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Genes and personality.


So far we have looked at genetic problems which give rise to specific clearly defined syndromes, such as phenylketonuria. They are not strongly linked to the environment, particularly the social one, though it does have an effect. The effects of PKU, in particular, are ameliorated by a change of diet. In a vegetarian culture, it might never be heard of.

It is when it comes to a socially defined behaviour, that studies become open to question.

The Aggressive Gene.

Jones(1) describes how one member of large Dutch family noticed a long history of violence among his relatives. Among those living, eight were found to have a substance in their urine which should have been broken down by the body.

The problem was traced to a defective gene which should have made a substance called monoamine oxidase. This has a number of complex functions in the body, among them to break down surplus chemicals which modify the action of the nervous system, adrenalin, and a chemical in the brain called serotonin. The gene turned out to be on the X chromosome, which is why the women in the family were unaffected.

It made headlines in the press. The researchers had found 'a gene for male violence' However, in spite of extensive testing of other violent people, it appeared to be confined to one family.

Moreover the effect was quite variable. Most of those with the gene had never committed any offence. Some had attacks of high blood pressure, sleeplessness or agitation after eating certain foods, since they could not digest certain substances found in red wine, chocolate and soft cheese.

Here we have behaviour linked to a single gene effect, one that is known.

The Homosexual Gene 

The "Gay" Gene

The likelihood, however, of there being single genes for complex behaviours is remote.

Hamer,(2) in 1993, was investigating possible genetic factors in the incidence of certain cancers in gay men with AIDS. As a result, he was able to trace links between homosexual men appearing among certain families for up to three generations. Since some were not directly related - they were uncles and cousins - he suggested that influence of upbringing was likely to be reduced.

Although highly publicised, his study was not the first. Pillard(3) had surveyed a number of families and found clusters of gay men. He also found a relationship with lesbian women, though it was not as strong. Bailey et al(4,5) carried out a survey in which he asked for responses from homosexual twins. For identical twins, he found a 52% chance that the brother was also homosexual, while with fraternal twins it was 22%. He also found strong evidence for the inheritance of lesbian sexuality.

With the indication that there might be a common genetic influence, Hamer(6) went on to obtain genetic profiles from forty homosexual men, and discovered a common gene marker in their X chromosomes. He was very careful in his conclusions. There is still a great deal of work to isolate the actual gene, or group of genes, involved. It would also be necessary to find out how such genes would exert their effect, in other words a precise causative link. He made it clear that there are probably other genes involved and, in any case, they would probably only impart a tendency towards becoming homosexual.

Although some of the subjects were not directly related, their upbringing would not be entirely dissimilar, and they lived in a similar culture. It would, of course, be no surprise that personalities tend to run in families.

Here we have a behaviour that has a name, though in fact its manifestations are extremely complex. What about even less well-defined behaviours?

The Socialisation Gene

In The Guardian of 12th., June, 1997, the headlines read: "Genes say boys will be boys and girls will be sensitive" This was followed by a bitter attack on the 14th. by Susie Orbach and Joseph Schwartz. The source of this uproar was a study by Skuse et al(7)

The Skuse team's actual report was much more sober and circumspect. It did not, in fact, specify any particular behaviour, using an abstract measure which they termed "social cognition".

The report simply noted that boys are more at risk of requiring remedial teaching and of being autistic. The reasons for this are, in any case, a matter for fierce debate. One problem is that figures for children in special education do not usually distinguish academic ability from behavioural problems, and the two do not necessarily go together. Skuse's study looked for a similar measure of Turner's children and noted any difference.

Skuse hypothesised that, if there were a gene on the X chromosome that coded for social behaviour and if in addition the presumed genes on the maternal X chromosome were inactivated, by 'imprinting', in ordinary boys and girls, then this could account for such differences. The father's X chromosome would affect daughters but not sons. Since a Turner's child has its single X chromosome from its mother or its father, the relevant gene would either be activated or not.

The parents of a group of 88 Turner's syndrome girls and young women, were asked to complete a "social cognition" questionnaire.

It was this which raised Orbach and Schwartz's hackles. Parents were asked to rate their children on a scale of zero to two as to "whether they are very demanding, disruptive of family life. and unresponsive to commands." The questionnaire was tested with control group of parents of 70 ordinary boys and 71 ordinary girls aged 6-18 years.

Their complaint was that the measured difference was very small and that it was based on parents' subjective assessments of their own children. They suggested that we need to know more about differences in appearance which may evoke responses on the part of parents which might inhibit or facilitate their capacity to relate easily to their children and encourage their sociability, particularly in regard to their condition.

To be sure, the team also used more rigorous and well characterised psychological tests. However these tests themselves do not necessarily imply a genetic origin, or eliminate an experiential one. A point could, however be made that boys are socialised as boys, whereas all Turner's syndrome children are socialised as girls.

What we have is that there appears to be a difference between boys and girls, which is paralleled in the Turner's children, correlated with a hypothesised gene effect, in a generalised Western environment, but it is not clear what that difference is.

Here we have three studies said to link genes to different behaviours, but we need to remember correlation does not necessarily imply causation. Only in the first has an actual causation been shown and, even then, it was variable. The gene concerned might be said to be polymorphic, that is to say it has different effects. More precisely its phenotypic expression is variable. Neither of the second two studies showed direct causative links. Homosexuality is a social construction, while what the Skuse study measured was an artifact of the study itself.

Bibliography and good reading.

  1. Jones, S., (1996) In The Blood: God, Genes and Destiny, (p231) London: Harper Collins
  2. Hamer,D. et al. reviewed in LeVay, S., (1993) The Sexual Brain Massachusetts Institute of Technology
  3. (not read)Pillard, R.C., Weinrich, J.D.,(1986) Evidence of Familial Nature of male homosexuality. Archives of General Psychiatry 43:808-812 in LeVay
  4. (not read)Baily, J.M., Pillard, R.C. (1991) A genetic study of male sexual orientation. Archives of General Psychiatry 48: 1089-1096 in LeVay.
  5. (not read)Baily, J.M., Pillard, R.C., Agyei,. Y. (1993) A genetic study of female sexual orientation. Archives of General Psychiatry 48: 1089-1096 in Le Vay
  6. Hamer, D.H., Hu, S., Magnuson, V.L., Hu, N., Pattatucci, A,M.L., (1993)A Linkage Between DNA Markers on the X chromosome and Male Sexual Orientation, Science, p321-327, vol 261, 16th. July, 1993
  7. Skuse, D. H., James, R.S., Bishop, D.V.M., Coppin, B., Dalton, P., Aamodt-leeper, G., Bacarese-hamilton, M., Creswell, C., Mcgurk. R., Jacobs, P. A. (1997) Evidence from Turner's syndrome of an imprinted X-linked locus affecting cognitive function. Nature 387, 705-708

Newspaper articles:
Radford, T, (1997) Genes say boys will be boys and girls will be sensitive, The Guardian, Thursday, 12th., June, 1997
Orbach, S., Schwartz, J., (1997) Why are little boys slugs and snails while little girls are all things nice? The Guardian, Saturday, 14th., June, 1997

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Bland, J., (2002) About Gender: Genes and personality.
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28.03.99 Last amended 21.07.02