Animal information processing and decision making are often considered to be adaptations that allow individuals to behave optimally under particular ecological conditions. Numerous examples demonstrate how cues from the biotic and abiotic environments affect the ways in which animals process information and make decisions. Information gained from interactions with living organisms is the most complex because individuals have to respond to heterospecifics or conspecifics which may decide on what to do depending on the behaviour of a focal individual. Evolutionary conflicts of interest include: (i) interactions between hosts and parasites, predators and prey, and between competitors; (ii) sperm competition interactions between females, male mates and male non-mates, and (iii) interactions between mate-searching females and their potential mates. Brains may evolve particularly rapidly under the influence of evolutionary conflicts and they may enhance the importance of adapted psychologies in these contexts.