For example, chimpanzees, which are continuous breeders, show a significant increase in testosterone levels and virulent male interactions when there are recipe and fertile females. [91] At present, no research has specified a link between the modified challenge hypothesis and human behavior or the human nature of hidden ovulation, although some believe this may be the case. [88] A new proposal combines military experience with anger and aggression, develops aggressive reactions, and studies these effects on those who possess the qualities of a serial killer. Castle and Hensley say, “The army provides the social context in which soldiers learn about aggression, violence, and murder.” [162] Post-traumatic stress disorder (PTSD) is also a serious problem in the military, which is thought to sometimes lead to assaults on soldiers who suffer from what they saw in combat. They return to the civilian world and can still be haunted by flashbacks and nightmares that cause significant stress. In addition, it has been claimed that in the rare minority, which is claimed to be prone to serial murder, violent impulses during the war can be strengthened and refined, which could create more effective assassins. [Citation needed] a great encounter, argument or fight that ends up settling a disagreement between people or proves who is the best A number of classifications and dimensions of aggression have been suggested. [59] It was also found that men tend to use unintended assaults more frequently than women.