Adverse interest

What does Adverse interest mean in American Law?

The definition of Adverse interest in the law of the United States, as defined by the lexicographer Arthur Leff in his legal dictionary is:

Numerous statutes use the term “adverse interest,” especialy in the formulations “parties [or “persons”] with an adverse interest [or “adversely interested”].” (The same idea is often expressed more concisely by reference to “adverse parties.”) In general, it is reasonably easy to determine who a person with an “adverse interest” is, e.g., if X and Y both claim title to a piece of land, or X is suing Y for breach of contract, then X and Y are persons or parties with interests adverse to each other’s. But

sometimes adverseness of interest is not that obvious, e.g., if a statute provides that all “adverse parties” be served with a notice of appeal, is a coplaintiff an “adverse party” when the other co-plaintiff decides to appeal? Inasmuch as he might be actually disadvantaged by the appeal, some courts have said yes. But the answer is not foreordained. As usual, whether a person or class of persons is to be included in the term as used in any particular statute will have to depend on the terms of

the statute, and on what one can glean from it about its purpose.