The "military theory" has been proposed as an explanation for why serial murderers kill, as some serial murderers have served in the military or related fields. According to Castle and Hensley, 7% of the serial killers studied had military experience. This figure may be a proportional under-representation when compared to the number of military veterans in a nation's total population. For example, according to the United States census for the year 2000, military veterans comprised 12.7% of the U.S. population; in England, it was estimated in 2007 that military veterans comprised 9.1% of the population. Though by contrast, about 2.5% of the population of Canada in 2006 consisted of military veterans.