Advances in understanding behavioral phenotypes in neurogenetic syndromes.

Syndrome-specific behavior was proposed by Langdon Down in his first clinical descriptions. Research interest followed but waned during the eugenics era when antisocial behavior was attributed to people with intellectual disability (ID) and the US Supreme Court legalized involuntary sterilization. When these claims were refuted and behavioral treatments introduced, their focus on environmental determination minimized the importance of biological research. The modern era began with the recognition that patterned behavior, for example, self-injury in Lesch-Nyhan syndrome and hyperphagia in PWS, was syndrome-specific, and when parent support groups pointed out syndrome-specific behavioral similarities in their children. Syndrome-specific rating scales and methodologies followed to allow behavioral comparisons between syndromes. The focus initially was on specific behaviors but with refinements in neuropsychological tests has expanded to include neurocognitive profiles. Greater clarification in genetic diagnoses has led to mutant mouse behavioral models and neurophysiologic and neuroimaging strategies have made possible the study of brain circuits. There is growing interest in investigating the developmental trajectory of behaviors from infancy to adulthood and old age. Because anxiety, mood disturbance, repetitive behaviors, and social deficits commonly occur in people with severe ID, those affected are often given multiple psychiatric diagnoses. This has led to considerable confusion in the literature. It is critical to focus on specific behaviors and cognitive patterns in research and not confuse psychiatric symptoms that lack precise definitions and involve multiple genes, the so-called psychiatric phenotype, with the more specific behavioral phenotype. New treatments based on knowledge of underlying neurobiology call for more fine-grained definition of behavior.