Ejectives, which occur in approximately 20 percent of the world's languages, are consonants produced when air is compressed in the mouth and pharyngeal cavity and then released in a burst. Unlike most sounds, they are not produced using air from the lungs.

For Caucasian languages, many of which are known for their rich inventories of sounds, ejectives are a characteristic feature.

Linguists and befuddled visitors alike have described these sounds as hisses, spits, and even mini-explosions in the mouth.

Georgian has six ejectives: Circassian may hold the record, with at least 10: The sounds appear in everyday terms, like the word for "bridge" in Chechen: Or "head scarf" in Avar:

In his research, Everett considered the distribution of 567 world languages in relation to six high-elevation "zones." Those zones were defined as major regions greater than 1,500 meters in altitude, plus the surrounding 200 kilometers. They included the Caucasus range and the Javakheti plateau, the Rocky Mountain region in North America, the southern African plateau, and others.

Of Everett's sample of languages, only a small portion contained ejectives. However, he found that nearly two-thirds of those that did were located in the high-altitude zones.

The only zone in which ejectives were absent was the Tibetan plateau. "In fact it strikes us as remarkable that only one region presents an exception," Everett writes.

Languages occurring in each zone, from the Caucasus to the Andes, were also from multiple, often unrelated language families. The Caucasian language Abkhaz contains ejectives, but so do several dialects of Armenian, a language from the entirely distinct Indo-European family. Such evidence, Everett says, goes far in arguing that geography, and not genetic relations, is behind the trend.

Explaining why such a correlation might exist is a more challenging proposition. Everett offers his best guesses, albeit tentatively.

"Hypothetically, these sounds should be easier to make at high altitudes because they require the compression of ambient air," Everett says. "Since air pressure is lower at higher altitudes, the sounds should be easier to make. That was my first hypothesis."

He also suggests that use of the sounds may be a biological adaption to the dryness of high-altitude locations. "Because you don't have to expel air from the lungs to produce ejective sounds, they should theoretically reduce the amount of water vapor lost during speech," he says.

Bernard Comrie, the director of the linguistics department at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, says Everett may be on to something, at least in showing that a real correlation exists between altitude and ejectives.

Like others, however, he is more skeptical when it comes to Everett's suggested explanations, but says they are not "unreasonable."