Giraffe group sizes appear to vary in response to localized ecological and environmental factors, but there has been little investigation of how social factors or predation risk affect group size in giraffes. We studied two adjacent, enclosed populations of Rothschild’s giraffes in Kenya, and used 591 records of groups to determine the relative influence of a series of variables on group size. One population was free from any risk of predation, while the other area contained a high density of lions. Mean group size was smaller in the population with lions, but a series of Generalized Linear Mixed Models accounting for habitat and age/sex class of individuals showed that the presence of high numbers of juveniles in the area free from lions artificially inflated group sizes. Removing juveniles from the analysis showed that contrary to the existing creche hypothesis, adult females were found in smaller groups when they had calves. We found no evidence that predation risk influenced grouping behaviour. Rather, recruitment and habitat type had a stronger influence on group sizes, but the results were complex and varied between different age and sex classes of individual. We conclude that predation is not an important driver of giraffe grouping, and that further research is necessary to understand the complex behaviour and ecology of this prominent yet understudied species.