In the late 1920s and early 1930s it was realized that substantial changes in intelligence could sometimes occur in response to changes in the environment. This discovery provoked intense speculation on the relative importance of heredity and environment in deterrnining intelligence; this 'nature vs. nurture5 debate still continues today, although it has become very much more sophisticated. It is now clear that there are both genetic and environmental contributions to intelligence, personality and temperament, and that there is a lifelong interaction between them.
It is difficult to define intelligence. It can be regarded as 'the ability to solve unfamiliar problems'. Cynics observe that it is nothing more than the quantity measured by intelligence tests. Some psychologists believe that there is a common factor in all mental processes, irrespective of the nature of the task, and that this can be identified in intelligence tests and distinguished from the influence of individual skills. Spearman, the psychologist who introduced this concept in 1904, called the common factor or underlying intelligence 'Neurophysiological studies show that some brains do process information more rapidly than others, suggesting that there might indeed be a 'g' factor, analogous to the overall processing power of a computer.
On the other hand, the human brain is capable of an amazing range of skills and only a few of these are assessed in standard intelligence tests; the manual dexterity of a craftsman, musical and artistic creativity and excellence in sport are just a few examples of skills which are often ignored when intelligence is assessed and which are not invariably accompanied by other evidence of exceptional intellectual prowess.
Questions about the nature of intelligence have also been raised by observations on so-called 'idiot savant' children, who have an extraordinary ability to carry out mathematical processes at high speed or perform prodigious feats of memory and transcription, and yet in all other respects have very limited intellectual powers. Whatever the truth of the matter, it is evident that any psychological assessment which ignores talents outside the narrow domains of traditional intelligence testing is likely to do the child a disservice.
Readers who require a detailed analysis of these issues should consult one of the many comprehensive volumes on the subject. For the developmental paediatrician, the following issues are of immediate clinical relevance.
Is the genetic contribution the major determinant of development and intellect? If it is, a child's developmental progress would be expected to follow a predetermined trajectory which could not easily be altered by environmental factors, therapy or teaching.
How much do biological influences such as intrauterine malnutrition or birth trauma affect intellectual development?
If environmental factors are important, what aspects of the environment are most closely related to development?
How can the clinician assess a child's development and the interactions between the factors which affect it?
How should the results of this assessment be summarized and presented to the parents and to other professionals?
Is intervention effective, and in which form, and for what indications?
Much of the following discussion will focus on intelligence, since this has been more extensively studied than other parameters; however, similar points can be made about the development of language, personality, temperament, etc. The issues listed above will be briefly examined in turn.
