We argue that crosslinguistic variation regarding verbal reflexivization is parametric, reflecting a broader lexicon-syntax parameter: arity operations—operations on θ-roles, which affect the valence of a predicate—can apply in the lexicon or in the syntax. The significant empirical coverage of this parameter supports the view that the lexicon must be an active component of the grammar. The discussion focuses mainly on the formation of reflexive verbs. We argue that the prevailing view that reflexive verbs have an unaccusative derivation cannot be maintained. Rather, the reflexivization operation bundles a θ-role with an external θ-role, forming a combination that must merge externally. Next, we also briefly review other arity operations: (a) reciprocalization, (b) decausativization, and (c) saturation, which is involved in the formation of passives, middles, and impersonals. Variation in auxiliary selection, owing to the application of reflexivization or other arity operations, is independent of the lexicon-syntax parameter and follows under our approach from a structural accusative Case parameter.