Age-of-acquisition (AoA) is cited increasingly as an important variable in verbal tasks, largely due to the work of Ellis and his co-workers. The notion that words learned earlier in life are faster to name than later-acquired words was first addressed experimentally by Carroll and White (1973), who examined picture-naming latencies. For a long time, however, the interest in AoA was limited to a few researchers, predominantly from the United Kingdom. The vast majority of researchers did not take the variable into account, and considered it as a confound of word frequency (in that earlier-acquired words tend to occur more frequently in adulthood). In a provocative article, Morrison and Ellis (1995) reopened the issue and reported that word frequency no longer affected word naming times when AoA was controlled for, whereas AoA kept on having a strong impact when word frequency was controlled for. On the basis of these findings, Morrison and Ellis concluded that all reported effects of frequency in lexical tasks may be AoA effects in disguise. Although subsequent studies have shown that Morrison and Ellis’s claim was too strong because combined effects of frequency and AoA on word naming latencies have been obtained, a growing number of researchers become convinced that AoA plays a basic role in lexical tasks.

The consensus that seems to have emerged from recent studies is that AoA is the critical variable in word production; this is the so-called phonological completeness hypothesis. In general, reference is made to Brown and Watson’s (1987) idea that early-acquired words are stored in their entirety within the phonological output lexicon, but that the representations of late-acquired words may be more fragmented. The extra time required to assemble the dispersed representation of late-acquired words would account for their slower naming speed. Two reasons are given for situating the AoA effect at the speech output stage. First, AoA is a significant variable in all tasks that require the production of a word to describe the presented stimulus (i.e., visual word naming, picture naming, word finding problems in aphasia), but is not always a significant variable in binary, manual decision tasks (e.g., object classification; see the following). Second, Gerhand and Barry (1998) found a strong effect of AoA on pronunciation duration, with a smaller and less reliable effect of frequency. In this task, participants were presented with spoken words, one at a time, and were requested to repeat each word 10 times as fast as they could while still pronouncing each word correctly and clearly. The time taken to repeat each word 10 times was measured by the experimenter.

Van Loon-Vervoorn (1989), however, suggested another possible origin of the AoA effect. According to her, the order of acquisition is the most important organisational principle of the semantic system, with the meanings of later-acquired concepts being built on those of earlier-acquired concepts. Empirical evidence for her position was provided by van Loon-Vervoorn (1989, Chapter 10). She used a discrete word-associate generation task to tap into the semantic system. In this task, participants are asked to say the first word that comes to their mind when seeing a stimulus word. The task has also been used by Chumbley and Balota (1984) and de Groot (1989) to assess the nature of the semantic system. Van Loon-Vervoorn presented 60 one-syllable words that allowed her to assess the independent effects of AoA, word frequency and imageability (IMA). She obtained a reliable effect of AoA (earlier-acquired words: RT=1440 ms; later-acquired words: RT=1681 ms), IMA (high=1445 ms; LOW=1677 ms), and no effect of frequency (high=1539 ms; LOW=1583 ms). On the basis of her findings, van Loon-Vervoorn (1989) concluded that AoA is a semantic variable rather than a lexical variable.

Van Loon-Vervoorn’s work has not been incorporated in the recent discussion on the importance of AoA, partly because it was published in Dutch but also because Morrison et al. (1992) had failed to obtain an AoA effect in a semantic task in which participants classified pictures of objects as naturally-occurring (e.g., apple) or man-made (e.g., anchor).

On the other hand, the possibility of a semantic origin of the AoA effect is appealing, because it would explain a number of findings. First, though there is an important, negative correlation between AoA and frequency, nearly all studies have reported a more pronounced correlation between AoA and other semantic variables. Rubin (1980), for instance, reported a correlation of ?0.40 between AoA and frequency, together with a correlation of ?0.59 between AoA and IMA. The same was true for Whaley (1978) who reported correlations of, respectively, ?0.52 and ?0.69. In both studies, factor analysis indicated that AoA loaded most on a semantic factor that included variables such as imagery, concreteness and number of meanings. Using a more objective AoA-measure (obtained by asking children of different ages to name line drawings), Morrison, Chappell & Ellis (1997) obtained a correlation of ?0.47 between their real AoA measure and the logarithm of the Cobuild frequency, compared to a correlation of ?0.55 between AoA and imageability.

A second finding that is in line with a semantic interpretation of the AoA effect is the robust AoA effect in object naming latencies, as picture naming requires not only the correct name to be produced but also semantic activation to connect the pictorial input with the verbal output. Third, a semantic interpretation of the AoA effect may explain why the AoA effect in oral reading of visually presented words seems to be particularly strong when naming latencies are long, because there has been some speculation that semantic variables may affect word naming times when these are long enough. Finally, AoA has a strong effect on lexical decisions times and since the work of Chumbley and Balota (1984) it is known that lexical decisions involve semantics. Morrison and Ellis (1995) and Gerhand and Barry (1998) provided a phonological explanation of this AoA effect by assuming that lexical phonology contributes to the word/non-word decision, but further research (Gerhand & Barry, 1999) has shown that the AoA effect in the lexical decision task remains significant when efforts are made to interfere with the phonological processing (such as using only pseudo-homophone non-words or using articulatory suppression).
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On the basis of these considerations, it occurred to us that researchers may have rejected van Loon-Vervoorn’s semantic interpretation of AoA too rapidly. At least, it seemed worthwhile to investigate the importance of AoA for a number of semantic tasks, and see how these findings relate to more “lexical” tasks, such as word naming and lexical decision. To do so, we made use of a set of six lists of 24 words recently assembled by Brysbaert et al. (2000) and validated in a naming and a lexical decision experiment. These lists consist of three pairs of lists that differ in AoA, frequency or IMA, and are matched on the other variables. We used these stimuli in two semantic tasks: a discrete word-associate generation task (Experiment 1) and a “word with a definable meaning” vs. “given-name” classification task (Experiment 2). The former is a replication of van Loon-Vervoorn (1989); the latter has been inspired by Taft and van Graan (1998).