Theories of similarity generally agree that the similarity of a pair i
ncreases with its commonalities and decreases with its differences. Re
cent research suggests that this comparison process involves an alignm
ent of structured representations yielding commonalities, differences
related to the commonalities, and differences unrelated to the commona
lities. One counter intuitive prediction of this view is that it shoul
d be easier to find the differences between pairs of similar items tha
n to find the differences between pairs of dissimilar items. This pred
iction is particularly strong for differences that are related to the
commonalities. We tested this prediction in two experiments in which s
ubjects listed a single difference for each of a number of word pairs.
The results are consistent with the predictions of structural alignme
nt. In light of these findings, we discuss the potential role of struc
tural alignment in other cognitive processes that involve comparisons.