Previous studies have established that family social background and individual mental ability and educational attainment contribute to adult social class attainment. We propose that social class of origin acts as ballast, restraining otherwise meritocratic social class movement, and that education is the primary means through which social class movement is both restrained and facilitated. We tested these proposals using data from three generations. Study participants were men born in 1921 who participated in the Scottish Mental Survey of 1932 and thereafter in the Lothian Birth Cohort 1921's initial assessment in 1999-2001. In addition to their own data on mental ability at age 11 and educational and occupational attainment, they reported educational and occupational data on their fathers and offspring. We used these data to construct structural equation models of participant and offspring social class attainment and odds of moving up or down in social class. Consistent with our proposals, parental social class contributed to educational attainment, which in turn contributed to participant social class attainment, suggesting that educational attainment contributed to social class stability. Childhood mental ability contributed to participant educational and social class attainment but did not transfer to offspring. Education was also important to social mobility and, where measured, mental ability contributed to educational attainment. Education thus appeared to play a pivotal role in the association between ability and social class attainment. (Contains 3 figures and 7 tables.)