We have reviewed what is known about the genetic basis of endophenotypes and shown that their genetic architecture may be as complex as that of psychiatric disease. This does not mean there is no advantage to the use of endophenotypes for genetic studies. We have pointed out that the robust, quantitative measures that are typical of many endophenotypes means that they may be suitable for collecting the large samples needed for genetic analysis of complex traits, and may afford more statistically reliable data. We suggest that, along with the frequency and penetrance of a disease-causing allele given in Table 3, the ease and reliability of phenotyping should be factored into power calculations.
