The evidence that cognitive impairments are present, and in at least
some individuals are progressive, before the onset of psychosis
suggests that they may reflect an early and ongoing pathogenic
process. For example, as early as the 4th grade of school, individuals
who would later be diagnosed with schizophrenia scored, on average,
more poorly than their peers on standardized tests of scholastic
performance, and in many domains their performance relative to peers
worsened through the school years (Keefe & Fenton, 2007). Similarly,
mean childhood IQ scores were lower in individuals who grew up to
meet diagnostic criteria for schizophrenia relative to comparison
groups composed of either those who were later diagnosed with major
depression or those who did not develop a psychiatric illness
(Reichenberg et al., 2010). In particular, relative to both of these
comparison groups, individuals later diagnosed with schizophrenia
failed to show the normal degree of improvement in working memory
between ages 7 and 13 years (Reichenberg et al., 2010).