Similarity comparisons are highly sensitive to judgment context. Three
experiments explore context effects that occur within a single compar
ison rather than across several trials. Experiment 1 shows reliable in
transitivities in which a target is judged to be more similar to stimu
lus A than to stimulus B, more similar to B than to stimulus C, and mo
re similar to C than to A. Experiment 2 explores the locus of Tversky'
s (1977) diagnosticity effect in which the relative similarity of two
alternatives to a target is influenced by a third alternative. Experim
ent 3 demonstrates a new violation of choice independence which is exp
lained by object dimensions' becoming foregrounded or backgrounded, de
pending upon the set of displayed objects. The observed violations of
common assumptions to many models of similarity and choice can be acco
mmodated in terms of a dynamic property-weighting process based on the
variability and diagnosticity of dimensions.