Association of Eating Disorder Symptoms with Internalizing and Externalizing Dimensions of Psychopathology among Men and Women

Eating disorders (EDs) are pernicious, multi-factorial conditions whose complexity is further compounded by their high rate of comorbidity with many other psychiatric disorders,1,2 especially depression, anxiety, personality, and substance use disorders. In the National Comorbidity Study-Replication, 47.9% of individuals with a history of anorexia nervosa (AN) had a history of an anxiety disorder, and 42.1% had a history of a depressive disorder. Among those with a history of bulimia nervosa (BN) these rates were 80.6% and 70.7%, respectively, and in individuals with a history of binge eating disorder (BED), 65.1% and 46.4%, respectively. The prevalence of a substance use disorder among individuals with AN, BN, and BED was 27.0%, 36.8%, and 23.3%2. One of the more influential contemporary models of psychiatric comorbidity is the internalizing/externalizing model, which organizes diagnoses on the basis of their associations with underlying liabilities. These higher-order liability factors, termed “internalizing,” representing mood and anxiety psychopathology, and “externalizing,” representing substance use disorders, antisociality, and conduct disorders, may account for a large proportion of the covariation among mental disorders3,4. Factor analytic studies generally have supported the hypothesis that common psychiatric disorders load onto either a latent internalizing or externalizing factor. For example, Krueger et al.4 found evidence for a two factor model among both women and men: major depression, panic disorder, simple phobia, and social phobia loaded onto the internalizing factor; and alcohol dependence, cannabis dependence, drug dependence, and adult antisocial behavior loaded onto the externalizing latent factor. This basic structure has been replicated in many factor analytic investigations, although in some models, the internalizing factor is bifurcated into two correlated subfactors: fear (panic disorder, simple phobia, and social phobia) and distress or anxious/misery (major depression, dysthymia, and generalized anxiety disorder). A recent examination of a model in which common psychiatric disorders loaded onto distress/anxious misery, fear, and externalizing found that this model was invariant for women and men, suggesting that the structure of psychiatric comorbidity may be equivalent across genders3. Until recently, factor analytic models of internalizing and externalizing dimensions of psychopathology have not included eating disorders. Forbush and colleagues5, in a sample of female twins, estimated a series of models to determine the location of eating pathology along the internalizing-externalizing dimension. They used a combined BN and BED variable and AN as indicators of a latent ED variable. The model with latent factors representing distress, fear, and EDs loading on a higher-order internalizing factor fit the data better than models in which the EDs loaded onto externalizing, or cross-loaded onto both latent variables. Forbush and Watson6 replicated this finding in an expanded internalizing/externalizing model that included both common and uncommon mental health diagnoses in a nationally representative sample of men and women. The underlying association between the internalizing and externalizing dimensions and EDs is of particular relevance to the study of gender in the etiology of EDs. It has long been thought that EDs affect women at much higher rates compared to men. However, population-based studies have shown that men comprise a higher percentage of ED cases than previously believed2. Further, men and women have fairly comparable rates of binge eating and BED7,8. In general, men have a higher prevalence of externalizing disorders, such as substance use disorders, while women have a higher prevalence of internalizing disorders, including depression and anxiety3,9,10. Thus, there may be important gender differences in patterns of comorbidity across the EDs and other disorders that reflect the influence of these latent dimensions. One population that has been understudied with respect to EDs is veterans. Preliminary findings suggest that rates of these conditions among military and veteran samples are comparable to those in the general population, if not higher11. The high overall rates of psychiatric morbidity among veterans, including depression and substance use disorders12,13, suggest the need to further evaluate the EDs in this predominantly male population. To date, few extant studies have included EDs in comprehensive models of internalizing/externalizing, and none have examined measurement invariance across women and men5,6. This study estimated a series of models, including the four tested by Forbush et al.,5 of mood, anxiety, substance use disorders, and ED symptom severity, among a sample of male and female veterans and civilians to determine the location of the ED symptoms along the internalizing and externalizing dimensions. Analyses were conducted on data from a study originally collected to evaluate the latent structure of PTSD comorbidity in trauma-exposed veterans and their intimate partners. Because of the selected nature of this sample, and high prevalence of PTSD, PTSD was not included in the models of psychiatric comorbidity, and the final model controlled for lifetime trauma exposure, as described below. Based on previous research14, we hypothesized that AN, BN, and BED would load onto the internalizing latent factor. In addition, we hypothesized that although mean symptom levels of disorders would differ significantly for men and women (termed scalar non-invariance), the overall factor structure and association between these EDs and the latent psychopathology dimensions would be similar among men and women (termed metric invariance).