Intelligence

I

Introduction

Intelligence, that facet of mind underlying our capacity to think, to solve novel problems, to reason and to have knowledge of the world. In psychology the term is used in two different senses. The first sense is to think of it as the emergent property of the cognitive system (or mind) as a whole. This motivates us to ask questions about how the mind is structured such that “intelligence” is possible at all, and in principle we could try to answer these questions by careful testing of a single individual. In this sense the study of intelligence is primarily the province of cognitive psychology. However, the more common usage of the term in psychology is in the study of individual differences. Here the fundamental question is: what makes one individual more intelligent than another? To answer this question we need to study many individuals and many factors that might contribute to the differences between them.

II

The Measurement of Intelligence

Francis Galton, a cousin of Charles Darwin and one of the polymath geniuses of the 19th century, believed that differences in intelligence are caused by differences in the speed at which information is transmitted through the nervous system and this, in turn, is due to differences in biology. Galton attempted to develop a measure of intelligence based on reaction time to tactile stimuli (for example, how quickly you respond to a pin-prick by removing your hand). Galton’s approach failed to catch on in psychology, in part because a completely different approach proved very successful. In France an educationalist, Alfred Binet, was asked to develop mental tests to identify students who might benefit from special education. Binet’s view, contrary to that of Galton’s, was that intelligence is a high-level property of complex mental events such as reasoning, logic and knowledge, and therefore tests of intelligence should tap these processes (and not speed of response to simple pin-pricks). Binet realized that the commonplace observation that older children can solve more difficult problems than younger children can be used to derive a measuring scale of intellectual performance. The difficulty of items on an intelligence test could be determined by observing how old a child had to be (on average) to pass the item—the older the child the more difficult the item. In turn, the logic could be reversed—once the test had items that were scaled in this manner any child taking the test could be given a score that matched the average chronological age of children who typically obtain this score. This measure became known as mental age. It was William Stern who then derived the intelligence quotient, or IQ, by dividing a child’s measured mental age on the test by their actual age and multiplying the result by 100. This means that the average child whose mental age is the same as their actual age will have an IQ of 100. The distribution of IQ scores is such that two thirds of people will have an IQ between 85 and 115, and only 2.3 per cent of the population will have an IQ lower than 70 or above 130. It is one of the major diagnostic characteristics of mental retardation that IQ is less than 70. Moreover, IQ is the single best psychological variable for predicting real life success and is often used in job selection.

Binet’s tests provide the template for all modern tests of intelligence, the most famous being the Stanford Binet and the Wechsler scales. It is interesting, however, that in recent years all new revisions of intelligence tests require that the scores be scaled downwards to maintain the average performance at 100. This effect was discovered by James Flynn and this phenomena of “rising IQ” across generations is still to be explained.

III

Theories of Intelligence

British psychologist Charles Spearman observed that if we take a sample of the population and give them a diversity of intellectual tests then we find that someone who is good at one intellectual ability is likely to be good at all others too. This is the single most important empirical fact discovered by research on intelligence. Spearman drew the conclusion that the reason all abilities are related is because all tests measure to some extent a general intelligence factor, or g. Spearman conceptualized this g as being like mental energy, with those higher in g having more mental resources to devote to intellectual tasks. He also supposed that each test might in addition measure some unique ability, and consequently Spearman’s theory became known as a “two-factor” theory. In the early 20th century statistical techniques designed to analyse patterns of relationships between scores, known as factor analysis, were developed apace and this led to the second major theory of intelligence, that of Louis Thurstone. Using these new statistical techniques Thurstone claimed to identify not two factors but seven primary mental abilities—verbal, numerical, spatial, memory, reasoning, word fluency, and perceptual speed. Most importantly though, Thurstone claimed that in fact there was no single general intelligence. Whether there is such a thing as general intelligence, or g, has remained the central issue for theories of intelligence ever since. An important development of the idea was proposed by Raymond Cattell, who distinguished between fluid g, that which determines current, on-line ability to think and reason, and crystallized g, which is related to stored knowledge. The former was proposed to be more influenced by biological states and the latter by processes of acculturation. While it has not been resolved to everyone’s satisfaction the current consensus is that g is real and is most probably related to something such as the speed with which we process information.

IV

The Development of Intelligence Across the Lifespan

The psychometric data that describes intellectual development is reasonably clear. The period of infancy is something of a dark-age, basically because the pre-linguistic child is difficult to test. Between the onset of language in the child and going to school there is an explosion in cognitive abilities. From school age to around young adulthood there is a steady but diminishing increase in measured intelligence that reaches a plateau around late adolescence or mid-teens. Thereafter there is very little increase in “fluid” intelligence, though some crystallized abilities (for example, vocabulary scores) can still show measured increases late in life. There is some dispute about what happens in the middle years and later ageing. Some view the middle years as a period of relative intellectual stability with only serious decline beginning as we approach our death (and consequently brought about in the main by illnesses that affect the central nervous system). Others though see it as a slow remorseless decline from early adulthood onwards.

Some researchers see developmental changes as operating in specific knowledge domains (for example, the child as a linguist, the child as a mathematician, the child as a physicist, and so forth) and largely independent of each other. Indeed the correlation between measures of intelligence in the pre-school child (particularly in infancy) and later IQ differences are usually very low. Yet there are strong correlations between intelligence test scores in the school-age child and later adulthood (even when the knowledge content of the respective tests is very dissimilar). So the contrasting view is that a single global process might account both for IQ differences between individuals and the changes in intellectual level over the lifespan of a single individual. For example, individuals with fast speed of processing will have higher IQs than those with slow speed and during child development speed increases and then slowly declines through adulthood into old age. However, a number of striking facts about development suggest that the complete picture must be somewhat more complex than this. For example, not all children of the same IQ are intellectually equivalent (for example, children with Down’s syndrome and children with autism); some children with normal IQs can have specific learning disabilities (for example, dyslexia); and in old-age some abilities seem to decline faster than others.