I suggested in my last post that in the future, the current system of academic peer review should be replaced by post-publication peer review. A recent publication in PNAS (Proceedings of the National Academy of Sciences) provides a good opportunity for me to attempt such a review myself. Of course, an article published in PNAS has already gone through peer review, but the close relationship between authors and editors there often results in a product that – even more than always – might benefit from some outside criticism.
"Climate, vocal folds, and tonal languages: Connecting the physiological and geographic dots," by Caleb Everett, Damián Blasi and Seán Roberts, is an ambitious, though short paper that argues for a connection between low humidity and temperature and the absence of tone, particularly complex tone (defined as three or more levels of phonemic pitch contrast), in languages around the world.
To make this argument, Everett et al. first cite literature, particularly from laryngology, supporting the idea that pitch distinctions are more difficult to make under dry and cold conditions. And if complex tone is "maladaptive (even in minor ways)," they predict that languages spoken in comparatively arid and/or chilly locales will be more likely to "lose/never acquire" complex tonal contrasts. The majority of the paper is then devoted to demonstrating that the predicted correlation exists: globally, within large language families, and across linguistic isolates.
To me, it was helpful to think of the paper's structure in the opposite direction. Clearly, if the correlation between climate and tone does not exist – or if it does exist, but could be due to chance – then as far as this particular claim is concerned, we could stop right there. However, it would still be worth thinking about the proposed explanation. While ease of articulation and perception are certainly important forces driving language change, the idea that these forces themselves might vary based on totally extra-linguistic factors (like climate) is very intriguing.
But if we do accept Everett et al.'s argument, we might also expect that many other small differences in people's environments should differentially favor changes to their language over the long term. So assuming more phonetic predictions of this type can be made, the theory would have a problem if the geographical correlations don't pan out as well as they seem to in this case. Also, if we extend our interest to small differences in people's anatomy in different parts of the world, we might be treading on ground that is considered, at least since the mid-twentieth century, rather dangerous.
Returning to the specifics of the article, the argument that dry and cold air negatively affects the production of precise pitch differences is well-supported, but the magnitude of these effects is never made clear. While the evolutionary argument does not depend on the effects being large, it would have been nice to know, for example, if the increased imprecision in pitch when "jitter measurements increased by over 50%" were comparable in magnitude to tonal pitch differences, or not. It is also relevant that language hearers typically "normalize" or compensate for phonetic differences of considerable size in the speech of their interlocutors. This point, and more generally the relationship between pitch (a phonetic property) and tone (a phonological one) was not considered.
Surprisingly, in the section about the geographic correlation, no quantitative estimate is ever given of the effects of humidity (or temperature) on the likelihood of a language having complex tone. This information is presented in a cumulative distribution plot (Figure 2), which does have the advantage (from the authors' point of view) of maximizing the appearance of the effect.
When numbers are presented, they compare the climate properties of tonal vs. non-tonal languages, rather than treating climate as the explanatory variable it is claimed to be. This may seem like a quibble, but it makes it rather difficult to understand just how strong an association is being shown. For example, when we read that "the average [humidity] for isolates with complex tone is 0.017, whereas the average for other isolates is 0.013," this measures a difference in average climate (whatever that means), depending on the language type. What the reader deserves to know is how different the tonal properties of languages are, depending on the climate.
Although the bulk of this section quite correctly attempts to eliminate areal effects as an explanation for the association between climate and tone, the final paragraph seemingly does an about-face, suggesting that "tone spreads across languages more effectively via
interlinguistic contact in regions with favorable ambient conditions" and less effectively in cold/dry regions. This expands the scope of the hypothesis beyond language transmission to include language contact, without any additional evidence, and possibly at the risk of circularity.
I would have expected that what linguists already know about tonogenesis would be more relevant to this topic. Mentioning it for the first time in their discussion and conclusions section, Everett et al. say only that this literature does not predict any effect of climate. Actually, this might make perfect sense if languages in dry, cold climates only tend to lose tone, rather than "lose/never acquire" it (to return to the authors' curious conflation). But in this case, some discussion of how tone is ordinarily thought to be lost might have been worthwhile, even if the influence of climate could be independent.
In summary, I found the argument for the geographic correlation itself to be fairly strong, although I did not really look into the details here. The link between the proposed phonetic effect and language change was plausible, but needed more grounding in research on language change in general and the loss of tone in particular. But I was less convinced that the physiological (or phonetic) effects of dry and cold air are really an obstacle to producing phonological tone. Like Everett et al., I too hope "that experimental phoneticians and others examine the effects of ambient air conditions on the production of tones and other sound patterns, so that we can better understand this pivotal way in which human sound systems appear to be ecologically adaptive." Unless they are too busy.