Although the point had been made on several occasions (see e.g. Eaves and Eysenck 1977; Purcell 2002), a recent paper (Eaves 2014) reiterated the implications of common problems of measurement in psychiatric genetics for the detection of interaction between measured environmental covariates and random genetic effects in twin studies. In particular, it was demonstrated that the use of symptom counts, characteristic of attempts to quantify clinical outcomes, would almost certainly generate statistical evidence for G × E when the underlying genetic and environmental causes of variation in liability were purely additive. Furthermore, because such interactions depend purely on the units of measurement rather than biology, they are almost certain to replicate, a sine qua non for publication.
