Traditionally, linguists have assumed that geography doesn't play a role in shaping languages, with the exception of vocabulary specific to certain environments or wildlife. A handful of small studies have suggested that languages in warm climates use more vowels than languages in cold climates, but the findings are controversial. [10 Things That Make Humans Special]
Everett set out to investigate how other aspects of geography, namely altitude, might be linked to certain sounds, or phonemes, in a language. Specifically, he looks at ejectives, a class of sounds (not present in English) produced by puffs of air in the mouth as opposed to the lungs. Everett suspected these sounds might be more common at high altitudes, where the lower air pressure would make them easier to produce.
To test this hypothesis, Everett analyzed phoneme data on 567 languages from the World Atlas of Linguistic Structures Online. He compared the data to the altitudes where the languages were spoken, obtained using geographic mapping software.