Why use twins to study psychosis?

Professor Robin Murray: If you take identical twins, they are identical in many ways, but it’s very interesting if they are not identical for a disease. If they have a wholly genetic disease, then both of them will get it, but in psychiatry and in schizophrenia, it’s quite common to find that one twin will develop the illness and another twin will not develop the illness.

Identical twins are absolutely the same genetically, but if one person, one twin develops schizophrenia, there’s only about a 45 per cent chance that the second twin will develop schizophrenia, even though they have exactly the same genome. So this demonstrates that you don’t actually inherit the schizophrenia, you inherit an increased risk of the schizophrenia, and you may or may not develop it.

We have studied identical twins who are discordant for schizophrenia – one has schizophrenia and the other doesn’t. And we look to see what has happened to the twin who has developed schizophrenia, as compared with the twin who remains well.

So, for example, the twin who develops schizophrenia is more likely to have had something developmentally abnormal happen to them. Maybe, in the days when obstetrics wasn’t so good, they were the second twin and maybe they had some oxygen deprivation at birth, or maybe that they had some childhood illness like meningitis, or maybe they, but not their brother, their co-twin, started abusing drugs, or some adverse life event happened to them.

So in a sense, if you have the predisposition, and you lead a charmed life, and everything goes well in your life, you may well never develop the illness, but on the other hand somebody who has either physical or social adverse effects – that’s more likely to bring on the pre-disposition.