Descriptive statistics for time-invariant variables by cognitive ability in four cohort studies.

Mean (SD) and N (%). Cognitive ability group refers to an individual’s cognitive ability relative to the average level of cognitive ability in their household (i.e., whether they have above or below average cognitive ability among their siblings). Note, this is why the number of households is the same for below and above average cognitive ability groups—cognitive ability group is defined within each household (i.e., whether a given sibling is above or below average cognitive ability for that household).