Acoustic exaggeration of vowels in infant-directed speech: a multimethod meta analytic review

There is a long-standing debate about the extent to which vowel hyperarticulation, the production of acoustically exaggerated vowels, occurs in infant-directed speech (IDS). This exaggeration has been argued to result in clearer speech sounds that are easier for infants to process and might be positively related to infants’ linguistic outcomes. However, previous findings regarding the presence of vowel hyperarticulation in IDS are seemingly inconsistent and contradictory, making it hard to advance our understanding of the role of hyperarticulation in language development and, consequently, the potential functions/roles of IDS in language acquisition. Thus, we adopted a systematic review and meta-analytic approach to investigate the robustness of vowel hyperarticulation in IDS and identify sources of het erogeneity in the literature. We employed four complementary meta-analytic approaches and evaluated the robustness of results across the different choices. We performed both traditional (Frequentist) and Bayesian meta-analyses first on methodologically consistent studies (20 studies, 42 effect sizes) and then on all studies of vowel hyperarticulation in IDS irrespective of the method (35 studies, 80 effect sizes). Findings indicate the presence of vowel hyperarticulation in IDS compared to adult-directed speech (effect sizes ranging from 0.41 to 0.69), as well as systematic and unsystematic variability due to, for example, cross-linguistic variability and methods employed, making it difficult to identify specific factors associated with stronger vowel hyperarticulation. The quantitative results combined with a systematic review of the literature also enable important methodological insights, which we summarize into recommended practices such as enlarging sample sizes and explicitly incorporating sources of heterogeneity in analyses.