Distance runners tend to have fewer male children, compared to the population at large. Author asks if distance running can determine the sex of your children. Result of studies: Mileage proved to be the only factor which had an impact on offspring gender, as running intensity (average training speed), overall life stress, paternal age, social status, manual vs. on-manual labour, competitive success, and the time interval between marriage and conception all had no significant effect. High-volume running tends to produce dips in testosterone, which in turn produces a decline in the output of boys (because of lowered testosteron levels). Higher training mileage levels of the mother should also drive female sex-hormone concentrations downward, increasing the chances of having a female.