Before revision:  In their seminal work, Vytlacil and Yildiz (2007) showed how to identify and estimate the average treatment effect of a dummy endogenous variable in a weakly separable model under a monotonicity condition. We also consider similar weakly separable models , but relax the monotonicity condition for identification.
After revision:  In their seminal work, Vytlacil and Yildiz (2007) showed how to identify and estimate the average treatment effect of a dummy endogenous variable when the outcome is weakly separable in a single index. Their identification result builds on a monotonicity condition with respect to this single index. In comparison, we consider similar weakly separable models , but relax the monotonicity condition for identification.