How animals choose to organise themselves socially has consequences for their subsequent survival and reproductive output, shaping evolutionary processes. However, the mechanisms by which such organisation influences their life history are complex and remain poorly understood; for example, the behaviour of reproductive partners, fine-scale social connections within non-reproductive groups, the broader social environment, and the physical environment all play a role in shaping the expression of social behaviours at the individual scale. In this thesis, which is separated into two parts, we consider the influence of social behaviour on reproductive choices. In the first part, we explore how researchers define a social association between individuals and then test the relationship between individual social behaviour and fitness at the annual and lifetime scales. In the second part, we apply concepts from animal social research to the question of female extra-pair paternity, which has long puzzled evolutionary ecologists. We conducted our research using a long-term study on wild house sparrows Passer domesticus, breeding on Lundy Island, and using a twenty-year genetic pedigree. We also used data from three other systems to compare the influence of association definition on social network structure. Our results suggest that individual sociality shapes reproductive choice in the short term, where the number of opposite-sex associates is correlated with reproductive success, but that social traits are likely to be subject to stabilising selection over a lifetime. Likewise, the inclusion of the social environment and social partner’s behavioural preference as an indirect genetic effect on female behaviour increased variance in quantitative genetics models for female extra-pair paternity, suggesting a role for social behaviour. Finally, we found a correlation between the female divorce rate (or propensity to switch social mates) and the solicitation of extra-pair males. Together, our results suggest that the social preference of individual animals, when appropriately defined, is a powerful force in the mediation of behavioural traits and subsequent fitness.