Socioecology has traditionally assumed that differences in social form between species derive from selected responses to particular evolutionary environments, while differences within a species reflect plastic responses to contemporary differences in ecology. The issue of behavioural or social plasticity is particularly salient for primatologists as it ties into conceptualizations of the evolution of cognitive capacity. In this regard, savanna baboons Papio hamadryas have provided a focus for research by virtue of their ecological plasticity and a problem for researchers by virtue of marked social differences in one subspecies – hamadryas baboons P. h. hamadryas – that cannot be explained solely by reference to current ecology. It is argued that the problem extends beyond hamadryas and that at least some differences between other populations also reflect selected responses to restricted, local evolutionary conditions. Of particular importance to the understanding of contemporary differences in baboon sociality is the extent to which evolutionary environments have constrained group size and perturbed adult sex ratios and, thereby, structured responses to the ever-present threat of infanticide. The view that there is much to be gained from a phylogenetic approach to population differences in behaviour is endorsed.