The classification of substance use disorders in young adults

International evidence has consistently shown that the prevalence of substance use disorders is highest amongst young adults in the general population. However, questions have been raised as to whether the apparent ‘youthful epidemic’ of substance use disorders is real or a methodological artefact of the prevailing classification systems. Recent reviews of the literature indicate three unresolved issues in this field of study: 1) What is the epidemiology of substance use disorders amongst young adults in the general population?; 2) Is there evidence of age-related bias in the substance use disorder criteria?; and 3) What are the possible sources of age-related bias in the substance use disorder criteria? The current thesis addresses these issues by developing and employing sophisticated statistical analyses and novel investigative techniques. This thesis empirically examines the apparent ‘youthful epidemic’ of substance use disorders in the general population. At the diagnostic level, substance use disorders were found to be highly prevalent, comorbid, severe and, in the case of alcohol use disorders, disabling amongst young adults (16-24 years olds) in the Australian general population. Criterion level analyses also indicated that young adults in the general population endorse almost each of the substance use disorder criteria at considerably higher rates than their older counterparts. However, differential item functioning, as implemented within the framework of item response theory, identified age-based bias in some of the substance use disorder criteria. To understand the sources of this bias, techniques informed by survey methodology indicated that young adults may misinterpret some of the substance use disorder criteria. This misinterpretation may lead to an inflation of the rates of substance use disorders in younger age groups in the general population. Thus, whilst the ‘youthful epidemic’ of substance use disorders was supported by empirical evidence, some of the substance use disorder criteria appear problematic when applied to younger age groups. This thesis concludes with recommended modifications to the criteria that will increase the applicability of the substance use disorder criteria across all age groups in the general population