Whether and why male parents should adjust the amount of care they provide to young if the female partner is unfaithful has been discussed for more than two decades. While there is general agreement that such a relationship exists across species, empirical results for within-population effects are ambiguous. The problem partially lies in the difficulty to disentangle between-individual from within-individual processes. We use an exceptionally well-suited dataset on wild house sparrows (Passer domesticus) to revisit this problem. https://sites.google.com/site/evolbehavecol/ http://www.orn.mpg.de/218867/Research-Group-Schroeder http://lundysparrows.blogspot.de/