The social brain hypothesis proposes that haplorhine primates have evolved relatively large brains for their body size primarily as an adaptation for living in complex social groups. Studies that support this hypothesis have shown a strong relationship between relative brain size and group size in these taxa. Recent reports suggest that this pattern is unique to haplorhine primates; many nonprimate taxa do not show a relationship between group size and relative brain size. Rather, pairbonded social monogamy appears to be a better predictor of a large relative brain size in many nonprimate taxa. It has been suggested that haplorhine primates may have expanded the pairbonded relationship beyond simple dyads towards the evolution of complex social groups. We examined the relationship between group size, pairbonding, and relative brain size in a sample of 19 lemurs; strepsirrhine primates that last share a common ancestor with monkeys and apes approximately 75 Ma. First, we evaluated the social brain hypothesis, which predicts that species with larger social groups will have relatively larger brains. Secondly, we tested the pairbonded hypothesis, which predicts that species with a pairbonded social organization will have relatively larger brains than non-pairbonded species. We found no relationship between group size or pairbonding and relative brain size in lemurs. We conducted two further analyses to test for possible relationships between two nonsocial variables, activity pattern and diet, and relative brain size. Both diet and activity pattern are significantly associated with relative brain size in our sample. Specifically, frugivorous species have relatively larger brains than folivorous species, and cathemeral species have relatively larger brains than diurnal, but not nocturnal species. These findings highlight meaningful differences between Malagasy strepsirrhines and haplorhines, and between Malagasy strepsirrhines and nonprimate taxa, regarding the social and ecological factors associated with increases in relative brain size. The results suggest that factors such as foraging complexity and flexibility of activity patterns may have driven selection for increases in brain size in lemurs.