It's a stark reminder that there are two classes of Himalayan mountaineers—those who pay to climb, and those who get paid to support them. The people spending money, to the tune of tens of thousands of dollars each, are foreign adventure enthusiasts. The people earning money, typically several thousand dollars each (or more for head Sherpas), are natives of Nepal for whom Everest expeditions provide lucrative livelihoods to support extended families.

Throughout the Himalayas, avalanches are the leading cause of death for Sherpas. The chart below, assembled from numbers collected by the Himalayan Database, shows that both Sherpas ("hired") and their customers/bosses (expedition "members"—a telling word choice, implying that Sherpas, though essential embeds in climbing groups, are set apart from the rest of the team) are more likely to die from falls or avalanches than other circumstances.

The Himalayan Database counts 608 "member" deaths and 224 "hired" deaths on mountains in Nepal, including Everest, between 1950 and 2009. Almost 50 percent of hired deaths were due to avalanches, while nearly 40 percent of member deaths were attributed to falls.

These patterns have a lot to do with who does what, and where, on mountains like Everest. Sherpas spend much of their time establishing and supplying camps in avalanche-prone zones. Paying expedition members move through those zones as quickly and efficiently as possible to save their energy for summit bids, where the risk of avalanches is lower but the air is thin and falls are more likely to occur. The graph below, again from the Himalayan Database researchers, shows deaths by altitude.

The data speaks to the most illuminating lesson of the recent tragedy on Everest—how the (growing) divide between Sherpas and the Western climbers they work for affects each group's mortality on the mountain.

In 2008, a study in the British Medical Journal examined this dynamic, tracking Everest death patterns between 1921 and 2006. The chart below is based on deaths between April and June from 1982 to 2006.

Most commercial expeditions approach Everest's summit from the Nepalese side, relying heavily on Sherpas to set up camps and transport gear below the South Col. The summit has become the most dangerous place for expedition members—notice how many of them are killed on their way down from the summit to the South Col, and how great the risk of falling is up there. But, arguably, these patterns exist because expeditions members are largely shielded from the mountain's other dangers—the Khumbu Icefall before "Ice Doctors" (Sherpas) have established safe routes through it with ladders and ropes; the exhaustion of carrying equipment between Base Camp, the Icefall, and higher camps; the repeated, precarious steps over crevasses and under ice shelves during multiple gear shuttles between camps. The Sherpas bear the brunt of these risks.