Despite improvements in diagnostic techniques and progress made in mapping genes associated with syndromal mental handicap, the estimation of recurrence risks in non-syndromal mental retardation is still dependent on empirical data. Unfortunately, few studies are available to guide the clinician and their results differ significantly. For example, recurrence risks to all sibs of a male index patient with severe mental retardation vary between 3.5% and 14% in commonly quoted series. The present review highlights the problems involved in interpreting the previous work in this area and discusses the definition of mental retardation according to the degree of severity, phenotype, and its pattern of inheritance. In planning future studies, an appreciation of these issues should allow us to derive accurate and comparable risk figures for use in counselling affected subjects and their families.
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