Deep frying has a long history supported by evidence from ancient cultures all over the world including Russia, Mexico, Japan and Brooklyn. Fritters had already existed in Europe since medieval time, and fried chicken was known as pollo fritto in Italy, Ga Xao in Vietnam, etc. before it became a culinary habit in the Southern United States. The Scots, and later Scottish immigrants to many southern states had a tradition of deep frying chicken in fat, unlike their English counterparts who baked or boiled chicken. There is also evidence of deep frying in West Africa. It is uncertain if deep frying existed in that region before European contact.
After the development of larger and faster growing hogs (due to crosses between European and Asian breeds) in the 18th and 19th century, in the United States, backyard and small scale hog production provided an inexpensive means of converting waste food, crop waste, and garbage into calories (in a relatively small space and in a relatively short period of time). Many of those calories came in the form of fat and rendered lard. Lard used for almost all cooking and was a fundamental component in many common homestead foods (many that today are still regarded as holiday and comfort foods) like biscuits and pies. The economic/caloric necessity of consuming lard and other saved fats may have lead to the popularity of fried foods, not only in the US, but worldwide. In the 19th century cast iron became widely available for use in cooking. The combination of flour, lard, a chicken and a heavy pan placed over a relatively controllable flame became the beginning of today's fried chicken.
When it was introduced to the American South, fried chicken became a common staple. Later, as Africans were brought to work on southern plantations, the slaves who became cooks incorporated seasonings and spices that were absent in traditional Scottish cuisine, enriching the flavor. Since most slaves were unable to raise expensive meats, but generally allowed to keep chickens, frying chicken on special occasions spread through the African American communities of the South. It endured the fall of slavery and gradually passed into common use as a general Southern dish. Since fried chicken traveled well in hot weather before refrigeration was commonplace, it gained further favor in the periods of American history when segregation closed off most restaurants to the black population. Fried chicken continues to be among this region's top choices for "Sunday dinner" among both blacks and whites. Holidays such as Independence Day and other gatherings often feature this dish.