Physiology of an individual.
Almost all men age 18-25 who are U.S. citizens or are immigrants living in the U.S. are required to be registered with Selective Service. U.S. law calls for citizens to register within 30 days of turning 18 and immigrants to register within 30 days of arriving in the U.S.
That is true, think about actions and consequences; everyone has the free will to choose to do what is right.