This study of 1555 adult mono- and dizygotic twins reared together estimates the heritability of temperament traits in a Polish and a German sample. We test whether the etiology of temperament traits differs between the two cultures and between different temperament traits. We assessed temperament traits with the Formal Characteristics of Behaviour–Temperament Inventory (FCB-TI), the Pavlovian Temperament Survey (PTS), the Revised Dimensions of Temperament Survey (DOTS-R), and the Emotionality–Activity–Sociability Temperament Survey (EAS-TS). Taking error of measurement into account, genetic sources of variance explained about 50% of the variance of temperament traits. We found neither reliable cultural differences nor robust differences in the etiology of the traits. However, the four questionnaires differed systematically with respect to the proportion of genetic and environmental influences on their scales. Copyright © 2002 John Wiley & Sons, Ltd.