the same period; in fact, more than half of all births in Sweden in the mid-1980s were from unmarried women. In the United States, the nuclear family fell from 44% of all households in 1960 to 29% in 1980. But for African Americans, the figure was higher. In 1991, single mothers had given birth to the majority of children (70%) and headed a majority of households (58%). Parts of the reason why marriages were delayed or avoided were economic. People who entered the workforce during the 1970s and 1980s made less than their fathers did in the 1950s. Fertility rates fell as a result. The rise in the number of cohabitating couples was also a factor. These couples claimed cohabitation helped assess the suitability of a mate before marriage. While sociologists had known for a long time that humans tend to select mates of the same ethnicity, social class, personality traits, and educational level, a phenomenon known as assortative mating, during the 1970s and 1980s, an even stronger correlation between the educational levels of a married couple was observed in the United States. In fact, both the age at (first) marriage and the amount of time spent in school increased in the 1970s. Whereas in the past women had typically looked for men of high social status while men had not, by the late twentieth-century, men also looked for women of high earning potential, resulting in an even more pronounced educational and economic assortative mating. More generally, higher rates of university attendance and workforce participation by women affected the marital expectations of both sexes in that both men and women became more symmetrical in what they desired in a mate. The share of marriages in which both spouses were of the same educational level rose monotonically from 47% in 1960 to 53% by the mid- to late-1980s. Similarly, the share of marriages in which both spouses differ by at most one level of schooling increased from 83% in 1940 to 88% by the 1980s. Part of the reason why people increasingly married their socioeconomic and educational peers was economic in nature. Innovations that became commercially available in the late twentieth century such as the washing machine and frozen food reduced the amount of time people needed to spend on housework, which diminished the importance of domestic skills. Moreover, it was no longer possible for a couple with one spouse having