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).

均值（标准差，Standard Deviation）与频数（百分比）。认知能力组（Cognitive ability group）指个体的认知能力相对于其所在家庭的认知能力平均水平，即该个体在其同胞中的认知能力是否高于或低于家庭平均认知水平。请注意，这也解释了为何认知能力低于平均水平与高于平均水平的两组对应的家庭户数完全一致——认知能力组是在每个家庭内部定义的，即给定的某位同胞在其所在家庭中的认知能力是否高于或低于该家庭的平均认知水平。