This study falls in the areas of both differential psychology and twin psychology. Using the EFT and the WCST (computerized version), we examined 11 MZFF pairs between 18 and 35 years of age. The aim was to establish the genetic and/or environmental determination of global-analytical cognitive style as well as some characteristics of conceptualization linked to field dependence. The research strategy consisted of introducing three other groups of the same size to control the weight of environmental factors different from those determined by subject selection. The results seem to support the hypothesis of genetic determination of field dependence of the MZFFs, probably linked to the XX chromosome combination. The "couple effect" and the attitude of parents and others toward two identical female subjects may contribute to full expression of the genome. The characteristics of conceptualization revealed by the WCST show that MZFFs persevere in errors typical of a global approach to experience.