Long-term, or longitudinal, studies allow researchers to look back in time to see what factors early in people's lives are associated with them developing mental-health problems later. People who develop schizophrenia later in life tend to have lower IQs when they are younger and a team of researchers led by Annick Urfer-Parnas at Hvidovre Hospital in Denmark looked to see whether the same was true for other mental-health problems. People in Denmark take an IQ test when they are conscripted into the military at 18 and, in a sample of 28,017 people the IQ scores of the 7,486 people who went on to be hospitalised with a mental illness were compared to those of the 20,531 people who didn't. The study found that the people who went on to develop mental-health problems had significantly lower IQs than those who did not. The people with affective disorders had the highest IQ at 18 and those with personality disorders the lowest. For each mental-health problem a lower IQ was associated with an increased risk of becoming a patient.
Urfer-Parnas, A. ... [et al] - Pre-morbid IQ in mental disorders: a Danish draft-board study of 7,486 psychiatric patients Psychological Medicine April 2010, 40(4), 547-556