lactose intolerance causes difficulty in digesting dairy products that contain lac- tose (milk sugar). it is particularly common among people of african and asian ancestry. in the united states (ignoring other groups and people who con- sider themselves to belong to more than one race), 82% of the population is white, 14% is black, and 4% is asian. moreover, 15% of whites, 70% of blacks, and 90% of asians are lactose intolerant.19 suppose we select a u.s. person at random.