Different types of cognitive abilities can lead to a variety of knowledge that can help an animal to find, access, and guard food and mates. One approach to gain insight into the evolution of such cognitive abilities is by inferring cognitive performances from observed behaviours across closely related species and to compare them. By linking differences in cognitive performances with differences in current socio-ecological circumstances, hypotheses about the evolutionary pressures that contributed to the selection of these abilities can be tested. This can then provide answers to the question why a trait, such as the ability to plan for the next day, evolved. Drawing inference about cognitive abilities from behaviour is, however, not straightforward. In her latest paper researcher Karline Janmaat describes a set of different approaches, addressing where and how one can make such inferences in a variety of species, with the focus on primates.