The decade of the seventies dawned with the United States' invasion of Cambodia, antiwar demonstrations, and the tragic killing of four students at Kent State University by the Ohio National Guard, all in 1970. Hannah Arendt wrote On Violence in the same year. In 1973 the U. S. Supreme Court ruled that abortion was legal. The United Nations declared 1975 the International Year of the Woman, and a world conference on women was held in Mexico City. It was a time of pluralism and eclecticism, in which individuals and individual groups emerged outside the mainstream. In 1976 Ringgold created a series of pieces entitled The Wake and Resurrection of the Bicentennial Negro, which focused on events from the private lives of black citizens. During the 1970s, the "number of unmarried couples who were living together tripled; among those age 25 and under with no children, it increased eightfold.