Empirical specification Within-twin-pair estimations Twins: log(earnings) of twins in family Vi=X,+B,Edui+Z1 B2+u:+a y2=Xa+BEu2+22B2+1+E2 (3) Taking the within-twin difference we have y1-y21=B(Eh1-E2)+(21-Z2)B2+E1-E2(4) Note that the unobservables u has been removed9 Empirical specification: Within-twin-pair estimations ◼ Twins: log(earnings) of twins in family i ◼ Taking the within-twin difference, we have ◼ Note that the unobservables, has been removed (3) (2) 2 1 2 2 2 2 1 1 1 1 2 1 i i i i i i i i i i i i y X Edu Z y X Edu Z           = + + + + = + + + + ( ) ( ) (4) 1i 2i 1 Edu1i Edu2i Z1i Z2i 2 1i 2i y − y =  − + −  + − 