The evolutionary ecology of sex ratio at birth: evidence at the local, national and international level

Evidence is presented for the facultative adjustment of sex ration at birth in response to maternal condition, for a food-stressed population in Ethiopia. National level data from Ethiopia also suggest that malnutrition may be biasing sex ratio at birth towards daughters. International variation in sex ratio at birth is also explored. Data are not available on all the relevant measures, but high birth rate does appear to be associated with low sex ratio at birth. Various explanations are discussed, but in each case, higher costs to the mother associated with male babies might explain these patterns. Reasons why other studies of human populations have not previously found such results are also discussed.