Greg Mankiw reports that Chetty/Saez’s recent study on income mobility finds that “91 percent of the variance [in a person’s income] is unexplained by [their] parents’ income.” “Based on adoption studies,” Mankiw speculates that “conservatively…half…of that 9 percent… [stems from] genetics rather than environment… [which] leaves only 4.5 percent of the variance attributed directly to parents’ income.” He notes that even if universal preschool could “completely neutralized the [non-genetic] effect of parent’s income… the standard deviation [i.e. the variation] of income would [only] fall to 0.977 [i.e. 98%] of what it now is.”