It is well established that animus can never constitute a legitimate state interest for purposes of equal protection analysis. But neither precedent nor scholarship has stated conclusively how animus is properly defined, what counts as evidence of animus in any given case, or the precise doctrinal significance of a finding of animus. The U.S. Supreme Court has explicitly addressed the question of animus only a handful of times, and these cases do not appear to be particularly congruent with one another, at least on the surface. Further, while a number of scholars have discussed animus in terms of moral philosophy, no one has attempted to articulate a unified theory of animus as a matter of doctrine—particularly in the post-Lawrence era.

This Article systematically examines Supreme Court precedent to distill a coherent standard for identifying the presence of animus in various forms of state action. What emerges is that the animus analysis the Court actually employs provides a more vigorous alternative to the thoroughly criticized “tiers-of-scrutiny” framework, which has defined and limited the scope of contemporary equal protection jurisprudence. In short, the doctrine of unconstitutional animus gives life to the strong anti-caste mandate of the federal Equal Protection Clause. The time is ripe to understand the nature of unconstitutional animus, as animus may well play a critical role in the Court’s decisions on the constitutionality of different forms of prohibitions against same-sex marriage.