Wednesday, January 21, 2015

My First Post-Publication Review: "Climate, vocal folds, and tonal languages" by Everett et al. (2015)

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.


  1. Leaving the geographical correlation aside for a moment, I'm struggling with the phonetic underpinnings. Certainly, dry air will affect our ability to voice "cleanly", as does age, particulate in the air, smoking, allergy, and other sorts of vocal pathology. This is unassailable, and well known. The authors point to increases in jitter (cycle-to-cycle variability in *frequency*, the frequency moves around), and shimmer (cycle-to-cycle variability in *amplitude*, the amplitude of each cycle changes), which is plausible. As far as I know, shimmer shouldn't be relevant to tone at all, barring the complete loss of cycles. Jitter, pitch variability, may.

    The trouble is that the authors fail to show, as Daniel points out, that the *degree* of jitter is sufficient to cause tone confusion. *Everybody* has non-trivial jitter in speech, and some populations (e.g. the elderly, Parkinson's sufferers, residents of cold deserts), have more. Yet, as the authors acknowledge, this doesn't prevent the use tone in tonal languages by these populations, nor singing (which requires greater precision in tone than natural language).

    Our ability to deal with speaker-by-speaker and moment-to-moment variation in tone (as in vowel formants, nasality, and so forth) is vast. Given that elderly smokers are still able to produce tone languages understandably, even with probable pathological levels of jitter, well above the threshold discussed here, this doesn't seem to prevent tone perception by adults. Similarly, children in Colorado's (sizable) Hmong-speaking population are able to learn tones from their (aging) parents, even when born and raised in our (dry) climate. Thus, this effect isn't enough to prevent the acquisition of tone, the other means to stop it.

    The authors argue that this is a subtle effect, strong enough to prevent the genesis of tone (or cause its disappearance) over time, but too weak to cause synchronic perceptual issues or phonological changes. The problem with this approach is that the argument becomes somewhat circular: this subtle, diachronic tone-limiting climate/larynx interaction is the authors' justification for the climate-based statistical analysis, but the analysis is the sole indication of such an unusual interaction.

    This study, then, simply *begs* for a perceptual study. For tone to be lost (or prevented from developing), children must be unable to pick it up, or it must be so unreliably produced as to no longer be able to carry functional load. This is not shown to be the case in the paper. If the authors could show *any* change in tone perceptibility with jitter variations of the degree caused by these climate differences, it would bolster their claim significantly.

    Certainly, the statistics seem to show a strong link to climate, and it certainly merits further consideration. But, without some additional evidence that this degree of jitter can prevent or reverse tonogenesis, I'm afraid the authors have a convincing correlation without a convincing story to explain it.

    (replaced to fix a typo)

    1. Thanks! I agree re: loss of tone, not sure small climate effect would "add up" there. If it's non-acquisition of tone (see below) gradual "pressure" account makes more sense.

  2. Many thanks for this thoughtful review. The main point brought up is about whether dry air can sufficiently affect tone in a way that leads to (or prevents) language change. The only evidence we have is the muscle tension disphonia paper on vietnamese speakers, who report some difficulty with producing tones given symptoms similar to a dehydrated larynx (though this isn't terribly convincing). I believe Caleb is currently running perceptual experiments that may be more informative.

    There's also the point about the effect size. This is tricky to work out for two reasons. First, the effect size in terms of language distribution can't be easily related to the effect size in acoustic/perceptual terms. There could be a very small difference in perception that is amplified by cultural evolution into a larger cross-cultural difference. Also, we 're not using regression.

    The main Monte-Carlo test can be interpreted in this way: When taking an independent sample of languages, the mean humidity will be lower for tonal languages than non-tonal languages 93% of the time. The mean difference in humidity quantiles between complex and non-complex is something like 0.0012, or about 6.6% of the total range of humidity. (95% confidence intervals: 0.6%, 11.4% of the range). The closest comparison in the data base is Finland (Humidity=0.005) and British Columbia (Humidity=0.004), though this doesn't help me very much!

    Regarding the mechanisms of change, it's true that they're not fleshed out in the paper. My approach is that tone is less likely to emerge in dry areas, due to tonal contrasts being less likely to replicate in conversation (rather than change across genrations due to learning). But that remains to be seen. I'm hoping to explore historical change further very soon.

    However, with regards to your comment on the ordering of the paper: we felt like we were setting up a prediction based on previous research, then testing that prediction. I've written before about how correlational studies can be used to jump-start hypotheses, but that's not really what happened in this case. Of course, now that we've established some kind of connection, there's more of an impetus to explore this idea further.

  3. Unless the air quality affects both tonogenesis and "tonoexodus", you'd expect a difference between the large language families where tone is reconstructed in the proto-langauge and where it's not.

    1. This is exactly what we'll test with phylogenetic models. The only thing I haven't figured out is how to model humidity (aspects of language are culturally transmitted, but humidity probably needs a Brownian motion model or something).

    2. Could you use phylogeographic methods to place the ancestor languages In space and use current climate models as a proxy for the humidity?

    3. Yes, the geo-phylo methods could do this. There's also a literature in biology on adaptation to humidity, which I'm currently reading