Evolutionary theory predicts that mothers of different condition should adjust the birth sex ratio of their offspring in relation to future reproductive benefits. Published studies addressing variation in mammalian sex ratios have produced surprisingly contradictory results. Explaining the source of such variation has been a challenge for sex–ratio theory, not least because no mechanism for sex–ratio adjustment is known. I conducted a meta–analysis of previous mammalian sex–ratio studies to determine if there are any overall patterns in sex–ratio variation. The contradictory nature of previous results was confirmed. However, studies that investigated indices of condition around conception show almost unanimous support for the prediction that mothers in good condition bias their litters towards sons. Recent research on the role of glucose in reproductive functioning have shown that excess glucose favours the development of male blastocysts, providing a potential mechanism for sex–ratio variation in relation to maternal condition around conception. Furthermore, many of the conflicting results from studies on sex–ratio adjustment would be explained if glucose levels in utero during early cell division contributed to the determination of offspring sex ratios.