Blueprint: 1

by George Hatjoullis

An interesting book has just been published titled ‘Blueprint’ by Robert Plomin. It is subtitled, ‘How DNA makes us who we are’, so by now you know what it is about. Plomin is a psychologist that has devoted his career to examining the influence of inherited DNA on individual behaviour. It is a controversial field and often characterised as the ‘nature versus nurture’ debate. It was skimmed over and largely dismissed in the two degrees in psychology that I received, as my department was firmly in the nurture camp. I kept an open mind and have returned to this controversial subject, in part, because advances in DNA sequencing have given us new research tools.

Plomin concludes that typically 50% of the variation in a population across a specific trait can be attributed to genetic differences. He argues that this is the largest systematic influence on behaviour. The remaining 50% he claims is unsystematic environmental effects. This creates the possibility of meaningful prediction of behaviour from DNA. In contrast, environmental factors, being unsystematic in their influence, offer little predictive value.

The original evidence is indirect and comes from twin and adoption studies. The idea is to hold the environment or the genetic make-up constant and look at the trait variation in a population that is explained by what is left. The best test is of adopted Monozygotic twins (MZ). The latter come from the same egg which is fertilised by the same sperm. The DNA is near identical. They share a common pre-natal environment. If they are adopted at birth and brought up in different environments then trait similarities can be attributed to genetics. The problem is that sample sizes are of necessity very small. Plomin demonstrates that for weight MZ twins reared apart are similar to those reared together. This is evidence supporting DNA as a large systematic influence on weight variation. Plomin suggests this is also widespread among psychological traits, and hence behaviour.

The common criticism of adoption studies still applies. Adoption is not random. There may be systematic similarities between the families chosen for adoption. There may be an unconscious desire to place the twins into similar families. The environments may thus not be completely different and if there are systematic similarities then these might be captured by any statistical analysis and attributed to genetics. It is such criticism that have often resulted in the downgrading of twin and adoption studies, though quantification of the significance does not always accompany the critique. There is a strong sociopolitical influence on the status of such studies.

Another type of twin study is the comparison of MZ with DZ (dizygotic) twins both reared together. The latter come from two eggs fertilised by different sperm so are only, on average, 50% similar in DNA. The argument is that both types of twin have the same pre-natal conditions and grow up in the same home, so have the same environment (or so it is argued). One can correlate the extent to which both types of twin are similar for a given trait (in a given sample). Plomin offers data for the physical trait ‘weight’ for illustration. The weight of MZ twins correlates 0.84 and DZ twins 0.55. The difference suggests genetic influence (environment is assumed constant for each pair of twins). The difference is 0.29. If we double this we get the contribution of genes to the weight variation between twins. In this case it is 0.58 or 58% of the variation can be attributed to genetic influence.

The is known as the Heritability of weight for this sample. Interestingly, as the twins in the sample get older (it is from a longitudinal study over many years) the heritability rises to 80%. This is one of the interesting findings of the book. Heritability typically increases with age. However, you are probably wondering why we doubled 0.29. First consider what it would mean if the correlation of both sets of twins was the same. This would imply genes make no difference and heritability is zero. By the same token heritability must be 100% if genes explain all the variation in the two twin sets. However, since DZ twins are only 50% similar the correlation can only be 0.5 whilst that of the MZ twins would be 1.0. The difference is 0.5 which is doubled to give 100%. Intuitively it is because DZ twins are only 50% related through genetics that we need to double the difference to see the amount of weight variation across the two samples that is explained by genes.

Adoption studies compare variation of a trait among children and how the trait correlates with adoptive and biological parents. If genes are important for the trait then the correlation should be greater with biological than adoptive parents and vice versa. The criticism of all such studies is the assumption that environments are different or at least any similarities are unsystematic. Adoption is not  random process. Nor indeed are twins reared together necessarily in the same ‘environment’. Parent/child interactions can be quite different even with MZ twins. Provided these environmental pollutions of the data are unsystematic it should make no difference to the conclusions.

One of the most powerful observations of this book is that heritability increases with time. This supports the idea that environmental influence is unsystematic. Early year environmental influence seems to fade as the child matures. An interesting example of this has emerged from a study at Kings College, London. Shared environment factors can explain a meaningful amount of variation among children in getting to university but none of the variation in their performance thereafter. As the Americans might say, DNA will out.

It is likely this book will be misunderstood as indeed the whole area of such research has been. It offers potential sociopolitical conclusions that many will fear and suppression may be the first instinct. The indirect evidence of twin and adoption studies has been accumulating for decades and now forms a powerful body of evidence. It does not however allow individual prediction. What makes this book compelling is the advances in DNA technology that has opened up the possibility of individual prediction. This will be the subject of my next blog followed by a discussion of the ethical issues that arise. What do we do with this knowledge? I rarely read books the whole way through (notes and all) let alone start reading them again. I also rarely recommend books to anyone. This is the exception. If you only read one book this year, I suggest it is this one.