Fig. 1.

Bimodality of the superficially unimodal Gaussian distribution of IQ in the population. In addition to the normal distribution of IQ in the general population, a second distribution, at the very low end, reflects the existence of a separate population of individuals with specific, etiologically defined, syndromes, such as fragile X syndrome, Rett syndrome, Angelman syndrome, Down syndrome (adapted from McHugh and Slavney [15]). Note that we are not claiming that other disorders (such as schizophrenia) have Mendelian origins, but simply that a quantitative trait with a superficially Gaussian “dimensional” distribution can conceal disparate subcategories and etiologies best seen as categorically distinct, and using the disease model. Illustrator: Joan M.K. Tycko.