, measures verbal and performance skills (including vocabulary, general knowledge, arithmetic, social awareness, similarities, picture completion and arrangement, block design and jigsaw type puzzles).
Sir Francis Galton (1822-1911) was the first to investigate individual differences in mental ability. Alfred Binet (1859-1911), a French psychologist, first devised an intelligence test that predicted academic success. In 1906 he was asked by the French government to design a test to identify slow learners in school so they could receive special instruction. Binet and collaborators created the concept of mental age as opposed to chronological age. If a 10-year-old answered questions that, on average, could be answered by other 10-year-olds, but could not answer questions appropriate for 11-year-olds, the child was assigned a mental age of 10. However, a 10-year-old answering questions appropriate for a 12-year-old was assigned a mental age of 12.
Later, mental age was expressed as an intelligence quotient (IQ), where the mental age was divided by the chronological age and the quotient multiplied by 100. A 10-year-old answering questions appropriate for a 12-year-old would have an IQ of 12/10 x 100 = 120. This formula is no longer used in modern tests. A score is now given that reflects how far the person's performance deviates from the average performance of others of the same age. By convention the score is still called IQ. Most tests define the average IQ score as 100.
The heritability of a trait is the proportion of the trait's variation in a population that is attributable to genetics and this is estimated by comparing the IQs of people who have differing degrees of genetic relationship. To my knowledge, no study has ever failed to find significant heritability of IQ. Overall, the studies show that heritability accounts for 40 to 80 per cent of variability in IQ scores.
The study of twins has particularly illustrated the extent to which intelligence is inherited. Twins that develop from a single egg are genetically identical, whereas twins that develop from separate eggs (fraternal twins) are no more genetically related to each other than to other brothers and sisters, ie they each have about half their genes in common. The IQs of identical twins raised together are close, and much closer than those of fraternal twins raised together.
The IQs of identical twins adopted into different homes and reared apart are also remarkably similar. In this case, differences in IQ scores can only be due to environmental differences and the similarity in IQs is very strong evidence for heritability of intelligence. Other studies have shown that IQs of adopted children have little similarity to the IQs of their adoptive parents, but are more similar to the IQs of their biological parents.
There is also evidence for environmental effects on intelligence. For example, schooling affects intelligence and children who attend school intermittently do less well on IQ tests than regular school attendees. It might be thought therefore that early intervention educational programmes can prevent lowered IQ that might be caused by deprivation. The US has such a programme called Head Start. These programmes have significant initial effects on improving IQ scores, but the gains seem to fade out after the interventions have run their course.
In recent years some theorists have proposed that the standard intelligence tests measure only a fraction of the human abilities that constitute intelligence. In 1983 the American psychologist Howard Gardner proposed the theory of multiple intelligences that seeks to broaden the traditional definition of intelligence. He originally proposed seven intelligences: linguistic (aptitude with language); logical/mathematical; spatial (to perceive visual/ spatial information and conceptualise the world, as in art); musical; bodily/kinesthetic (use of body in various activities, eg athletics); interpersonal (understanding others); intrapersonal (understanding self).
Gardner's ideas are popular with educators because they imply that students who are slow by conventional academic measures can still excel in other areas. However, there is much evidence of a tendency for different abilities to correlate and Gardner's theory does not accommodate this.
What we traditionally measure as intelligence is "cleverness". It is undoubtedly important, but not the only important thing. There are other capabilities that IQ tests do not directly measure: creativity, wisdom, social skills and practical knowledge. These are important abilities that also help us to adapt to the world.
As Albert Einstein said: "We should take care not to make the intellect our God; it has, of course, powerful muscles, but no personality."
- William Reville is associate professor of biochemistry and public awareness of science officer at UCC - http://understandingscience.ucc.ie.