Several studies have reported that the age at which a word is learned affects skilled reading. This age-of-acquisition effect is potentially important for theories of reading and learning. The effect has been difficult to pin down, however, because the age at which a word is learned is correlated with many other lexical properties. Zevin and Seidenberg (2002) analyzed these phenomena, using connectionist models that distinguished between cumulative frequency (the total number of times a word is experienced) and frequency trajectory (the distribution of these experiences over time). The models prompted a reevaluation of the empirical literature on this topic. The present research tested and confirmed three behavioral predictions derived from these models. First, cumulative frequency has an impact on skilled word naming, more so than standard measures of frequency derived from such norms as those of Kucera and Francis (1967). Second, frequency trajectory affects age of acquisition: The timing of exposure to words affects how rapidly they are learned. However, frequency trajectory does not affect skilled reading aloud, because the consistencies in mapping between spelling and sound eventually wash out the effects of early differences in frequency of exposure. Thus, in skilled performance, the timing of exposure to words is less important than the amount of exposure. The results clarify the conditions under which age-dependent learning effects occur in reading aloud.
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