Lifetime prevalence and co-morbidity of externalizing disorders and depression in prospective assessment

Mental illness creates major public health and economic burdens while causing considerable pain to patients and their families. Collectively, mental disorders account for the worldwide loss of 185 million years of healthy life due to disability and premature death (Murray et al., 2012). Estimates of the lifetime prevalence of mental illness—the percentage of individuals who have ever experienced a mental disorder—vary widely (Moffitt et al., 2010). Such estimates indicate the scope of mental illness, aid measurement of its economic burden, guide decisions on resource allocation toward mental health services and research, and inform the development of etiological theories. Accurate estimates are therefore imperative. Most prevalence estimates come from epidemiological surveys, like the Epidemiological Catchment Area program (ECA; Regier et al., 1984), the National Comorbidity Survey (NCS; Kessler et al., 1994), the National Comorbidity Survey Replication (NCS-R; Kessler et al., 2005), and the National Epidemiologic Survey on Alcohol and Related Conditions (NESARC; Compton et al. 2007; Hasin et al. 2007). The major strength of these surveys is their use of nationally representative samples, but they are believed to underestimate the lifetime prevalence of mental disorders because of documented recall failure in one-time, retrospective assessment, with respondents forgetting specific symptoms associated with remote episodes of mental illness (Simon & VonKorff, 1995; Angst et al., 2005). Epidemiological surveys might also underestimate lifetime prevalence due to a lack of rapport between researchers and participants, which may prevent the latter from disclosing private information during brief meetings with an unfamiliar interviewer. Prospective prevalence studies, which assess individuals repeatedly over time to detect who develops a mental disorder, can minimize problems with both recall failure and rapport. Prospective Prevalence Estimates In an early prospective study of 591 Zurich adults assessed six times over the course of 20 years, Angst et al. (2005) found that their cumulative prevalence rates of mental disorder categories were comparable to rates in the NCS. Subsequent studies have yielded different results. In a study of 352 New Englanders assessed three times between ages 21 and 30, Tanner et al. (2007) reported considerably higher lifetime prevalence rates. In 2010, Moffitt et al. found that cumulative prevalence rates in a New Zealand cohort (N = 1000) assessed four times between ages 18 and 32 were twice as high as rates in the NCS, NCS-R, and the New Zealand Mental Health Survey. Finally, in their study of a multi-cohort North Carolina sample (N = 1420) assessed up to nine times from a minimum age of 9 to age 21, Copeland et al. (2011) concluded that whereas “[o]nly a small percentage of young people meet criteria for a DSM disorder at any given time, … most do by young adulthood” (p. 252). In sum, while only a few studies have estimated the lifetime prevalence of mental disorders prospectively, most of these studies suggest that many more people experience mental illness than is commonly believed. Two of the four prospective studies reviewed above examined change in the lifetime prevalence of mental illness over time. Tanner et al. (2007) found that the lifetime prevalence of all disorders increased significantly between ages 21 and 30. Using imputed data, Copeland et al. (2011) found that the cumulative prevalence rate of any well-specified mental disorder increased from around 14% at age 9 to 70% at age 21 in the longest-studied cohort; the cumulative prevalence rate of any mental disorder (including “Not Otherwise Specified” disorders) increased from around 28% to 90%. Both studies thus suggest that the lifetime prevalence of mental disorders increases substantially throughout adolescence and young adulthood. Sex Differences and Comorbidity The NCS indicated that the lifetime prevalence of mental illness varies by sex (Kessler et al., 1994). Some prospective studies have examined sex differences. Consistent with extant research, Angst et al. (2005), Tanner et al. (2007), and Copeland et al. (2011) found higher rates of substance use disorders in males than in females, and the first two studies found higher rates of depression and anxiety in females. Compared to males, females had higher odds of a lifetime diagnosis of depression, PTSD, and phobia at age 30 and, in the case of PTSD, at ages 21 and 26 (Tanner et al.). Males had higher odds of a lifetime diagnosis of an alcohol use disorder at all three ages and had higher odds of a lifetime diagnosis of a drug use disorder at ages 26 and 30. Additional prospective research is necessary to confirm the magnitude and temporal stability of these sex differences. The NCS also revealed, “the vast majority of lifetime disorders … were comorbid disorders” (Kessler et al., 1994. p. 11). Few prospective studies have investigated comorbidity in representative community samples. In a prospective study that re-assessed a probability subsample of NCS participants 10 years later (NCS-2), Swendsen et al. (2010) examined which baseline mental disorders predicted the subsequent onset of substance use disorders. They found that behavioral disorders and preexisting substance use disorders were the best predictors, with strong support also for certain anxiety and mood disorders. Researchers have studied if comorbidity rates vary by age group in retrospective assessment. Using NCS-R data, King-Kallimanis et al. (2009) investigated comorbidity of MDD with anxiety disorders and dysthymia in older (65+ years) versus younger (18–64 years) adults. They found that 12-month and lifetime comorbidity rates generally did not vary by age group. In those rare instances where there were significant differences, comorbidity rates were higher among younger adults. The authors concluded that comorbidity is high across the lifespan. Prospective research that can assess comorbidity over time in the same sample is needed. Limitations of Existing Research Despite its important contributions, current prospective research has limitations. In particular, prospective prevalence studies have assessed either only a few common mental disorders or only classes of disorders (e.g., “any behavioral disorder”). Additionally, Moffitt et al. (2010) and Angst et al. (2005) used international samples, provided past-year appraisals of mental disorders at each assessment, and did not consider mental disorders developing before the age of 17 and 19, respectively. Copeland et al. (2011) inquired about the three months preceding each assessment and did not assess participants past the age of 21. Thus, although these studies show that the aggregation of cross-sectional data over repeated assessments can lead to increased prevalence rates, missing from the literature are estimates of lifetime risk when assessments are aggregated such that all intervening time is accounted for. Furthermore, there is a need for prospective research on sex differences in the lifetime prevalence of mental disorders, on comorbidity between disorders, as well as on changes in prevalence and comorbidity patterns as individuals age. The Present Study We extended existing research by examining the lifetime prevalence of and comorbidity between alcohol dependence, cannabis dependence, nicotine dependence, antisocial personality disorder (ASPD), and major depressive disorder (MDD) in a statewide Minnesota sample assessed four times between ages 17 and 29. We focused on externalizing disorders and depression because they are common, typically begin in adolescence to young adulthood, pose major public health, economic, and societal problems, and were measured throughout the duration of the study. Additionally, the ages studied encapsulate the period of greatest risk for the development of externalizing disorders, making these disorders a suitable target. First, we examined the lifetime prevalence of all disorders at ages 17 and 29 in the full sample as well as separately in males and females. Second, we studied the lifetime comorbidity between disorders at ages 17 and 29 in the full sample. Our aim was to obtain accurate estimates of the percentage of individuals who have ever experienced a mental disorder and of the co-occurrence among disorders within individuals’ lifetimes. Furthering this aim, our study has the following strengths: A large (N = 1252) sample that is representative of the Minnesota population and has very low attrition (<10%), reliable and comprehensive assessment methods, evaluation of previously unexamined mental disorders, and coverage of all the time between assessments and before the initial assessment.