In sharp contrast with Britain and most E.U. countries, the board of a U.S. takeover target firm has virtually total discretion on whether to offer post-bid resistance. There has long been an extensive debate among legal scholars about the normatively optimal level of board discretion in this context. We accordingly investigate empirically whether U.S. target firm boards function as bona-fide fiduciaries for shareholders when they exercise this exceptional level of discretion. We use a research design that enables causal inferences. Exploiting well-documented, directly relevant, and plausibly exogenous instrumental variables, we find a positive causal relationship from existing antitakeover provisions (ATPs) to post-bid resistance, and no causal relationship from bid premiums to post-bid resistance. Alongside our accompanying conceptual framework, our results enable us to infer that the target board’s decision to resist is, on average, not motivated in the best interests of shareholders, but by entrenchment considerations. Our empirical results underscore a need to revisit the issue of board discretion and director primacy in relation to takeover resistance in U.S. law and practice.