Issues of clinical relevance with child rearing: The genetic contribution: Heredity makes a significant contribution to intellect and also to other qualities such as temperament and personality, but the exact extent of the genetic contribution is controversial.
The inheritance referred to here is biological, that is, it is carried on the genes rather than transmitted socially through the quality of the environment created by the parents. The distinction between biological and social inheritance has always presented difficult research problems.
Data which show a relationship between the intelligence quotient (IQ) of parents and that of their offspring could indicate either biological transmission or the effects of social and environmental variables. Studies of identical and non-identical twin pairs reared separately and together suggest that up to 80% of the variance in intelligence in a Western European or North American population can be attributed to genetic transmission. It must be emphasized that this is a statistical statement about a population selected for study.
It does not preclude the possibility that, in one particular subject or subpopulation at a particular point in time, the environment may be of much greater or much less importance in determining intelligence. For instance, very wide variation in environmental stimulation in a particular population would increase the environmental effect. The genetic influence is not confined to the effect on total IQ scores.
Pathological influences on intelligence Intrauterine and postnatal pathological factors such as malnutrition must be regarded as environmental influences on brain development, and therefore on neurological and intellectual function. Pathological factors which affect the physical structure and growth of the nervous system could be responsible both for developmental disorders and for at least a part of normal variation. For example, some developmental problems might result from subtle perinatal brain damage, which could be either focal or diffuse. The literature relating brain disorders to problems of intellect, learning and behaviour is diverse and confusing. Clearly, the relationships are very complex. It is naive and misleading to attribute developmental disorder to ill-defined 'brain damage' without considering what this means.
Environmental influences: In some circumstances intelligence is clearly related to environmental factors. For example, Rutter noted that 'there is abundant evidence that mild mental retardation is extremely common among children brought up in city slums. Intelligence develops and is not a "given" capacity. Its development is a social process strictly dependent upon the quality and organisation of the human environment.'
In extreme circumstances, environmental manipulation can lead to substantial gains in intelligence; some relevant studies are described later in this chapter. It is more difficult to discover if this is also true in families where the parents are of normal intelligence, material conditions are adequate and family life is stable. Environmental measures such as social class, income or quality of housing are no more than crude indicators of child-rearing styles and are too insensitive to be useful in unravelling this problem. It seems likely that the skill of the parents in encouraging and extending their child's development throughout childhood is the key factor. The term 'micro-environment, is useful to describe the innumerable small details of daily child-rearing and experience, the extent to which the child is encouraged to play and to practise skills, and so on.
Measurement and analysis of the microenvironment are difficult, but various methods have been devised, such as the home stimulation inventory. Correlations are found between environmental measures and the child's abilities and developmental progress, but the nature of the association is not always clear. The correlation may simply indicate that parents find it more rewarding to teach a child who is good-natured and intelligent. Nevertheless, there is some evidence that microenvironmental factors do influence intellect, personality and temperament.
