The primary factor, B say the study's authors: U.S. troops are subjected to multiple tours of high-intensity guerrilla warfare and face the constant threat of death or dismemberment from improvised bombs.

这份研究的作者表示，最主要的因素在于--美军参与许多高压且紧张的游击战任务，并不断面临死亡或是被土制炸弹炸断四肢的威胁。