Clinical neuroscience conditions represent a heterogeneous group of conditions with varying contributions from genetic and environmental influences. It has been common to view some of these conditions under the disease model presumed to represent a specific pathophysiology, tissue pathology (i.e. Alzheimer’s disease, Huntington’s disease). Other conditions have been classified as representing primarily a disorder of behavior (i.e. anorexia nevosa, substance use disorders).
The disease model is thought to be something outside of an individual’s control—individuals have the condition and the manifestations of the disease represent the effects of the underlying pathology. More behavioral disorders are thought to have a volitional component. Individuals with behavioral disorders are thought to contribute more to their condition and therefore assume some responsibility for having the condition.
Bienvenu, Davydow and Kendler recently published an examination of the validity of this type of classification approach. They noted that diseases with involuntary symptoms would typically have a stronger genetic contribution that behavioral disorders where personal choices contribute to the condition. One method to examine the genetic contribution to a particular disorder is the twin study. Using identical and fraternal twin, examination of the concordance rates for a particular illness will provide an estimate of the relative contribution of genetic factors in the illness. Heritability ranges between zero (no genetic contribution) to 1 (purely genetic condition).
The authors reviewed high-quality twin studies in six clinical neuroscience conditions felt to be diseases and six clinical neuroscience conditions with a strong behavioral contribution. The heritability estimates for the 12 conditions are shown in the graph adapted from data in the manuscript:
The authors note that the review fails to support the disease versus behavior distinction. Behavioral disorders appear to have as significant of a genetic contribution as a group as do those more typically classified as diseases. They note that some may argue that the diseases with lower heritabilities (major depression and generalized anxiety) are really not diseases. If you take away major depression, panic disorder and generalized anxiety disorder from the analysis, you do get the three remaining diseases as having the highest heritability.
Nevertheless, the authors correctly point out that many behavioral disorders carry significant genetic contributions to risk. This finding should reduce some of the stigmatization of these behavioral disorders. They note “we humans do not seem to be equally free in our decisions”.
The study also underscores that mental disorders considered the most disease-like, bipolar disorder and schizophrenia, have heritabilities as high as Alzheimer’s disease. Few would argue that Alzheimer’s disease is just a behavioral disorder without a brain-based pathological contribution. Increasing evidence supports incorporating bipolar disorder and schizophrenia in the brain disease model category.
Bienvenu OJ, Davydow DS, & Kendler KS (2011). Psychiatric 'diseases' versus behavioral disorders and degree of genetic influence. Psychological medicine, 41 (1), 33-40 PMID: 20459884
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