The last decade witnessed the development of algorithms that completely solve the identifiability problem for causal effects in hidden variable causal models associated with directed acyclic graphs. However, much of this machinery remains underutilized in practice owing to the complexity of estimating identifying functionals yielded by these algorithms. In this paper, we provide simple graphical criteria and semiparametric estimators that bridge the gap between identification and estimation for causal effects involving a single treatment and a single outcome. First, we provide influence function based doubly robust estimators that cover a significant subset of hidden variable causal models where the effect is identifiable. We further characterize an important subset of this class for which we demonstrate how to derive the estimator with the lowest asymptotic variance, i.e., one that achieves the semiparametric efficiency bound. Finally, we provide semiparametric estimators for any single treatment causal effect parameter identified via the aforementioned algorithms. The resulting estimators resemble influence function based estimators that are sequentially reweighted, and exhibit a partial double robustness property, provided the parts of the likelihood corresponding to a set of weight models are correctly specified. Our methods are easy to implement and we demonstrate their utility through simulations.
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