Twin studies shed light on human behaviour

A woman's tendency to be unfaithful is influenced by her genes, according to a new study of twins. A second study reveals that genes are also partly to blame for snoring and sleep disorders. The surveys of identical and non-identical sets of twins found that the genetic contribution (heritability) to both infidelity and poor sleeping is surprisingly high. Scientists from St Thomas's Hospital in London carried out the studies, which will both be published in the journal Twin Research.

In the first study, the researchers quizzed more than 1,600 pairs of female twins, and found that a tendency to be unfaithful has a heritability of 41 per cent. Genes also appear to influence the number of sexual partners that a woman has, a trait estimated to have a heritability of 38 per cent. Team leader Tim Spector believes the findings suggest that infidelity and other sexual behaviours persist because they gave women a reproductive advantage at some point in human evolution.

Spector stressed that is no single 'infidelity gene', but probably 50-100 genes that make people respond to situations in different ways. 'For example, it may be important for a woman's survival to be unfaithful to a violent husband or when she sees she is unlikely to produce children - there is evidence for having affairs with people with higher standing or better genes', he said.

In the second study, the researchers questioned 1,937 pairs of twins on sleep disorders such as obstructive apnoea (snoring) and restless leg syndrome. They found high heritabilities for disruptive snoring (42 per cent), daytime sleepiness (45 per cent), restless legs (54 per cent) and jerking legs (60 per cent). The results took account of other factors that can trigger snoring, such as being overweight or smoking. Spector said that the genetic variations responsible for disruptive sleep may have survived because poor sleep patterns are linked to weight gain. 'These genes may have helped our ancestors through periods of famine and the Ice Age', he said.

Scientists have studied twins for many years, in order to measure the influence of genes and environment on a wide range of traits. They compare identical twins, who have identical genes and similar backgrounds, with non-identical twins - who also share similar backgrounds, but are no more genetically alike than ordinary siblings. In this way, researchers can estimate how much of the variation seen for a particular characteristic is accounted for by genes - a measure of 'heritability'. A trait that is completely controlled by genes, for example eye colour, has a heritability of 100 per cent. At the other extreme, a characteristic that has nothing to do with genes, such as a scar, will have a heritability of zero.

The new studies are both part of an ongoing project taking place at the Twin Research Unit at St Thomas's, which has 10,000 twins on its register. Another survey carried out by the team looked at religious belief, and found that although belonging to a particular religion depends entirely on upbringing, belief in a god, or 'spirituality', had a heritability of 40 per cent. Any twins aged over 15 (either identical or non-identical) who would like to take part in the research should call 020 7188 5555.