How effective is second language incidental vocabulary learning? A meta-analysis

In answer to the first research question, the analysis revealed that L2 meaning-focused input contributed to large learning effects for vocabulary knowledge on both first and second posttests. This finding is important because it clarifies the value of encountering L2 meaning-focused input on vocabulary learning. Moreover, when considering this result, it is useful to also reflect on aspects of vocabulary learning that were not included in the meta-analysis, but may also occur through encountering L2 meaning-focused input. For example, in the research literature, incidental vocabulary learning is limited to gains in knowledge of whichever target words are tested. These words are typically the lowest frequency words that are least likely to be known by most participants. However, there are likely to be many non-target words that are unknown or partially known that may also potentially be learned in these studies. In addition, the value of encountering L2 input for vocabulary learning might primarily be that it reveals how words can be used. While there are a small number of studies (e.g., Chen & Truscott, Reference Chen and Truscott2010; Pigada & Schmitt, Reference Pigada and Schmitt2006; Webb, Reference Webb2007, Reference Webb2008) that have investigated the contribution of meaning-focused input on learning aspects of knowledge other than form-meaning connection (e.g., collocation, grammatical function, association), these studies do reveal that gains in vocabulary knowledge will typically extend beyond form-meaning connection. Thus, it is important to note that the studies included in this meta-analysis were unlikely to reveal the full extent of gains in L2 vocabulary knowledge through encountering meaning-focused input.

In answer to the second research question, the mean proportions of target words learned as indicated by the three test formats (form recognition, meaning recognition, form recall) ranged from 9–18% on the first posttest, and 6–17% on the second posttest. Higher mean proportions of target words learned for meaning recognition (15% -> 17%) and meaning recall (9% -> 12%) on delayed posttests suggest practice effects may often occur from immediate to delayed posttests. Researchers should try to control for practice effects by counterbalancing target items between immediate and delayed posttests and measuring knowledge of half of the target items in the immediate posttest and the remaining items in the delayed posttest. Teachers should also be aware of the positive effects and potential gains that can occur through assessing knowledge of target vocabulary encountered in L2 input.

The mean proportions of target words learned in the different modes of input on the first posttest were 7% for viewing, 13% for reading while listening, 15% for listening, and 17% for reading. On the second posttest, the mean proportions of target words learned were 15% for reading, 13% for listening, 17% for reading while listening, and 5% for viewing. The results suggest that the proportions of words learned through reading, listening, and reading while listening may be similar, but that all of these modes of input might lead to greater gains than viewing on immediate and delayed posttests. This may be owing to: (a) learners’ familiarity with learning from L2 written and spoken input in the classroom in comparison to audiovisual input, (b) the ability for researchers to more easily manipulate the frequency of occurrence of target items in spoken and written input than in audiovisual input, and (c) the use of spoken and written learning materials designed for L2 learning in studies in comparison with audiovisual materials created for L1 viewers’ entertainment, education, and interest.

The analyses indicate that incidental vocabulary learning gains are likely to be smaller than occur through intentional vocabulary learning activities. The findings of the present study suggest that less than 20% of target words are likely to be learned through encountering meaning-focused input on delayed posttests. In comparison, a meta-analyses of intentional vocabulary learning tasks revealed the mean proportions of target words retained was 39.4% on meaning recall delayed posttests (Webb et al., Reference Webb, Yanagisawa and Uchihara2020). It is important to note, however, that the incidental vocabulary learning gains reported in the present meta-analysis accounted for possible learning effects by calculating the degree to which the gains of experimental groups exceeded those of control groups. The lack of control groups in L2 intentional vocabulary learning studies limited Webb et al. (Reference Webb, Yanagisawa and Uchihara2020) from carrying out a similar calculation. Thus, we urge readers to be cautious when interpreting the difference in proportional gains between the two studies.

The present study and Webb et al.'s (Reference Webb, Yanagisawa and Uchihara2020) meta-analyses of intentional learning activities reveal that both meaning-focused and intentional learning activities have large positive effects on vocabulary learning. However, both approaches have advantages and disadvantages, and neither is likely to contribute to comprehensive knowledge of words on their own over a short period of time. There is a great deal to learn about a word including its spelling, pronunciation, derivations, associations, meanings, collocations, grammatical functions, and constraints on use (Nation, Reference Nation2013). Research indicates that learners make incremental gains in word knowledge and that developing comprehensive knowledge of words is a slow process in which gains and losses occur (Webb & Nation, Reference Webb and Nation2017). Thus, the gains demonstrated through tests in any cross-sectional study likely represent a fraction of the vocabulary knowledge that could be gained about each word. Rather than suggesting that any one approach to learning is best, there is greater value in determining the degrees to which different types of vocabulary knowledge (e.g., form-meaning connection, collocation, word parts) are gained over different lengths of time though the different learning approaches.

In answer to the third research question, the results revealed that learner, material and activity, and methodological variables all moderated the incidental learning gains. Of the learner variables, the results showed that more proficient learners make larger gains than less proficient learners; however, the difference between the proficiency levels diminished on delayed posttests. Institutional level did not significantly moderate gains (although it should be noted that the size of gains showed a similar trend i.e., university (g = 1.36), secondary school (g = 0.87), and primary school (g = 0.72)). The reason why more proficient learners make larger gains is likely owing to better comprehension of L2 input allowing them to devote greater processing resources towards understanding unfamiliar language. Lower proficiency learners may have to devote greater processing resources towards understanding the input as a whole, thus making it more difficult to attend to unfamiliar words. The difference in findings between the two learner variables indicates the greater precision of proficiency tests as an indicator of level than institutional background. This should be expected because there is likely to be greater variation among learners within an institutional level than among those within a proficiency band.

Of the material and activity moderator variables, the analyses of immediate posttests revealed that incidental gains were larger for narrative texts than expository texts, texts oriented towards L2 learners rather than L1 learners, and through reading and reading while listening, followed by listening and then viewing. The greater effects of learning from narrative text than expository text may be because the former provides better conditions for word learning than the latter; narrative text consists of a higher proportion of higher frequency words and is less lexically dense than expository text (Gardner, Reference Gardner2004). Greater vocabulary gains occurring from exposure to materials created for L2 learners rather than those oriented towards L1 learners should be expected, because L2 materials are designed to be at the appropriate lexical and syntactic levels for L2 learners while L1 materials are not. Materials that are less linguistically demanding should allow learners to attend more to unfamiliar words and increase the potential for vocabulary learning. The results of the delayed posttests revealed significant positive effects of learning occurring over multiple sessions rather than in a single session, and through learning in each of the different modes of input. Greater incidental vocabulary learning gains through learning in multiple sessions on delayed posttests is supported by research on distributed practice, which suggests that spacing study sessions has a positive effect on learning particularly when there is an interval between learning and testing (Kim & Webb, Reference Kim and Webb2022). This finding demonstrates the value of extensive learning programs that involve regular meaning-focused learning over longer periods of time.

The comparisons between the different modes of input varied on first and second posttests. On immediate posttests, reading contributed to the largest effect size, whereas on delayed posttests, reading while listening contributed to the largest effect size (although it should be noted that there were only two reading while listening studies that included delayed posttests). It might be expected that written materials contribute to greater gains than aural and audiovisual materials, because in the EFL context, participants are most likely to be familiar with learning with written texts, followed by aural and audiovisual materials. However, the results tended to show medium to large effects for all modes (except a small effect for viewing on immediate and delayed posttests). Further incidental vocabulary learning studies of listening, reading while listening, and viewing would be useful as there were far fewer studies investigating these modes of input than for reading.

The moderator analyses also revealed greater effects on learning when pre-knowledge control involved pseudowords followed by pretests, pilot testing, and no pretests. There are several possible reasons for this result. First, pseudoword forms may be more salient than real word target items because the former have never been encountered before while the latter may be unfamiliar (i.e., partially known but not to the degree that participants can demonstrate knowledge on pretests) rather than completely unknown. Second, it may be that practice effects in which participants gain knowledge of target items through pretests reduce the extent to which gains are revealed in pretest-posttest designs. However, it is important to note that pretest control was found to be the most common form of pre-knowledge control, and it contributed to large effect sizes at both retention intervals. Thus, there appears to be strong support for the use of pretests for pre-knowledge control. Third, although pilot testing other learners with a similar profile to participants also revealed medium and small effect sizes on immediate and delayed posttests, these smaller effect sizes in relation to the other two pre-knowledge control options suggest that this is the least effective of the three approaches perhaps owing to variation in knowledge of pilot test and experimental participants.

The moderator analysis did not reveal a significant difference between the three test formats. Large effect sizes were found at both retention intervals for meaning recognition and meaning recall while large and small effects were found for form recognition on the immediate and delayed tests, respectively. The similar effect sizes between meaning recognition and recall might be considered surprising because recall formats are less sensitive to knowledge than recognition formats (e.g., Laufer & Goldstein, Reference Laufer and Goldstein2004; Webb, Reference Webb2007), and may limit the degree to which incidental learning gains are found (Nagy et al., Reference Nagy, Herman and Anderson1985). However, it may be that because L2 incidental learning studies tend to include optimal conditions for word learning (i.e., higher than normal frequencies of occurrence of target items), meaning recall tests have been similarly effective for revealing word learning. A second reason for the similarity in findings between these test formats may be that guessing on meaning recognition pretests limits the degree to which gains are found through exposure to input.