Sex differences in physical attributes such as muscle size and strength and circulating haemoglobin levels give male athletes an insurmountable competitive advantage over female athletes in sports where size, strength and power matter. 

These advantages (which translate, in athletics, to an average 10-12% performance difference across all disciplines) make competition between men and women as meaningless and unfair as an adult competing against a child or a heavyweight boxer competing against a flyweight. Only men would qualify for elite-level competition; the best female athlete would not come close to qualifying.

The evidence gathered by the experts consulted by the IAAF (both peer-reviewed research and observational data from the field) suggests that having levels of circulating testosterone in the normal male range rather than in the normal female range, and being androgen-sensitive gives a female DSD athlete a performance advantage of at least 5-6% over a female athlete with testosterone levels in the normal female range (which is an enormous difference in events where milliseconds count). 

The effects are most clearly seen in races over distances between 400m and one mile, where the combination of increased lean body mass and elevated circulating haemoglobin appears to have the greatest combined impact.

- IAAF