Sex segregation in competitive sports is regarded as fair. Before puberty boys and girls do not differ in height, muscle and bone mass. Testosterone (T) exposure during puberty leads to an ultimate average greater height in men of 12-15 cm, longer and larger bones and muscle mass and strength and higher hemoglobin levels. Postpubertal androgen ablation reverses, at least in part, previous anabolic effects of T on muscle, bone mineral density and hemoglobin but the long bones remain longer and wider. T administration dose dependently increases muscle mass and maximal voluntary strength. Therefore, exogenous androgens, being performance enhancing drugs, are banned for all athletes. An issue is the participation in competitive sports of people with errors of sexual differentiation and particularly transsexuals who have been sex-reassigned. In view of the effects of T, a clear demarcation is whether sex reassignment has taken place before or after hormonal puberty. Pubertal effects of T are in part reversible but there is no reliable evidence as to its completeness. The International Olympic Committee (IOC) has taken an inevitably arbitrary decision with regard to participation of sex-reassigned transsexuals in elite sports: sex reassignment must have taken place at least two years earlier, hormone treatment must be appropriate for the reassigned sex and the reassigned sex must be legally recognized. The IOC policy is not binding for other organizations.

Keywords: sports, sex difference, gender, testosterone, muscle

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