If you're arguing for particular significance of a variable (grit), you can't go around controlling for all other variables with the same effect. It's not meani

If you're arguing for particular significance of a variable (grit), you can't go around controlling for all other variables with the same effect. It's not meaningful to see significance of a variable in a homogeneous cohort and translate that to the the variable being significant in the general population. It might make sense that West Point students differ largely by grit, especially if all other variables are controlled for. That does not prove that the success of people generally is significantly affected by their grit.