Schizophrenia may affect up to 1 percent of the population, it is diagnosed primarily in the teenage or early adult years, and is associated with problems in mental ability and memory.
People who have a greater familial genetic link to schizophrenia are more likely to see a fall in IQ as they age, even if they do not develop the condition, according to psychologists in a new paper.
Genome-wide association studies (GWAS) have not found genetic causes for schizophrenia but familial instances are considered a risk factor. The psychologists used genetic analysis matched with IQ test results to reach their conclusion on how thinking skills change with age.
They compared the IQ scores of more than 1,000 people from Edinburgh who were tested for general cognitive functions in 1947, when the subjects were aged 11, and again when they were around 70 years old. The researchers were able to examine people's genes and calculate each subject's "genetic likelihood" of developing schizophrenia, even though none of the group had ever developed the illness. This term warrants a little skepticism, since obviously if science knew how to accurately calculate a genetic likelihood they could know if schizophrenia was caused by a lot of common things, one high impact mutation or that it is a lot less genetic than psychologists believe. What they mean is they determined whether or not it 'runs in the family', some common polygenic instances that researchers have proposed as genetic candidates.
With the polygenic similarity results, they compared the IQ scores of people with their high and low risk of developing schizophrenia. They found that there was no difference at age 11, but people with a greater genetic risk of schizophrenia had slightly lower IQs at age 70.
Those people who had more "genes linked to schizophrenia" also had a greater estimated fall in IQ over their lifetime than those at lower risk.
Psychologist Ian Deary, Director of the University of Edinburgh's Centre for Cognitive Ageing and Cognitive Epidemiology, who led the research team, said, "Retaining our thinking skills as we grow older is important for living well and independently. If nature has loaded a person's genes towards schizophrenia, then there is a slight but detectable worsening in cognitive functions between childhood and old age."
Andrew McIntosh, of the University of Edinburgh's Centre for Clinical Brain Sciences, said, "With further research into how these genes affect the brain, it could become possible to understand how genes linked to schizophrenia affect people's cognitive functions as they age."
Citation: Andrew M. McIntosh, Alan Gow, Michelle Luciano, Gail Davies, David C. Liewald, Sarah E. Harris, Janie Corley, Jeremy Hall, John M. Starr, David J. Porteous, Albert Tenesa, Peter M. Visscher, Ian J. Deary, 'Polygenic Risk for Schizophrenia Is Associated with Cognitive Change Between Childhood and Old Age', Biological Psychiatry published online 18 February 2013 doi:10.1016/j.biopsych.2013.01.011