The likelihood of lactose intolerance depends on ethnicity. Lactose intolerance is the normal phenotype.  The mutations associated with lactase persistence occurred, and were selected for, when there was an increased reliance on dairy as a food source.

Many ethnicities are mostly lactose intolerant, including 90% of Asian Americans, 70% of African-Americans, 95-100% of Native Americans and 50% of Mexican-Americans.

In contrast, less than 25% of Caucasians and only about 5% of people of Northern European ancestry are lactose intolerant, because dairy has been a food staple in their past.