Complex cognitive abilities have evolved several times independently in primates, yet whether convergent behavior reflects modifications to common neural systems remains unclear. We examined cerebellar lobular expansion across 51 primate species (103 specimens) using phylogenetic comparative methods (pANCOVA, AICc model comparison) to test four a priori hypotheses about posterior lobular expansion relative to the anterior lobe.Results demonstrate strong support for parallel expansion of Crus I, a prefrontal-projecting lobule, in cognitively complex platyrrhines (Cebinae, Atelinae) and independently in cercopithecoids, apes, and humans. Primarily (pre)motor-projecting lobule HVI shows generalized anthropoid and ape expansion without Cebinae/Atelinae-catarrhine convergence. This lobular specificity suggests that cognitive convergence depends on targeted prefronto-cerebellar circuit expansion, not undifferentiated cerebellar growth. Crucially, relative Crus I size explains 82% of variance in behavioral measures of self-control and 50% of variance in general cognition, confirming its association with behavioral complexity. We further demonstrate that within-species variation is substantially smaller than between-grade differences, validating phylogenetic analysis at the species level. We suggest that heterochronic shifts extending cerebellar development, coupled with prolonged juvenile periods characteristic of cognitively complex lineages, enable experience-dependent refinement of expanded prefrontal-cerebellar circuits.