September 25, 2013. People who later develop schizophrenia have thinking and memory (or "cognitive") difficulties that disrupt their everyday functioning both early in life—long before other symptoms appear—and throughout the course of their illness. The difficulties begin at different times for different types of cognitive functions, according to the most comprehensive long-term study of these problems to date, published online September 13, 2013, in the American Journal of Psychiatry.
Terrie Moffitt of Duke University in Durham, North Carolina, is the associate director of an ongoing project looking at the physical and mental health of all babies born at one hospital in Dunedin, New Zealand, between 1972 and 1973. The Dunedin study brings participants back to the research center every few years for evaluations (most recently at age 38). In the most recent study, first author Madeline Meier and colleagues found that 4 percent of the children eventually developed schizophrenia, and by comparing their scores on cognitive tests at several time points across development to children who did not get a mental illness, the researchers could examine how their mental functioning changed over time.
Moffitt and colleagues found that the overall general intellectual ability (IQ) of subjects who later developed schizophrenia was lower than that of healthy controls in childhood (before illness onset) and even lower in adulthood (after illness onset). When the researchers used more specific tests to examine different kinds of mental functioning, they also found declines over time. The specific pattern depended on the mental process being tested, with some, like verbal difficulties, developing during early childhood and remaining constant through adulthood. Mental processing speed, on the other hand, appeared normal at age seven in those who would go on to develop schizophrenia, but declined slowly between age seven and 38. The findings suggest that the two patterns of cognitive decline may have different underlying causes. (For more details, see the related news story.)—Allison A. Curley.