We describe a behavioural genetic extension of the classic multitrait-multimethod study design that allows estimating genetic and environmental influences on method effects in twin studies (MTMM-T). Genetic effects and effects of the environment shared by siblings are interpreted as indicators of convergent validity. In an application of the MTMM study design, we used self- and peer report data to examine the higher-order structure of the NEO-PI-R. Structural equation modelling did not support a general factor of personality in multimethod data. The higher-order factor Stability turns out to be, at most, a weak trait factor. Genetic effects on method factors indicate that especially self-reports but also peer reports show convergent validity between twins but not between methods. Copyright © 2010 John Wiley & Sons, Ltd.