Genes caused upward social mobility to come to a grinding halt after the 1970s, according to some theories. The poorest people score lowest on IQ tests and the poor marry each other, as do the rich. The theorists rarely say it, but they mean that the genetic sludge sinks to the bottom of the gene pool and stays there. What is the evidence for this?
Studies of identical twins show that IQ is half heritable. But there is a major problem with these studies: they are completely unreliable. To take one of many examples, it is presumed that identical and non-identical twins are equally likely to be nurtured in the same way.
Yet this "equal environment assumption" has been disproved in numerous studies. Anyone who has come into contact with identical twins will know how hard it is to tell them apart. That is bound to lead to parents treating them more similarly than non-identicals, therefore producing biased results.
Ah yes, but what about those studies where the twins were separated at birth? In fact, there is only one large study, by Thomas Bouchard, which has had acres of newspaper coverage and hours of TV programming devoted to it (including a BBC series by Robert Winston). Bouchard's study claims to show that identical twins raised apart have more similar IQ scores than identical twins raised together, suggesting both that genes do contribute to intelligence and that the genetic effect is far greater than the environmental effect. But his study can be questioned on a number of methodological grounds, such as the way in which he obtained his sample. He has also refused to allow independent scrutiny of his data.
The only truly reliable method is possibly the Human Genome Project. It proves that we have only 30,000 genes, rather than the 100,000 expected. In the words of Craig Venter, one of the project's heads, this suggests that "the wonderful diversity of the human species is not hard-wired in our genetic code. Our environment is crucial".
Subsequent research supports him, and in the past couple of years, leading figures in the field have admitted that there are no single genes for almost anything, certainly not for mental illnesses, such as schizophrenia or manic depression.
On a broader canvas, cross-national differences make a major role for genes seem unlikely. For example, by far the most reliable evidence of prevalence of mental illness (a recent WHO study of 15 nations) shows that English-speaking nations have twice as much as mainland Western Europe (23 per cent vs 11.5 per cent).
I am not claiming that the differences between your offspring or between your pupils are caused by upbringing, merely that the scientific evidence for genes is nowhere near as strong as is often suggested and widely credited. I do not doubt that all of us are born with a genetically-caused psychological repertoire for humour, anger, sexual desire and so on.
I also suspect that in some cases, extreme mental illnesses, such as autism, can be wholly or largely caused by genes, although even these can often be wholly caused by nurture - it is now an accepted fact that at least half of schizophrenics suffered sexual abuse, for example.
Overall, I suspect that in most respects we are not a product of "a bit of both" nature and nurture, that usually it is largely nurture, occasionally largely nature and occasionally a bit of both. But that's just an opinion.
And until we know for sure, look closely at the next media report you see of "a gene for" any psychological trait. If it is based on a twin study, ignore it, and if based on a test of DNA, only pay attention if it has been replicated by other studies. Most fundamentally, as teachers, all the evidence so far indicates that you can radically change your pupils because their destiny is not in their genes.
Oliver James is the author of Affluenza: How to be successful and stay sane. The second edition of his They F*** You Up: How to survive family life is out now.
Unreliability of twin studies: Joseph, J., The Gene Illusion (2003) and The Missing Gene (2006).
See also appendix 2, James, O.W., They F*** You Up (2007).
Unreliability of Bouchard study: James, O.W., They F*** You Up, appendix 1.
Sexual abuse and schizophrenia: Read, J. et al, 2005, Acta Psychiatrica Scandinavica, 112, 330-50.
WHO study: Demyttenaere, K. et al, 2004, Journal of the American Medical Association, 291, 2581-90; Oakley Browne, M.A. 2006, www.moh.govt.nzmoh.nsfpagesmh5223$filemental-health-survey.pdf