Most of the statistical techniques used in the social and behavioral sciences estimate the linear relationships between variables. These techniques often accurately model hypothesized relationships (e.g., a "dose-response" relationship between stress and depression). But not all research questions imply linear relationships and caution is needed to insure that linear models are not estimated in a pro forma way.
Some individuals experience events or conditions that are milestones or turning points in the life course. Conceptually this implies that certain events or conditions literally change the direction of life course pathways (Nagin, Pagani, Tremblay, & Vitaro, 2003; Ronka, Oravala, & Pulkkinen, 2003). Statistically, milestones and turning points are observed as deflections which change either the direction or rate of change in trajectories (Haviland & Nagin, 2005; Seidman & French, 2004).
Mental health problems experienced early in the life course can be milestones or turning points, resulting in a variety of alterations in the life course. This is another topic for which dichotomous measures of mental illness are especially salient. A milestone implies a discrete change in status or direction. The only way to study mental illness as a life course milestone is to identify the times at which individuals transition into mental illness and trace subsequent alterations in their lives. Two primary types of turning points have been studied to date.
The Consequences of Early Mental Illness for Adult Achievements. There is substantial evidence that mental health problems during childhood, adolescence, and/or early adulthood are associated with poor social and economic achievements in later adulthood. Early onset mental health problems predict lower educational attainment (e.g., Chen & Kaplan, 2003; Gore & Aseltine, 2003; Woodward & Fergusson, 2001) and lower occupational status and income (Chen & Kaplan, 2003; Wiesner, Vondracek, Capuldi, & Porfeli, 2003). Adult social bonds also are negatively affected; early onset psychiatric disorder predicts decreased likelihood of marriage, especially for persons with psychotic disorders (e.g., Walkup & Gallagher, 1999); earlier age of marriage for persons with affective and substance abuse disorders (e.g. Forthhofer, Kessler, Story, & Gottlib, 1996), earlier parenthood (e.g., Woodward & Fergusson, 2001), and higher rates of divorce (e.g., Wade & Pevalin, 2004).
The findings cited above are primarily from longitudinal studies that followed samples from adolescence through early to mid-adulthood. Although important, these studies cannot inform us about whether later onset of mental illness has similar effects on life course accomplishments. Logically, earlier onset would be more likely to decrease later achievements because both socioeconomic achievements, especially educational attainment, and family formation are typically established during young adulthood. Turnbull and colleagues compared individuals who experienced early (age 25 and younger) and later (age 26 and older) onset. They found that SES and family characteristics among those with late onset were no different from age peers who had no history of psychiatric disorder. Those with early onset, however, had significantly lower levels of SES and poorer family outcomes than their age peers who had no history of psychiatric disorder (Turnbull, George, Landerman, Swartz, & Blazer, 1990). Freud is credited with saying that the major tasks of adulthood are to love and to work. Clearly, early onset of mental health problems places successful fulfillment of those tasks at risk and can be a turning point in the life course.
The Consequences of Early Mental Illness for Adult Mental Health. Early onset of mental health problems also is a powerful predictor of recurrent mental health problems throughout adulthood. For example, Woodward and Fergusson (2001) report that anxiety disorder during adolescence is the strongest predictor of anxiety disorder in early adulthood. Similarly, Kim-Cohen and colleagues (Kim-Cohen, Caspi, Moffitt, Harrington, Milne, & Poulton, 2003), using data from a 15 year longitudinal study, report that 75% of the adults who experienced psychiatric disorders had also experienced mental illness as adolescents. Using data from the long-term study of Harvard undergraduate men, Vaillant and colleagues report that depression and excessive alcohol use during young adulthood are associated with increased risk of depression at ages 47 (Long & Vaillant, 1985) and 65 (Vaillant & Vaillant, 1990).
Delinquency during adolescence also increases the risk of multiple psychiatric disorders during adulthood. Using data from a sample of British women who were followed from birth though age 52, Kuh, Hardy, Rodgers, and Wadsworth (2002) report that adolescent delinquency predicted major depressive disorder in middle age. Similarly, Hagan and Foster (2003) report strong associations between adolescent delinquency and adult depression for both men and women and alcoholism for men. In both studies, the effects of juvenile delinquency were estimated with a wide variety of other predictors of mental health problems statistically controlled (e.g., educational attainment, adult SES, marital status). These authors posit that juvenile delinquency is a stronger predictor of depression and substance abuse than of antisocial personality, suggesting that what is called "conduct disorder" in psychiatric nomenclature is a generalized risk factor for a variety of mental health problems during adulthood.
There are undoubtedly multiple reasons for the strong relationships between early and later psychiatric problems. Genetics may play a role, although the often-observed shift from juvenile delinquency to affective or substance use disorders during adulthood seems to contradict the theory that specific genes lead to family histories of specific disorders. Unmeasured social factors may partially account for the strength of these associations. Obvious social risk factors (e.g., SES, demographic characteristics) have been controlled in the studies cited above, but those risk factors were measured at a single point. It is possible that more fine-grained patterns of change and stability would partially explain the pathways from early to subsequent mental health problems. Other scholars hypothesize that both early onset psychiatric problems and other adversities experienced early in life change brain anatomy and chemistry in ways that sustain vulnerability to mental health problems over time (e.g., Schulenberg, Sameroff, & Cicchetti, 2004).
It also is important to recognize that early adjustment problems, especially juvenile delinquency, often do not lead to mental health problems during adulthood. Two studies demonstrate this particularly well. In a series of research papers (e.g., Laub, Nagin, & Sampson, 1998; Sampson & Laub, 1990) and a book (Laub & Sampson, 2003) based on a sample of delinquent males born in the 1920s who were studied intermittently until age 70, Sampson and Laub demonstrate that the accumulation of social resources, especially stable jobs and supportive marriages, inhibit adult crime, psychiatric symptoms, and substance abuse/dependence. Long and Vaillant (1985) studied a sample of inner-city men from the time they were children until they were age 47. During childhood, boys from unstable homes exhibited substantially higher rates of delinquent behavior and psychiatric symptoms than boys in stable homes. By age 47, however, psychiatric and substance use symptoms were unrelated to childhood family stability. Both studies were based on high-risk samples where elevated rates of psychiatric problems during adulthood would be expected as a result of socioeconomic deprivations and high rates of delinquency. In both samples, however, surprisingly high proportions of study participants were able to overcome their early adversities and be productive and healthy adults. Thus, multiple trajectories describe both samples, with some pathways associated with continued psychiatric vulnerability during adulthood and other pathways demonstrating impressive resilience. Although early onset of psychiatric problems is a far from perfect predictor of mental health problems later in the life course, it is clear that early onset can be a milestone or turning point associated with chronic or recurrent psychiatric problems across adulthood.
The cross-fertilization of life course perspectives and the sociology of mental health is ripe for furthering our understanding of the conditions under which mental illness does and does not become a life course milestone. Although life course perspectives assume a link between past and current statuses, mental health theory informs us about the most likely pathways linking early mental illness to later achievements and mental health.
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