He ascribed his brother’s indifference at first to the disparity of their age and education.
There are no more uses of "disparity" in the book.
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In the 1980s, the Court rejected a constitutional challenge to imposing the death penalty on juveniles; upheld the death penalty for disabled people suffering from "mental retardation"; and, in a widely condemned opinion, found no constitutional violation in the extreme racial disparities that could be seen throughout most death penalty jurisdictions.
Bryan Stevenson -- Just Mercy
As much as possible they try to avoid any huge disparities in intelligence, temperament, social background, and age.