A critical issue in genetic research of complex phenotypes, including psychophysiological responses, is test-retest reliability. Clearly, only individually stable, trait-like characteristics can be heritable. As already mentioned above, test-retest reliability can be regarded as upper boundary for heritability. Psychophysiological variables show a very broad range of reliability, from very high such as resting EEG, to very low such as emotion-modulated startle reflex. Therefore, one important but often neglected step in genetic studies should be the selection of a reliable indicator of the function or process of interest. Some pilot studies may be needed to compare the utility of different psychophysiological indices, but this investment will pay off because a properly conducted genetic study requires large samples, and the availability of a reliable measure may reduce the sample size required to achieve the desired statistical power.
