This paper proposes a novel tool to nonparametrically identify models with a
discrete endogenous variable or treatment: semi-instrumental variables
(semi-IVs). A semi-IV is a variable that is relevant but only partially
excluded from the potential outcomes, i.e., excluded from at least one, but not
necessarily all, potential outcome equations. It follows that standard
instrumental variables (IVs), which are fully excluded from all the potential
outcomes, are a special (extreme) case of semi-IVs. I show that full exclusion
is stronger than necessary because the same objects that are usually identified
with an IV (Imbens and Angrist, 1994; Heckman and Vytlacil, 2005; Chernozhukov
and Hansen, 2005) can be identified with several semi-IVs instead, provided
there is (at least) one semi-IV excluded from each potential outcome. For
applied work, tackling endogeneity with semi-IVs instead of IVs should be an
attractive alternative, since semi-IVs are easier to find: most
selection-specific costs or benefits can be valid semi-IVs, for example. The
paper also provides a simple semi-IV GMM estimator for models with homogenous
treatment effects and uses it to estimate the returns to education