Sara McLanahan, Who Studied Single Motherhood, Dies at 81

While completing a postdoctoral fellowship at the University of Wisconsin, Dr. McLanahan came across “The Underclass,” a 60,000-word article by Ken Auletta that appeared in three consecutive issues of The New Yorker in 1981. (It was later expanded into a book.) In it Mr. Auletta argued that the “weakened family structure of the poor’’ was a major reason for poverty in the country.