Book Review: Understanding the Nature-Nurture Debate

Understanding the Nature-Nurture Debate
By Professor Eric Turkheimer
Published by Weidenfeld and Nicolson
ISBN-10: 1108958168, ISBN-13: 978-1108958165
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Eric Turkheimer is a professor of psychology at the University of Virginia who has worked in the field of behaviour genetics for around four decades. He is a past president of the Behaviour Genetics Association. He is, in short, very well qualified to guide us through this subject in 'Understanding the Nature-Nurture Debate'.

But is there much more to say about nature-nurture? Should we react to Professor Turkheimer's, and any more books on this topic, in the manner of Brenda from Bristol, when told about the snap general election called by Theresa May: 'You're joking – not another one!'?

In short, my answer is that yes there is more to say, and we should welcome another one (and more).

There are of course many historical and technical aspects to the nature-nurture issue which numerous books have covered before. Professor Turkheimer deals with these well for the general reader, despite (or maybe because) of the book's relative brevity. Chapters on Sir Francis Galton, statistical methods, the history of IQ controversies, and the past 30 years of gene hunting cover essential ground. But this book is not simply a handy primer for those new to the subject. It is an argument for a re-framing of the nature-nurture debate. For while Professor Turkheimer is a paid-up member of the community, he is also something of an outlier – a persistent critic of those within the field he believes have over-sold the explanatory and predictive power of genetic findings; indeed, of what genetics could ever explain and predict. Crucially, he thinks some of the latest research findings vindicate his long-standing views.

In this review I highlight two themes. The first is Professor Turkheimer's assessment that recent genomic studies show that heritability for some of the traits we most care about is far lower than was previously thought, and, choosing the example of IQ to develop his point, that this matters:

'GWAS [Genome Wide Association Studies] has turned the tables on the heritability of intelligence, from the 80 percent presumed by [Arthur] Jensen to something closer to 20 percent now; within-family analyses have reduced the heritability even further... Polygenic scores for intelligence, especially when they are properly corrected for family-level differences by estimating them within sibling pairs, don't work well enough to be useful to anyone, or to prompt anyone to think there are powerful deep-seated genetic causes of differences in cognitive ability.'

Is Professor Turkheimer obviously correct here? The original higher figures were derived using a variety of family studies, including twins, both identical and non-identical, living together and apart, as well as more than one kind of adoption study design.

'Missing heritability' was the term used for the fact that the proportion of total phenotypic variation accounted for by known genetic factors fell well short of what was regarded as the true value for heritability established by family studies. The twist is that now some people, including Professor Turkheimer, think it is not missing – it is rather not there to be found. The basic idea of the new approach is that the total genetic contribution to variance can be established by genomic methods even if all the specific genetic factors are not known.

When these methods are applied, typically, the values come in much lower than those derived from family studies not based on detailed genomic analysis.

Proponents of traditional family studies do however have some good arguments in response. The newer methods are, well, new, and we do not have that many studies yet. The values found via newer methods vary quite widely. The obvious shared environmental factors explain very little of variance, especially by adulthood, so the idea that values derived from twin studies are biased upwards by hidden shared environmental factors is difficult to sustain ('heritability is missing, but so is environmentality', as Professor Kathryn Paige Harden puts it).

Perhaps most significant of all, the values derived from twin studies roughly match those from adoption studies using correlations between the adoptee and their adopting parents as compared with correlations between the adoptee and their biological parents, a method which gets around some of the obvious possible biases in some twin studies.

Does it matter if heritability is 15 percent or less rather than 80 percent? There is a standard argument that it is a population-specific measure, so it doesn't matter – it is only telling us something about a specific population at a specific time and place. But I have some sympathy with Professor Turkheimer's view that there is a clear difference between 15 percent and 80 percent heritability in the implications for the kinds of societies we live in now and for the malleability of important traits. In many ways I would like Professor Turkheimer to be right. We can't, however, let the wish father the thought.

Time, perhaps not too much of it, will tell us where the consensus settles regarding heritability estimates for IQ and other important social characteristics. But even if the estimates settle higher than Professor Turkheimer's current estimates, he has more to say in this book about the pitfalls of heritability calculations, at a more conceptual level. This takes us to his second broad theme: focusing on heritability as such can side-track us from other, perhaps more important, issues.

In particular, does heritability as such tell us anything meaningful about what he believes we specifically care about in the nature-nurture debate: the character of, and variation among people in, our most obviously human traits; and what it means to be human?

Professor Turkheimer discusses his own divorce when he was a young man. Does the fact that divorce has a non-zero heritability tell us anything meaningful about the character of divorce in general or his own divorce in particular? He thinks not. On the other hand, he argues, the heritability of many genetic diseases tells us something meaningful about the disease itself and an individual's experience. He calls divorce an 'outcome heritability': 'arising as a statistical remnant in developmental systems that lack specific genes that cause specific outcomes'; and genetic disease an 'essence heritability': 'a system in which a heritable phenotype is represented in a specific way in a recognisable genetic system.'

It's a stimulating way to think about these issues, which for me brought to mind Evelyn Fox Keller's dissection of the conceptual connections and differences between a trait and trait differences – variations in factors may affect the latter without telling us much or anything about the nature of the former.

This distinction between outcome and essence connects to the biggest issue of all. At the start of the book, he tells us: 'post-Darwinian people know that we are one with nature, completely so in a scientifically literal sense, but we are also unable to give up the idea that we are apart from it.'

Later, he raises the issue again, focusing on conscious control: 'Are human beings just animals, or are we different in some way. Am I simply a product of the biology I was born with, or do I have some conscious control over the human being I become?'

At the end of the book the question marks and ambiguity have gone, and he declares his hand: 'Analysis of variance, clever and useful though it may be, is not capable of providing an answer'; 'we are animals, full stop. Yet we are not.'

Just as it is 100 percent certain this will not be the last book on nature-nurture, so it is pretty much certain that many people will disagree with Professor Turkheimer's reaching for a humanistic framing of the debate. But wherever you stand on that, this book is very much worth selecting as a guide, or provocation, from the wide variety of books available on this ever-fascinating topic.

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