Animus is a LawLatin term used in a variety of contexts to designate the motivations of a legal person.
Criminal law
In criminal law, animus nocendi refers to an accused's guilty state of mindwith respect to the actus reus of the crime. It is thus analogous to mens rea, a more commonly used term in common law countries. The term dates back Roman understandings of censorship, where it referred to an author's impermissible intention in writing a literary work. In Scots law, the term animus malus is sometimes used.
Family law
In family law, animus deserendi refers to one spouse's firm intention to leave the matrimonial home—and hence the marriage. When combined with the "factum of separation," it "constitute" desertion. Proof of desertion, in turn, has been grounds for divorce in some legal systems.
Property law
In property law, animus possidendi refers to a person's manifest intention to control an object, and is one of the two elements—along with factum possidendi —required to establish property in an object by first possession. In both civil and common law, animus revertendi distinguishes an animal in which one may have a property right from a wild animal by reference to the animal's habitual return to a person who cares for it. Blackstone describes the doctrine as follows:
The law therefore extends … possession farther than the mere manual occupation; for my tame hawk that is pursuing his quarry in my presence, though he is at liberty to go where he pleases, is nevertheless my property; for he hath animum revertendi. So are my pigeons, that are flying at a distance from their home … and likewise the deer that is chafed out of my park or forest, and is instantly pursued by the keeper or forester: all which remain still in my possession, and I still preserve my qualified property in them. But if they stray without my knowledge, and do not return in the usual manne, it is then lawful for any stranger to take them.
… if the constitutional conception of "equal protection of the laws" means anything, it must at the very least mean that a bare congressional desire to harm a politically unpopular group cannot constitute a legitimate governmental interest.
Referring to the concept of animus explicitly later in the same passage, the Supreme Court formulated the doctrine thus in United States v. Windsor :
seeks to injure the very class New York seeks to protect. By doing so it violates basic due process and equal protection principles applicable to the Federal Government.