Strong mothers bear more sons in rural Ethiopia.

In humans, there is evidence that the physiological cost to the mother of bearing sons is greater than of bearing daughters. Parents should manipulate the sex of offspring born in response to resource availability to maximize their reproductive success. Here, we demonstrate that, within a rural food-stressed community in southern Ethiopia, there is a strong association between the sex of the most recent birth and maternal nutritional status, measured either by body mass index or mid-upper arm muscle area (AMA) (measures of fat and muscle mass). The effect of muscle mass is very marked: those women in the upper 25th percentile of AMA were more than twice as likely to have had a recent male birth than those in the lowest 25th percentile.