Military enlistment is highly selective for reasons of both labor demand and supply. An early-twentieth-century evolution of military technology that shifted the demand for workers of different stature illustrates the importance of labor demand beyond the commonly discussed influences originating with labor supply. English-born soldiers in the Anglo-Boer War (1899–1902) were taller, on average, than those of World War I (1914–18), yet these differences cannot be attributed to standard of living or business cycle influences on the labor market. Rather, we argue, the mechanization and bureaucratization of warfare increased the relative value of shorter people permitting a decline in the average height of soldiers. Technological change over the period of these two wars affected labor demand in a way that must be recognized before using this evidence to test hypotheses about changes in population health.