Crab mentality, also known as crab theory, crabs in a bucket mentality, or the crab-bucket effect, is a way of thinking usually described by the phrase "if I can't have it, neither can you".

Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting tail-like abdomen, usually hidden entirely under the thorax. They live in all the world's oceans, in freshwater, and on land, are generally covered with a thick exoskeleton, and have a single pair of pincers on each arm. They first appeared during the Jurassic period, around 200 million years ago.