Although most historians count 87 members of the party, Stephen McCurdy in the Western Journal of Medicine includes Sarah Keyes—Margret Reed""s mother—and Luis and Salvador, bringing the number to 90.[177] Five people had already died before the party reached Truckee Lake: one from tuberculosis (Halloran), three from trauma (Snyder, Wolfinger and Pike), and one from exposure (Hardkoop). A further 34 died between December 1846 and April 1847: twenty-five males and nine females.[178][M] Several historians and other authorities have studied the mortalities to determine what factors may affect survival in nutritionally deprived individuals. Of the fifteen members of the snowshoe party, eight of the ten men who set out died (Stanton, Dolan, Graves, Murphy, Antonio, Fosdick, Luis and Salvador), but all five of the women survived.[179] A professor at the University of Washington stated that the Donner Party episode is a "case study of mediated natural selection in action".[180]

The deaths at Truckee Lake, Alder Creek, and in the snowshoe party, were probably caused by a combination of extended malnutrition, overwork, and exposure to cold. Several members, such as George Donner, became more susceptible to infection due to starvation,[181] but the three most significant factors in survival were age, sex, and the size of family group each member traveled with. The survivors were on average 7.5 years younger than those who died; children aged between 6 and 14 had a much higher survival rate than infants and children under the age of 6, of whom 62.5 percent died, including the son born to the Keseburgs on the trail, or adults over the age of 35. No adults over the age of 49 survived. Deaths among males aged between 20 and 39 were "extremely high" at more than 66 percent.[178] Men have been found to metabolize protein faster, and women do not require as high a caloric intake. Women also store more body fat, which delays the effects of physical degradation caused by starvation and overwork. Men also tend to take on more dangerous tasks, and in this particular instance, the men were required before reaching Truckee Lake to clear brush and engage in heavy labor, adding to their physical debilitation. Those traveling with family members had a higher survival rate than bachelor males, possibly because family members more readily shared food with each other.[177][18