Evolutionary theory suggests that offspring sex should be adjusted to environmental conditions in order to maximize future reproductive success. In several animal taxa environmental factors indeed affect the secondary sex ratio. In humans, changes in the sex ratio at birth have been associated with population stressors like war, environmental disasters or economic strife during pregnancy. Here we compared litter sex ratios of female guinea pigs, exposed experimentally to a stable and an unstable social environment. In the latter group composition was changed every three days. Under unstable social conditions the sex ratio was significantly more biased towards daughters than in the stable social situation. This finding was consistent among four independent experiments, conducted independently from each other. Life expectancy can be dramatically reduced under conditions of social instability. Hence mothers in such conditions should bias their investment towards the sex that reaches sexual maturity first, which is the female sex in this species. Thus, to shift the offspring sex ratio towards more daughters under conditions of social instability may represent a maternal strategy to maximize future reproductive success.