Thematic relations are an important source of perceived similarity. For instance, the "rowing" theme of boats and oars increases their perceived similarity. The mechanism of this effect, however, has not been specified previously. The authors investigated whether thematic relations affect similarity by increasing commonalities or by decreasing differences. In Experiment 1, thematic relations affected similarity more than difference, thereby producing a noninversion of similarity and difference. Experiment 2 revealed substantial individual variability in the preference for thematic relations and, consequently, in the noninversion of ratings. In sum, the experiments demonstrated a noninversion of similarity and difference that was caused by thematic relations and exhibited primarily by a subgroup of participants. These results indicate that thematic relations affect perceived similarity by increasing the contribution of commonalities rather than by decreasing the contribution of differences. (Contains 3 tables, 2 figures and 1 footnote.)