Abstract Parents should bias resource allocation towards the sex most likely to provide higher fitness returns by adjusting the birth sex ratio and/or through differential care of sons and daughters. Sex allocation research in mammals to date has been focused almost exclusively on maternal traits, but fathers may also play an important role. Future studies should investigate the influence of paternal quality on the fitness of sons and daughters, and possible conflicts of interest between mothers and fathers. There is also a crucial need for more studies examining whether relative levels of maternal care in sons and daughters depend on paternal quality.