Conventional social mobility research, which measures family social class background relative to only fathers' characteristics, presents an outmoded picture of families--a picture wherein mothers' economic participation is neither common nor important. This article demonstrates that such measurement is theoretically and empirically untenable. Models that incorporate both mothers' and fathers' characteristics into class origin measures fit observed mobility patterns better than do conventional models, and for both men and women. Furthermore, in contrast to the current consensus that conventional measurement strategies do not alter substantive research conclusions, analyses of cohort change in social mobility illustrate the distortions that conventional practice can produce in stratification research findings. By failing to measure the impact of mothers' class, the current practice misses a recent upturn in the importance of family background for class outcomes among men in the United States. The conventional approach suggests no change between cohorts, but updated analyses reveal that inequality of opportunity increased significantly for men born since the mid-1960s compared with those born earlier in the century. (Contains 22 footnotes, 5 tables, and 2 figures.)