In two experiments, we investigated the possibility that susceptibility to
proactive interference (PI) affects performance on memory span measures. We
tested both younger and older adults (older adults were tested because of
the suggestion that they are differentially susceptible to PI). We used two
different span measures and manipulated testing procedures to reduce PI fo
r these tasks. For older adults, span estimates increased with each PI-redu
cing manipulation; for younger adults, scores increased when multiple PI ma
nipulations were combined or when PI-reducing manipulations were used in pa
radigms in which within-task PI was especially high. The findings suggest t
hat PI critically influences span performance. We consider the possibility
that interference-proneness may influence cognitive behaviors previously th
ought to be governed by capacity.