Languages are louder in the tropics

The natural environment can shape languages as they evolve through time. Since we are surrounded by air when we speak and listen, the physical properties of the air can influence how easy speech is to produce and hear. On the one hand, the dryness that comes with cold air represents a challenge for the production of voiced sounds, which involve vibration of the vocal cords, and, on the other hand, warm air tends to limit unvoiced sounds by absorbing their high frequency energy. Such factors could favor higher sonority in warmer climates. Sonority refers to the loudness of speech sounds, which is influenced by the openness of the vocal tract. Qibin Ran and colleagues sought to verify this intuitively plausible relationship by leveraging the Automated Similarity Judgment Program (ASJP) database, which contains basic vocabularies for 5,293 languages. The authors found that languages with high mean sonority indexes (MSI) are concentrated around the equator and the Southern Hemisphere. Languages in Oceania have some of the highest MSIs. However, there were some exceptions to the trend. Mesoamerica and Mainland Southeast Asia showed lower MSIs despite being tropical. Overall, there was a positive correlation between MSI and mean annual temperature, averaged by language family. However, when looking at relationships within families, the authors found no clear pattern. According to the authors, the fact that the relationship is only predictable at the language family level is an indication that temperature effects on sonority evolve slowly, shaping language’s sounds only on timescales of centuries or even millennia of linguistic evolution—longer than the average lifespan of an individual language.

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