Changing, feeding, neuron pruning

Michael Rutter makes some mental notes on child development

All parents want to know what they can do to make their children happy and smart. This book seeks to provide some answers, but it is distinctive in setting about the task from the perspective of neuroscience research into brain development. Thus the blurb on the cover argues that it "demonstrates the innumerable ways in which parents can actually help children develop better brains". The background is provided by the claims (particularly in the United States) that the first three years are crucial for the development of the brain, that experiences sculpt the growth of the brain and that if good experiences are to make a difference to children's mental development it is crucial that they occur early.

Lise Eliot takes a more measured position on those claims, but her basic proposition is much the same. She states that babies' brains are learning machines and that they adapt to the environment at hand; their experiences take over control of the wiring of the brain to customise each child's hardware to his or her unique environment; that a young child's environment directly and permanently influences the structure and eventual function of the brain; and that once the neuronal pruning phase is over, the critical period is finished and the brain circuitry is fixed.

Some of the evidence on this proposition is well established. Initially, there is an overproduction of neurons in the brain; these migrate to other parts of the brain in a genetically programmed fashion. There is a proliferation of synapses (junctions between nerve cells) and there is then a selective pruning of cells in order to optimise function. Myelination (the development of covers for nerve fibres) occurs in parallel. All of this is accompanied by an increasing specialisation of brain functions and decreasing plasticity or flexibility. It has also been shown that experiences are important in this process of brain development. The findings have led to slogans such as "use it or lose it" and "cells that fire together wire together".

So far, so good. But that is not where the controversies (or research difficulties) lie. The best evidence on the role of experiential input on brain growth is provided by David Hubel and Torsten Wiesel's Nobel prizewinning research some 40 years ago (and the subsequent studies by others to which this gave rise) on the crucial role of visual input in shaping the growth of the part of the brain dealing with vision. The effects clearly matter, as shown by the evidence that unless children's squints are corrected early in life, the development of normal binocular vision is unlikely. It is not just vision that affects brain growth, however. Reference is made to William Greenough's finding that rats reared in an enriched environment had bigger, better brains (and were smarter) than rats reared in the standard impoverished laboratory environment.

This research is solid and well confirmed. Nevertheless, four serious questions have to be raised about the extrapolation to the effects of early intervention in humans to alter brain growth and thereby the functioning of the mind.

First, the findings on the role of experiences mainly apply to gross differences in the total amount of a particular form of sensory input. There is no doubt that these have effects on the brain. It is by no means so clear, however, that differences in the quality of the input are so influential. It is evident that babies suffer greatly from being reared without conversational interchange (and although not clearly demonstrated in humans, it is reasonable to suppose that this will affect brain structure), but does how parents talk with babies matter in the same way? Eliot quotes findings on associations between parental talk to children and their children's language development, but that evidence took no account of the possibility of genetic mediation and did not demonstrate any permanent effects on the brain.

Second, it is not known whether the critical period effect found for visual input applies to other types of experiences. The book has individual chapters dealing with touch, bouncing, smell, taste, motor function, hearing and socio-emotional input, as well as vision. Of course, all these make an impact. But do they do so in the same way as happens with vision? It is claimed that: "The millions of moments parents share with their children create a limbic legacy that may live for generations" - the limbic area being part of the brain. But is this true? Of course, in a real sense, all learning affects the brain. How else could learning take place? But effects on the brain are not synonymous with a permanent alteration of brain growth.

Third, to what extent are the effects of a lack of experiences on brain growth universal and permanent? Eliot claims (with respect to malnutrition) that: "After two years of age, a child will have lasting mental deficiencies, even if he is now adequately nourished and stimulated." Not so; follow-up studies have shown that, although lasting effects are more common if nutritional and experiential deprivation lasts for more than two years, major deficits are not found in all children.

Fourth, queries have to be raised about the assumption that there is little brain growth and little plasticity after the first few years of life. The book acknowledges that there is continuing development, but says very little about it, and does not discuss the implications for the postulates being put forward. Recent neuroscience research increasingly challenges the notion that the brain is fixed in infancy.

Eliot provides a mostly well-informed account of early brain development, but she glosses over the difficulties in many of the extrapolations and she does not bring out at all well the ways in which research tackles developmental questions. John Bruer's book, The Myth of the First Three Years , deals better with problems in Eliot's theoretical position (although his book is more polemical and much narrower in coverage); Patrick Bateson and Paul Martin's Design for a Life: How Behaviour Develops provides a superior account of the excitement of research challenges and how they may be met; and Ann and Alan Clarke's book Early Experience and the Life Path is better balanced with respect to the human evidence on behavioural continuities and discontinuities.

Where Eliot scores positively is on the breadth of topics covered. Her book, for example, deals extensively with the findings on prenatal physical experiences (with respect to features such as maternal alcohol use, smoking, use of recreational drugs, irradiation and infections, as well as the impact of microwaves, stress and hormones). It notes (rather briefly) the major changes over time in levels of psychological functioning (such as the Flynn effects with respect to the rise in IQ), but does not discuss their causes at all critically.

Curiously, given the book's focus, it is particularly weak on both genetic and psychosocial research. There is no clear demarcation between universal genetic effects that have arisen through evolution and which do not bring about individual differences (such as the effects on the sequence of brain development) and those that do bring about such differences. The main emphasis is on heritability, but occasionally the concept is misleadingly applied to individuals - such as "genes account for 50 per cent of a person's IQ". They do not. They account for about 50 per cent of individual differences in IQ, which is not the same thing at all. Also, there is little or no attention to the importance of gene-environment correlations and interactions - that is, of the various ways in which nature and nurture work together. Psychosocial research is discussed rather uncritically, without attention to the need to use designs that control for possible genetic mediation and without discussion of which sorts of parental actions make most difference. There is also little consideration of influences outside the home (from the peer group, for example).

The book is a useful source of findings on all sorts of influences on brain development and on the workings of the mind, but despite its extensive documentation (458 footnote references) it remains rather uncritical and lacking in a straightforward take-home message. Because the book has chapter titles such as "How to raise a smarter child" the balance cannot really be restored by a final sentence urging parents to relax a bit more and enjoy their children for who they are.

Sir Michael Rutter is professor of developmental psychopathology, Institute of Psychiatry, London.