Large brains are a defining feature of primates, as is a clear allometric trend

between body mass and brain size. However, important questions on the

macroevolution of brain shape in primates remain unanswered. Here we

address two: (i), does the relationship between the brain size and its shape

follow allometric trends and (ii), is this relationship consistent over evolutionary

time? We employ three-dimensional geometric morphometrics and

phylogenetic comparative methods to answer these questions, based on a

large sample representing 151 species and most primate families. We found

two distinct trends regarding the relationship between brain shape and

brain size. Hominoidea and Cercopithecinae showed significant evolutionary

allometry, whereas no allometric trends were discernible for Strepsirrhini,

Colobinae or Platyrrhini. Furthermore,we found that in the taxa characterized

by significant allometry, brain shape evolution accelerated, whereas for taxa in

which such allometrywas absent, the evolution of brain shape decelerated.We

conclude that although primates in general are typically described as largebrained,

strong allometric effects on brain shape are largely confined to the

order’s representatives that display more complex behavioural repertoires.