The Dunedin Study longitudinally ascertains mental disorders using a strategy akin to experience sampling: Every 2 to 6 years, we interview participants about past-year symptoms. Past-year reports maximize reliability and validity because recall of symptoms over longer periods has been shown to be inaccurate. It is possible that past-year reports separated by 1 to 5 years miss episodes of mental disorder occurring only in gaps between assessments. We tested for this possibility by using life-history calendar interviews to ascertain indicators of mental disorder occurring in the gaps between assessments, including inpatient treatment, outpatient treatment, or spells taking prescribed psychiatric medication (indicators that are salient and recalled more reliably than individual symptoms). Life-history calendar data indicated that virtually all participants having a disorder consequential enough to be associated with treatment have been detected in our net of past-year diagnoses made at ages 18, 21, 26, 32, and 38. Specifically, we identified only 11 people who reported treatment but had not been captured in our net of diagnoses from ages 18 to 38 (many of whom had a brief postnatal depression).
Symptom counts for the examined disorders were assessed via private structured interviews using the Diagnostic Interview Schedule (Robins, Cottler, Bucholz, & Compton, 1995 ) at ages 18, 21, 26, 32, and 38. Interviewers are health professionals, not lay interviewers. We studied
DSM-defined symptoms of the following disorders that were repeatedly assessed in our longitudinal study (see
Table S1 in the Supplemental Material available online): alcohol dependence, cannabis dependence, dependence on hard drugs, tobacco dependence (assessed with the Fagerström Test for Nicotine Dependence; Heatherton, Kozlowski, Frecker, & Fagerström, 1991 (
link)), conduct disorder, MDE, GAD, fears/phobias, obsessive-compulsive disorder (OCD), mania, and positive and negative schizophrenia symptoms. Ordinal measures represented the number of the 7 (e.g., mania and GAD) to 10 (e.g., alcohol dependence and cannabis dependence) observed
DSM-defined symptoms associated with each disorder (see
Table S1 in the Supplemental Material). Fears/phobias were assessed as the count of diagnoses for simple phobia, social phobia, agoraphobia, and panic disorder that a study member reported at each assessment. Symptoms were assessed without regard for hierarchical exclusionary rules to facilitate the examination of comorbidity. Of the 11 disorders, 4 were not assessed at every occasion, but each disorder was measured at least three times (see
Fig. 1 for the structure of psychopathology models and see
Table S1 in the Supplemental Material).
Elsewhere we have shown that the past-year prevalence rates of psychiatric disorders in the Dunedin cohort are similar to prevalence rates in nationwide surveys of the United States and New Zealand (Moffitt et al., 2010 (
link)). Of the original 1,037 study members, we included 1.000 study members who had symptom count assessments for at least one age (871 study members had present symptom counts for all five assessment ages, 955 for four, 974 for three, and 989 for two). The 37 excluded study members comprised those who died or left the study before age 18 or who had such severe developmental disabilities that they could not be interviewed with the Diagnostic Interview Schedule.
Caspi A., Houts R.M., Belsky D.W., Goldman-Mellor S.J., Harrington H., Israel S., Meier M.H., Ramrakha S., Shalev I., Poulton R, & Moffitt T.E. (2014). The p Factor: One General Psychopathology Factor in the Structure of Psychiatric Disorders?. Clinical psychological science : a journal of the Association for Psychological Science, 2(2), 119-137.
