In both Pavlovian conditioning and human causal judgment, competition betwe
en cues is well known to occur when multiple cues are presented in compound
and followed by an outcome. More questionable is the occurrence of competi
tion between outcomes when a single cue is followed by multiple outcomes pr
esented in compound. In the experiment reported here, we demonstrated block
ing (a type of stimulus competition) between outcomes. When the cue predict
ed one outcome, its ability to predict a second outcome that was presented
in compound with the first outcome was reduced. The procedure minimized the
likelihood that the observed competition between outcomes arose from selec
tive attention. The competition between outcomes that we observed is proble
matic for contemporary theories of learning.