The relationship between completed education and adult cognition is investigated using data from the Indonesia Family Life Survey. We compare adult siblings to account for shared, difficult-tomeasure characteristics that likely affect this relationship, including genetics and parental preferences and investments. After establishing the importance of shared family background factors, we document substantively large, significant impacts of education on cognition in models with sibling fixed effects. In contrast, the strong positive correlation between education and adult height is reduced to zero in models with sibling fixed effects, suggesting little contamination in the education-height association beyond factors common to siblings.