Within socially monogamous breeding systems, levels of extra-pair paternity can vary not only between species, populations and individuals, but also across time. Uncovering how different extrinsic conditions (ecological, demographic and social) influence this behavior will help shed light on the factors driving its evolution. Here, we simultaneously address multiple socio-ecological conditions potentially influencing female infidelity in a natural population of the cooperatively breeding Seychelles warbler, Acrocephalus sechellensis. Our contained study population has been monitored for over 25 years, enabling us to capture variation in socio-ecological conditions between individuals and across time and to accurately assign parentage. We test hypotheses predicting the influence of territory quality, breeding density and synchrony, group size and composition (number and sex of subordinates), and inbreeding avoidance on female infidelity. We find that a larger group size promotes the likelihood of extra-pair paternity in offspring from both dominant and subordinate females, but this paternity is almost always gained by dominant males from outside the group (not by subordinate males within the group). Higher relatedness between a mother and the dominant male in her group also results in more extra-pair paternity — but only for subordinate females — and this does not prevent inbreeding occurring in this population. Our findings highlight the role of social conditions favoring infidelity and contribute towards understanding the evolution of this enigmatic behavior.