How does compositionality challenge a behaviorist theory of word meaning?

Compositionality is the principle that the meaning of a complex expression is determined by and predictable from the meanings of its simpler constituents. This feature is essential for explaining the productivity and creativity of natural languages.1

Behaviorist semantics, proposed by B.F. Skinner, defines the meaning of an expression as the behavioral stimulus that produces it, the response it elicits, or a combination of both. Compositionality challenges this theory because the meaning of a complex phrase is not always a predictable result of the behavioral responses associated with its individual parts.2

For example, a person might exhibit a specific behavioral response—such as recoiling—when hearing the phrase "brown cow," even if they do not exhibit that same response when hearing the words "brown" or "cow" in isolation. Because the meaning of the complex expression (the recoiling) cannot be derived from the meanings of its constituents, the behaviorist account fails to explain how language users understand an infinite number of novel, complex expressions based on the meanings of their parts.21