Department of Physics and Materials Research Institute,The Pennsylvania State University, University Park, Pennsylvania 16802, USA(Dated: June 8, 2011)We report a magneto-optical Kerr e ect study of the collective magnetic response of arti cialsquare spin ice, a lithographically-de ned array of single-domain ferromagnetic islands. We nd thatthe anisotropic inter-island interactions lead to a non-monotonic angular dependence of the arraycoercive eld, and comparisons with micromagnetic simulations indicate that the two perpendicularsublattices exhibit distinct responses to changing magnetic eld that drive the magnetization reversalprocess. Furthermore, such comparisons demonstrate that island shape disorder plays a hithertounrecognized but essential role in the collective behavior of these systems.