Psychology has a WEIRD problem. It is overly reliant on participants from Western, Educated, Industrialized, Rich, and Democratic societies. Over the last decade this problem has come to be widely acknowledged, yet there has been little progress toward making psychology more diverse. This Element proposes that the lack of progress can be explained by the fact that the original WEIRD critique was too narrow in scope. Rather than a single problem of a lack of diversity among research participants, there are at least four overlapping problems. Psychology is WEIRD not only in terms of who makes up its participant pool, but also in terms of its theoretical commitments, methodological assumptions, and institutional structures. Psychology as currently constituted is a fundamentally WEIRD enterprise. Coming to terms with this is necessary if we wish to make psychology relevant for all humanity. This title is also available as Open Access on Cambridge Core.