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

There have been many studies demonstrating that L2 words can be learned incidentally through reading (e.g., Day et al., Reference Day, Omura and Hiramatsu1991; Pitts et al., Reference Pitts, White and Krashen1989; Waring & Takaki, Reference Waring and Takaki2003). However, research also reveals that vocabulary is learned incidentally through listening (e.g., Jin & Webb, Reference Jin and Webb2020; Pavia et al., Reference Pavia, Webb and Faez2019; Van Zeeland & Schmitt, Reference Van Zeeland and Schmitt2013a) and viewing (e.g., Montero Perez et al., Reference Montero Perez, Peters, Clarebout and Desmet2014; Peters & Webb, Reference Peters and Webb2018). Incidental vocabulary learning gains are perceived to be small in comparison with intentional vocabulary learning gains leading to suggestions that intentional learning is more effective (e.g., Laufer, Reference Laufer2003; Nation, Reference Nation2013). Laufer's (Reference Laufer2003) study provides some support for this as she found that three word-focused tasks contributed to greater gains in vocabulary knowledge than reading.

Studies of incidental vocabulary learning through reading have reported the following gains: 6.5%–8.6% (Pitts et al., Reference Pitts, White and Krashen1989), 7.2% (Zahar et al., Reference Zahar, Cobb and Spada2001), 18%–42% (Waring & Takaki, Reference Waring and Takaki2003), 28%–63% (Webb & Chang, Reference Webb and Chang2015a), 43%–80% (Cho & Krashen, Reference Cho and Krashen1994), 44% (Webb & Chang, Reference Webb and Chang2015b), 51% (Horst, Reference Horst2005), and 65% (Pigada & Schmitt, Reference Pigada and Schmitt2006). There is similar variation in gains in studies of incidental word learning from listening with results ranging from 3.29%–8.67% (Pavia et al., Reference Pavia, Webb and Faez2019), 2%–29% (Brown et al., Reference Brown, Waring and Donkaewbua2008), 7.8%–15.5% (Vidal, Reference Vidal2003), and 19%–29% (Van Zeeland & Schmitt, Reference Van Zeeland and Schmitt2013a). There are relatively few studies of incidental word learning through viewing, and gains have been inconsistent: 8%–14% (Peters & Webb, Reference Peters and Webb2018), 16–25% (Feng & Webb, Reference Feng and Webb2020), and 23%–30% (Rodgers & Webb, Reference Rodgers and Webb2020). The great deal of variation in the size of gains in studies of incidental vocabulary learning is likely owing to the many differences in participant, methodological, and treatment variables between studies.