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

There have been many studies investigating the extent to which words might be learned through exposure to second language (L2) input. Initially, most studies of L2 incidental vocabulary learning focused on reading (e.g., Cho & Krashen, Reference Cho and Krashen1994; Horst, Reference Horst2005; Pitts et al., Reference Pitts, White and Krashen1989; Zahar et al., Reference Zahar, Cobb and Spada2001). However, in recent years there has been an increasing number of studies investigating L2 vocabulary learning through listening to aural input and viewing audiovisual input (e.g., Peters & Webb, Reference Peters and Webb2018; Van Zeeland & Schmitt, Reference Van Zeeland and Schmitt2013a). The degree to which words are learned incidentally has varied considerably across studies. For example, studies of incidental vocabulary learning through reading revealed gains of less than 10% (e.g., Pitts et al., Reference Pitts, White and Krashen1989; Zahar et al., Reference Zahar, Cobb and Spada2001) and more than 40% (e.g., Cho & Krashen, Reference Cho and Krashen1994; Horst, Reference Horst2005). Studies of incidental vocabulary learning through listening have reported gains from 3.29% (Pavia et al., Reference Pavia, Webb and Faez2019) to 29% (Van Zeeland & Schmitt, Reference Van Zeeland and Schmitt2013a), while studies of incidental vocabulary learning through viewing television have reported gains as low as 8% (Peters & Webb, Reference Peters and Webb2018) and as high as 30% (Rodgers & Webb, Reference Rodgers and Webb2020).

The variation in gains across studies of incidental vocabulary learning is likely owing to the many differences in participant, methodological, and treatment variables. For example, participants in studies include primary (e.g., Pavia et al., Reference Pavia, Webb and Faez2019), secondary (e.g., Szudarski & Carter, Reference Szudarski and Carter2016), and university students (e.g., Rott, Reference Rott1999), with varying proficiency levels. Treatments involve learning from expository (e.g., Vidal, Reference Vidal2011) and narrative (e.g., Day et al., Reference Day, Omura and Hiramatsu1991) text through different modes of input such as reading (e.g., Day et al., Reference Day, Omura and Hiramatsu1991), listening (Van Zeeland & Schmitt, Reference Van Zeeland and Schmitt2013a), reading while listening (e.g., Webb & Chang, Reference Webb and Chang2015a), and viewing (e.g., Peters & Webb, Reference Peters and Webb2018) in a single session (e.g., Peters & Webb, Reference Peters and Webb2018), as well as through exposure to the input across multiple study sessions (e.g., Rodgers & Webb, Reference Rodgers and Webb2020). In addition, prior knowledge of target items has been controlled across studies through the use of pseudowords as target items (e.g., Webb, Reference Webb2007), pretests (e.g., Vidal, Reference Vidal2011), and pilot studies (e.g., Miyasako, Reference Miyasako2002), while gains in vocabulary knowledge have been assessed through different test formats including form recognition (e.g., Peters & Webb, Reference Peters and Webb2018), meaning recognition (e.g., Day et al., Reference Day, Omura and Hiramatsu1991), and meaning recall (Rott, Reference Rott1999).

The present study aimed at examining the overall effects of meaning-focused learning on incidental L2 vocabulary. A meta-analysis of 24 primary studies that provided 29 different effect sizes and a total sample size of 2,771 participants was used to determine overall rates of incidental vocabulary learning through reading, listening, reading while listening, and viewing. A secondary aim was to investigate which variables moderate the effects of meaning-focused learning on incidental vocabulary learning. The research should help to clarify the proportional gains that occur through meaning-focused learning, as well as the degree to which these gains vary across different modes of input. The results should also help to guide teachers and learners to optimize vocabulary learning by identifying variables that affect incidental vocabulary gains through meaning-focused learning.