Although the concept of mental disorder is fundamental to theory and practice in the mental health field, no agreed on and adequate analysis of this concept currently exists. I argue that a disorder is a harmful dysfunction, wherein harmful is a value term based on social norms, and dysfunction is a scientific term referring to the failure of a mental mechanism to perform a natural function for which it was designed by evolution. Thus, the concept of disorder combines value and scientific components. Six other accounts of disorder are evaluated, including the skeptical antipsychiatric view, the value approach, disorder as whatever professionals treat, two scientific approaches (statistical deviance and biological disadvantage), and the operational definition of disorder as "unexpectable distress or disability" in the revised third edition of the Diagnostic and Statistical Manual of Mental Disorders (American Psychiatric Association, 1987). The harmful dysfunction analysis is shown to avoid the problems while preserving the insights of these other approaches.

The Diagnostic and Statistical Manual of Mental Disorders (3rd ed., rev.; DSM-HI-R) operationally defines disorder essentially as "statistically unexpectable distress or disability" This definition is an attempt to operationalize 2 basic principles: that a disorder is harmful and that a disorder is a dysfunction (i.e., an inability of some internal mechanism to perform its natural function). However, the definition fails to capture the idea of "dysfunction" and so fails to validly distinguish disorders from nondisorders, leading to invalidities in many of DSM-III-R's specific diagnostic criteria. These problems with validity are traced to DSM-III-R's strategies for increasing reliability.

Adaptation and natural selection are central concepts in the emerging science of evolutionary psychology. Natural selection is the only known causal process capable of producing complex functional organic mechanisms. These adaptations, along with their incidental by-products and a residue of noise, comprise all forms of life. Recently, S. J. Gould (1991) proposed that exaptations and spandrels may be more important than adaptations for evolutionary psychology. These refer to features that did not originally arise for their current use but rather were co-opted for new purposes. He suggested that many important phenomena--such as art, language, commerce, and war--although evolutionary in origin, are incidental spandrels of the large human brain. The authors outline the conceptual and evidentiary standards that apply to adaptations, exaptations, and spandrels and discuss the relative utility of these concepts for psychological science.

The harmful dysfunction (HD) analysis of the concept of disorder (J. C. Wakefield, 1992a) holds that disorders are harmful failures of internal mechanisms to perform their naturally selected functions. S. O. Lilienfeld and L. Marino (1995) proposed instead that disorder is a Roschian prototype concept without defining properties. Against the HD analysis, they argued that many disorders are not failures of naturally selected functions because they are either designed reactions (e.g., fever) or failures of functions that are not naturally selected (e.g., reading disorder). The HD analysis is defended here against these and other objections and compared with the Roschian account. It is argued that the objections are based on conceptual confusions and can be turned around to provide strong new support for the HD analysis. A series of conceptual experiments demonstrates the superior explanatory power of the HD analysis and disconfirms the Roschian account.

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