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Research finds genetic link to personality trait

The first evidence that a normal personality trait is partially medicated by genes comes from two studies, one from Israel and the other from the United States (Nature Genetics 1995;12:78-80, 81-4). Both test the hypothesis that individual variation in the "novelty seeking" trait, a normal personality characteristic that we all possess, is associated with a genetically determined variability in dopamine transmission in the brain.

Both found, in different populations of normal subjects, a significant association between high test scores on validated questionnaires measuring novelty seeking -- characterised by excitable, impulsive, exploratory, and extravagant behaviour--and the presence of long alleles of D4 dopamine receptor gene exon III. Those with only short alleles had lower novelty scores, which are associated with more reflective, rigid, and orderly behaviour. The association was independent of ethnic group, age, and sex.

Although it has long been recognised, largely from studies on identical twins, that much of human behaviour is determined by heritable factors, this is the first time that molecular evidence has shown that variations in normal human behaviours are influenced by genes. The finding is strengthened by being shown by two independent groups of scientists using different personality questionnaires in ethnically distinct populations. The 124 Israeli subjects were unrelated volunteers, 69 males and 55 females, recruited from staff at the Ben-Gurion University and Beersheva Mental Health Centre. They completed a Hebrew version of the trimensional personality questionnaire.

In the US the subjects were participants in two existing National Institutes of Health studies looking at the X chromosome, and so were predominantly male. Of the 315 subjects in the study, 291 were siblings from 131 families.

In this study a personality inventory was used that did not include novelty seeking as a specific factor but did ask questions such as "Have you ever done things just for kicks or thrills?"

The commentary that accompanies the papers suggests that this is landmark research which should encourage a new research effort to map genes that are related to normal temperament. It will also boost research that is already under way to identify genes that are associated with complex unhealthy risk related behaviours such as aggression, excessive alcohol consumption, and smoking and with mental disorders such as schizophrenia.

"The logic for looking at the D4 dopamine receptor gene," said Dr Dean Hamer, one of the authors of the US study from the National Institutes of Health in Bethesda, Maryland, "stems from two lines of evidence which show that dopamine is an important neurotransmitter involved in pleasure and reward. One comes from experiments in mice and in rats where if you raise their dopamine levels the animals perk up, run around and explore, and if you lower them they just sit around. Indirect evidence in humans comes from the study of patients with Parkinson's disease. There, a reduction in dopamine levels is associated with a rather rigid personality and lack of interest in new things.

"It is important to realise, however, that not just one gene determines novelty seeking behaviour. Novelty seeking is a complex normally distributed trait, and we suspect that about 10 different genes are involved as well as non-genetic factors. It is also worth emphasising that this discovery does not suggest that a predictive genetic personality test is round the corner. Looking at people's lifestyle and assessing their personality by means of questionnaires is infinitely more informative than any genetics test would be."--TESSA RICHARDS, BMJ

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