AbstractMost Piagetian formal operational reasoning tasks show horizontal decalage; that is, subjects pass certain tasks and fail others that have the same logical structure. The study reported here analyzes the importance of individual difference variables, as postulated by the neo‐Piagetian theory of Pascual‐Leone, in explaining subject performance in formal reasoning. A sample of 72 freshman students were administered a test of formal reasoning having 20 items of different types of reasoning, and the tests of the individual difference variables. Results obtained from multiple regression analyses show that Pascual‐Leone's structural M‐capacity (Ms) is the most consistent predictor of success in the different formal reasoning tasks, followed by Witkin's cognitive style, and to a much lesser degree Raven's progressive matrices, and Pascual‐Leone's functional M‐capacity (Mf). It was found that in the total score on the 20 items of formal reasoning, Msaccounted for 23.3% of the variance (R= 0.483,F= 6.39,p= 0.014) and Witkin's Group Embedded Figures Test, increased the multipleRsignificantly (F= 7.77,p= 0.007) and accounted for 7.6% of the variance. Mfand the Raven test did not make a significant contribution to the regression equation. Correlation coefficients among most of the items having the same reasoning pattern but different content are generally low but statistically significant (p<0.01). Intercorrelations among items having the same formal reasoning pattern and content are fairly high (p<0.001). These results emphasize the importance of individual difference variables: information‐processing capacity (Pascual‐Leone) and oversensitivity to potentially misleading information (Witkin). It is suggested that in order to understand student performance in formal reasoning tasks, we should expect horizontal decalages as a rule and not the exception, as Piaget had postulated. Educational implica