Grouping in primary memory: the case of the compound suffix.

Appending a nominally irrelevant item, or "suffix," to the end of a short list of items impairs recall of the list. Appending a second such item, however, does not increase the impairment. The research reported here shows that the impairment can in fact be increased if the suffix items are physically dissimilar. Thus, Experiments 1-4 show that memory for a sequence of digits is impaired more by the addition of two zeros uttered in different voices than by either a single zero or two zeros uttered in the same voice. Experiment 5 shows a similar pattern of results in the visual modality, with physical similarity defined by typefont. The findings are contrary to at least two theories of the suffix effect but can be accounted for by assuming that physically similar items tend to form a cohesive group.