exist only in law and not in practice. Usually the requirement applies to younger male citizens, though it may extend to women and non-citizen residents as well. In times of war, the requirements, such as age, may be broadened when additional troops are thought to be needed. At different times and places, some individuals have been able to avoid conscription by having another person take their place. Modern draft laws may provide temporary or permanent exemptions from service or allow some other non-combatant service, as in the case of conscientious objectors. In the United States, males aged 18-25 are required to register with the Selective Service System, which has responsibility for overseeing the draft. However, no draft has occurred since 1973, and the U.S. military has been able to maintain staffing through voluntary enlistment. Enlistment Soldiers in war may have various motivations for voluntarily enlisting and remaining in an army or other armed forces branch. In a study of 18th century soldiers' written records about their time in service, historian Ilya Berkovich suggests "three primary 'levers' of motivation ... 'coercive', 'remunerative', and 'normative' incentives." Berkovich argues that historians' assumptions that fear of coercive force kept unwilling conscripts in check and controlled rates of desertion have been overstated and that any pay or other remuneration for service as provided then would have been an insufficient incentive. Instead, "old-regime common soldiers should be viewed primarily as willing participants who saw themselves as engaged in a distinct and honourable activity." In modern times, soldiers have volunteered for armed service, especially in time of war, out of a sense of patriotic duty to their homeland or to advance a social, political, or ideological cause, while improved levels of remuneration or training might be more of an incentive in times of economic hardship. Soldiers might also enlist for personal reasons, such as following family or social expectations, or for the order and discipline provided by military training, as well as for the friendship and connection with their fellow soldiers afforded by close contact in a common enterprise. In 2018, the RAND Corporation published the results of a study of contemporary American soldiers in Life as a Private: A Study of the Motivations and Experiences of Junior Enlisted Personnel in the U.S. Army. The study found that "soldiers join the Army for family, institutional, and occupational reasons, and many value the opportunity to become a military