It has been known that schizophrenia runs in families. This could be because family members share the same disadvantaged environment, but a large body of research evidence suggests that genetic factors are also important.
First-degree relatives (parents, siblings, offspring) share an average of 50% of their genes whilst second-degree relatives share around 25%. Kendler et al 1985 showed that first-degree relatives of those with schizophrenia are 18 times more at risk than the general population.
Family studies are often inconclusive as they are conducted retrospectively, that is, they are comparing a cross section of people who have already been diagnosed. A longitudinal study can provide more reliable data as it follows the same group of people over a period of time and make comparisons before and after any of the signs of illness occur.
Many studies have been conducted and have shown that monozygotic (identical) twins, who share 100% of their genes, have much higher concordance rates (the likelihood of both twins being affected) than dizygotic (non-identical) twins, who share only 50%. Researchers seek out MZ twins that have been reared apart where at least one has been diagnosed with schizophrenia. Gottesman & Shields 1982 used the Maudsley twin register and found 58% were concordant for schizophrenia. A study by Fischer 1971 found that 9.4% of offspring from a non-affected discordant MZ twin developed schizophrenia- much higher than the general population (1%). A study in London by Cardno found a 40%…