There are many different questions about intelligence that easily become confused. They concern its measurement, its psychological basis (if any), its heritability and its relevance to human group differences. Even a discussion of the importance of intelligence could range widely. (1) The evolution of intelligence might consider what selection pressures generate and maintain the higher levels of intelligence that humans are generally thought to possess. (2) The persistence of individual differences in human intelligence could consider whether such differences serve some ‘group’ function in establishing clear bases for social hierarchy: individuals might differ just because intelligence is irrelevant to fertility under conditions of social hierarchy. Or the differences might be temporary, because Western social hierarchies depend on intelligence differences that they will soon undermine. (3) Is intelligence an explanatory concept in psychology? Or must attribution of scholastic or other successes to ‘intelligence’ always yield to further analyses that somehow break down intelligence into hypothetical ‘components’? (4) Educational relevance might be the issue: is it important to take intelligence differences into account when deciding on how to educate children? (5) Or the concern might be with democratic sentiment and the importance people attach to intelligence for themselves and in their spouses and children.

Successes in science tend to be successful attempts at reduction and causal interaction. The reduction of the interaction of bodies to the laws of motion and the reduction of chemical phenomena to elements and their interactions are good examples. In these cases reductionism leads to observed phenomena being explained by a relatively small number of basic concepts and the laws which govern the interaction of the concepts. The spectrum of inquiry that stretches from common sense to scientific psychology is infused with a tendency toward reductionism. Hippocrates and Galen originated and developed, respectively, the notion that temperamental differences might be reducible to the amounts of four bodily fluids, or humours (black bile, yellow bile, blood and phlegm), that a person produced. Astrologers reduced stable personality differences to the positions of the stars and planets at the time of birth. And differential psychologists have reduced the personality trait adjectives contained in the lexicon to about five major dimensions.

It has long been known that people who have high scores on intelligence tests are also relatively fast at simple laboratory tests of ‘mental speed’ (Woodrow, cited in Woodworth, 1946). When first observed these correlations attracted little interest because, though ubiquitous, they are very modest, accounting for only 3–9% of variance between individuals (Sternberg, R. J., 1977). Correlations between intelligence test scores (TSs) and speed of performance on simple tasks were seen as less interesting than equally strong associations between TSs and performance on simple tests of memory and learning (Woodrow, 1938). The idea that intelligence can be defined as the capacity to learn new skills seemed a more fruitful guide to the development of individual differences in competence (Woodrow, 1946). Further, since scores on most intelligence tests represent the number of problems correctly solved within a time limit, it did not seem remarkable that people who can solve complex problems quickly should also be able to make simple decisions fast. Finally, evidence accumulated that, speed and accuracy of problem solving can be independent; for example, the times that individuals take to solve complex spatial problems do not predict their error-rates (Egan, 1978). Consequently mental speed was seen as only one of many different demands that solving intelligence test problems make on the cognitive system. Many distinguished investigators believe that better insights can be gained into the nature of individual differences in intellectual ability by studying correlations between TSs and performance on complex tasks that demand the encoding, organisation and transformation of information in working memory or successive stages in syllogism solution (e.g. Carpenter & Just, 1989; Daneman & Carpenter, 1980; Kyllonen & Crystal, 1990; Necka, 1992; Sternberg, R. J., 1977).

Health of school age children and the Partnership for Child Development

Until recently, health programmes in developing countries have focused on infant survival and on the health of adolescents and, consequently, the health concerns of children of school age, falling between the ages of infancy and adolescence, have been neglected. The World Bank Development Report of 1993, stated a need to move beyond the focus on survival and to capture the concept that community health depends on the quality of life and opportunities for development. In response, the Partnership for Child Development was set up in 1992 to address the problem of ill-health among school-age children in the developing world.

Unlike other organs the energy requirement of the brain is met almost exclusively by aerobic glucose degradation (Siesjo, 1978). The energy requirement of the brain is 20–30% of the whole organism at rest, although its weight is only 2%. The energy stores in the brain are extremely small when compared with the high rate of glucose utilisation: thus the brain is reliant on a continuous glucose supply. Only about 30% of glucose is required for direct energy production; much of the remainder is used for the synthesis of amino acids, peptides, lipids and nucleic acids (Siebert, Gessner & Klasser, 1986). Thus a source of glucose is essential for the synthesis of physiologically active amines such as serotonin, noradrenaline and acetylcholine. Although it is well accepted that hypoglycaemia can result in the disruption of cognitive functioning, this is a rare phenomenon and it has usually been assumed that levels of blood glucose, within the normal range, do not influence intellectual functioning. This assumption is discussed in this paper.

The possibility that early nutrition has long term consequences in man has been much debated. There have been limited opportunities to perform formal randomised studies on the effect of early nutrition in man and many studies have been flawed by problems with study design. Infants born preterm are a special group. At the start of this study in 1982 evidence on which to base choice of diet was inconsistent and related only to short term outcome, and diets available for such babies differed greatly in nutrient content. In this group it was both ethical and practical to conduct a formal, randomised trial of early diet and outcome and the results were clearly needed for management decisions.

A long term prospective outcome study was undertaken on 926 preterm infants randomly assigned to the diet received in the neonatal period. Surviving children have been followed at 9 months, 18 months and now 7½–8 years of age. The findings suggest that children fed a nutrient supplemented preterm formula perform better than those fed a standard formula milk, and also that human milk may contain factors which promote brain growth or development. Outcome data from the randomised trials show that a very brief period of dietary manipulation (on average for the first 4 weeks of life) influences later development.

An obvious requirement before embarking on molecular genetic investigation of a trait is prior evidence from ‘classic’ genetic studies that there is indeed a genetic component. Many behavioural traits are familial and these range from comparatively uncommon single gene disorders such as Huntington's disease which has a typical mendelian dominant pattern of transmission, to much commoner characteristics such as career choice or religious denomination which, it might be assumed, are heavily influenced by cultural factors. In between, there is a wide range of attributes including personality type, cognitive ability and liability to common disorders such as depression, that show a tendency to run in families, and which could conceivably be explained by shared genes, shared environment or a combination of the two.

It has long been known that mild mental retardation (meaning that associated with an IQ in the 50–69 range) shows a strong tendency to run in families and that there is a much increased recurrence risk if either a parent or a sib has mental retardation. For example, Reed & Reed (1965) found that, having had one retarded child, the chance of a further retarded child was 6% if both parents and their sibs were ‘normal’, 13% if both parents were normal but one had a retarded sib, 20% if one parent was retarded, and 42% if both were retarded. A somewhat similar gradient was found in the more recent study by Bundey, Thake & Todd (1989). The appreciation that mild mental retardation was so strongly familial had led Lewis (1933) some years earlier to refer to it as ’subcultural‘ (meaning that it was a normal variation), in order to differentiate it from severe mental retardation, which was viewed as ‘pathological’. Penrose (1938, 1963) made the same classificatory distinction and the utility of a two-group approach became generally accepted (Burack, 1990).

When asked whether he would discuss man in the Origins of the Species, Darwin replied, ‘I think I shall avoid the subject, as so surrounded with prejudices, though I fully admit it is the highest and most interesting problem for the naturalist’. Galton on the other hand replied to the same question, ‘I shall treat man and see what the theory of heredity of variations and the principles of natural selection mean when applied to man’ (Pearson, 1914–30, Vol. II, p. 86).

Of all the controversial issues surrounding IQ tests, few have generated more heat and less light than the question whether different groups in our society differ in average IQ. Those who study, and find evidence of, such differences believe that they are addressing a legitimate scientific issue and one whose implications, however unpalatable they may be, society ignores at its peril. Their critics insist that the question has no intrinsic scientific interest whatsoever, and conclude that those who study it are either naive, deceiving themselves or only too happy to find justification for their own comfortable position within an unjust and unequal society. In the case of sex differences in IQ scores, at least, one could argue that both sides have got it largely wrong. With one exception, there is relatively little social or political implication in such differences as there are. More importantly, however, arguments will be presented against the critics' view that such research is of no intrinsic scientific interest. On the contrary, it has already helped to answer some important scientific questions about the nature of IQ, and has the potential to answer more. Unfortunately, those who engage in such research have usually failed to see this.

‘From a black perspective…the notion that a black (who passes for white) might reclaim his ethnic identity to take advantage of preferential admissions can only trigger an almost inexpressible sense of outrage.’ (Livingston, 1979)

‘Turning an elephant loose in a crowd offers everyone, except the beast and his rider, equal opportunities of being trampled.’ (Tawney, 1931)

More than a century ago, Francis Galton, the father of behavioural genetics, coined the scientific use of the phrase nature–nurture. He believed that ability, especially individual differences in intelligence, is due primarily to nature rather than nurture, as implied by the title of his earliest and most famous work, Hereditary Genius (1869). In a paper on twins, he declared that ‘there is no escape from the conclusion that nature prevails enormously over nurture’ (1875. p. 404).