Scientists believe that females in opposite-sex twins are exposed to higher than normal levels of testosterone in the womb, which could affect their behavior.
Low levels of testosterone can make men lethargic, affect their libido and cause erectile dysfunction.
More than 400 couples will be involved in the trial, expected to take place in several different countries.
'We think this is one of the biggest survival benefits ever reported in a trial of an adult solid tumor,' said one expert.
Talented distance runners could possess evolutionary 'signal' from the days of hunter gatherers.
As all men age, their testosterone production lowers. So why do we blame A-Rod for seeking treatment?