Two concerns have fueled the recent interest in EPs in psychiatric genetics. First, our ‘exophenotypes’— PDs—are seen to be very complex from an etiological and genetic perspective. In seeking to follow Henry David Thoreau’s maxim of ‘simplify, simplify,’ the hope is that by studying EPs we will simplify our problems. Second, PDs seem to be a very long way from DNA variation. The number of physiological steps from a variant in a base pair sequence to a PD, such as schizophrenia, depression or alcohol dependence, is likely very large indeed. In studying EPs, we hope to ‘move closer’ to the ‘DNA level.’ For interest, Table 1 lists three representative definitions of the EP construct by Gottesman and Gould,2 Preston and Weinberger,3 and Cannon and Keller.1
