Lend an ear to an egg

Nature via Nurture
September 19, 2003

Ian Wilmut admires intuition and hard graft in a study of genetics

Which of us parents has not winced to see a less-than-ideal trait of our own in one of our children? Or perhaps as children wondered why it is that we resemble our parents? No doubt the argument whether this happens because of an effect of inheritance or environment has gone on since before the mechanisms of inheritance were understood - in fact, long before Francis Galton first coined the phrase "nature or nurture" in 1874.

Rather than enter the debate on which of these is more important, Matt Ridley has precisely defined a far more ambitious objective in his latest book: "I intend to make the case that the genome has indeed changed everything, not by closing the argument or winning the argument for one side or the other, but by enriching it from both ends till they meet in the middle. The discovery of how genes actually influence behaviour, and how human behaviour influences genes, is about to recast the debate entirely. No longer is it nature-versus-nurture, but nature-via-nurture."

Let us consider one or two of his examples to see how well he makes the case for this proposal. We all accept that animals have instincts. How else could a cuckoo mate with an adult of its own species when it has been reared by a different species entirely? Ridley also mentions the digger wasp that lays its egg on a caterpillar to provide food for the grub when it hatches. These innate behaviours often have to be triggered by environmental cues, such as the red spot on the beak of herring gulls that elicits begging behaviour in the young chick. What of such inherited ability in humans?

One method of assessing the importance of genes in determining human behaviour has been to study the differences between identical twins: in particular, to contrast the similarity between those who were separated soon after birth with those who were reared together. When these observations were started, it was expected that they would reveal that the similar behaviour of identical twins was due to their being raised in the same home. It was a shock to discover that many characteristics were similar, regardless of whether the children were brought up together or in different families. Such characteristics are said to be (comparatively) highly heritable. As an approximation, about 50 per cent of the variation in many personality traits is due to genes, 10 per cent to environmental influences and 40 per cent to the unique life history of the individual.

By contrast, IQ is influenced rather more by environment than other traits and in particular by socioeconomic circumstances. In poorer areas, the family environment limits the opportunity of the children to express their abilities and estimates of the heritability of intelligence are low. Only in richer areas do children have the opportunity and stimulus to express their different ability and, consequently, heritability estimates are higher. Ridley emphasises the important political conclusion that providing opportunities for children from the poorest families will do most to equalise attainment with important social benefits. He then moves on to describe the present understanding of the way in which human abilities and personality develop. This, too, has profound social consequences.

Scientific understanding comes from a mixture of intuition and hard graft laced with a dash of good fortune. Ridley recounts how the distinguished Spanish scientist Santiago Ramon y Cajal suggested that the nervous system was built up by nerves growing out toward chemicals that attract them.

Important understanding of these mechanisms came later from reductionist experiments on nerve growth in mice. There is a very precise number of olfactory sensors in the nose, each sending signals along fibres to specific sites in the brain. These neurons live for only 90 days before being replaced. The mechanisms that regulate their growth were analysed by using precise molecular tools to destroy specific populations of cells.

Briefly, growth of the cells is controlled by one gene in the neuron and another at the location to which it should grow.

Experiments of this kind might lead one to imagine, as others have done before us, that the brain is fixed and inflexible in both structure and function, but there is direct evidence that neither is true. Early social experience affects adult behaviour in species as different as nematode worms, mice and monkeys. Interestingly, the young inherit the same social behaviour non-genetically so that newborn mice adopted by affectionate (licking) mothers are themselves affectionate. Similar environmental effects on behaviour can even influence the performance of mice in tests of skill. These are clear examples of the way that both genes and environment influence the behaviour and ability of offspring, and this effect continues into adulthood.

Ridley assembles evidence that environmental factors may influence a wide range of human characteristics. These factors are often able to affect behaviour only during specific periods either before or after birth. He lists too many to describe here but they include: vulnerability to heart disease in later life; lack of sexual attraction to children of the opposite gender with whom you grew up (incest avoidance); the Pavlovian reflex; and an increased probability of homosexual tendencies in men. We may be sure that the list is not yet complete.

As research into the cellular and molecular mechanisms that regulate human behaviour is only just beginning, it may be that some present ideas are mistaken. Consider the apparently well understood imprinting behaviour in ducklings. In these species, an imprint on an adult bird requires audio as well as visual cues to establish a link with the hatchling's own species.

Surprisingly, this seems to depend on the duckling being able to make sounds while it is still in the egg. The present hypothesis is that the bird hears its own call while in the egg and it is this that determines the species to which the bird will attach. Suddenly, what had seemed clearly to be an instinct is now considered to be a learnt behaviour, although much remains to be understood in this specific case.

Comparable windows of opportunity exist in our ability to learn. Ridley describes dramatic effects upon learning to speak in tragic cases in which children have been deprived of human contact during their early years.

These sad cases reveal that we have an innate ability to learn languages, but it is limited to our early years. One individual who was in essence isolated in this way until the age of 13 has not been able to learn grammatical structures despite intensive examination and assistance. She has a considerable vocabulary, is intelligent and has excellent non-verbal means of communication, but not language. The accent in our speech that identifies the location where we grew up changes very little after a certain age. The suggestion of a limited period of life in which we can learn languages has implications for educational policies that seem not to be acted on. Around the time that we reach puberty, most of us lose the ability to learn other languages readily - just the age when most language instruction takes place. Do not imagine that these windows of opportunity are restricted to subjects learnt at school. Experiments with animals show that there is only a limited period in which they are able to acquire the skill to take advantage of the information provided by the eyes. This fact was confirmed recently in a human patient who had his sight restored by the transfer of stem cells when in his 40s, having been blind since the age of three. While he now has sight, he is unable to use visual information effectively.

Inevitably, Ridley's Nature via Nurture will be compared with Steven Pinker's recently published The Blank Slate: The Modern Denial of Human Nature , for both are concerned with the effect of genes and environment on human personality and ability. There the similarity ends. The tone of Pinker's book is often strident and offers a broad sweep across human personality, whereas Ridley has a more focused objective outlined at the start of this review.

Both authors consider the social and political implications of what is emerging from such research. They rail against the over-simplification of journalists who describe genes associated with specific characteristics as being genes that cause the trait in question. Most aspects of human behaviour are far too complex to be determined by a single gene: the genes in question are identified because a failure in the function of the gene prevents a specific characteristic from developing. Families with mutations in a gene known as FOXP2 are unable to acquire normal grammatical and speaking ability, for example. In that sense, the gene is essential for normal speech; but the gene alone is not sufficient for a person to have that ability.

Ridley ends with seven thought-provoking "paradoxical morals" in which he focuses on challenging issues such as the role of parents and the existence of free will. We are entering an exciting era in which many observations will be made on gene structure and function in different animals, including humans. These will be interpreted in the light of then-current views of human biology; and the historical perspective in both of these books reveals just how flawed our understanding has been in the past.

This book is part of the process of working out in some detail exactly how genes and environmental factors together build up human personality and ability. Ridley achieves his objective in revealing nature-via-nurture very well. Elegantly summarising his main point, he characterises genes as "devices for extracting information from the environment".

Ian Wilmut is head of the department of gene expression and development, Roslin Institute.

Nature via Nurture: Genes, Experience and What Makes us Human

Author - Matt Ridley
ISBN - 1 84115 745 7
Publisher - Fourth Estate
Price - £18.99
Pages - 328

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