Southern United States literature is defined as American literature produced about the South or by writers from the region. Literature published about the American South began during the colonial era and flourished greatly during and after the United States' time of slavery. After slavery, however, economic conditions in the South declined drastically, resulting in fewer opportunities for authors. Writers from the South have had a significant impact on world literature. Thomas Hardy, William Faulkner, John Grisham, Anne Tyler, J. D. Salinger, and Harper Lee are among the most famous current or past writers from the region. The New South movement in the 19th century and the Civil Rights Movement in the 20th both had an impact on southern writing. Novelists such as Charles Brockden Brown, Henry David Thoreau, and Walt Whitman were influenced by events occurring in the South. Many other important writers have been born in or lived primarily in the South, including Edgar Allan Poe, Nathaniel Hawthorne, Herman Melville, James Fenimore Cooper, Margaret Mitchell, Carson McCullers, Ralph Ellison, and Toni Morrison. Literary critics divide American literature into four periods based on major events that occurred within the country: pre-Columbian, colonial, post-revolutionary, and industrialization/urbanization. Southern literature is included in this category because most of it was written before the civil war.