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.