While food allergies have been observed by doctors since ancient times and virtually all foods can be allergens, research by the Mayo Clinic in Minnesota found they are becoming increasingly common since the early 2000s. Today, one in twelve American children has a food allergy, with peanut allergy being the most prevalent type. Reasons for this remain poorly understood. Nut allergies in general have quadrupled and shellfish allergies have increased 40% between 2004 and 2019. In all, about 36% of American children have some kind of allergy. By comparison, this number among the Amish in Indiana is 7%. Allergies have also risen ominously in other Western countries. In the United Kingdom, for example, the number of children hospitalized for allergic reactions increased by a factor of five between 1990 and the late 2010s, as did the number of British children allergic to peanuts. In general, the better developed the country, the higher the rates of allergies. Reasons for this remain poorly understood. One possible explanation, supported by the U.S. National Institute of Allergy and Infectious Diseases, is that parents keep their children "too clean for their own good". They recommend exposing newborn babies to a variety of potentially allergenic foods, such as peanut butter, before they reach the age of six months. According to this "hygiene hypothesis", such exposures give the infant's immune system some exercise, making it less likely to overreact. Evidence for this includes the fact that children living on a farm are consistently less likely to be allergic than their counterparts who are raised in the city, and that children born in a developed country to parents who immigrated from developing nations are more likely to be allergic than their parents are.