Diagnostic and Statistical Manual of Mental Disorders , Fifth Edition ( DSM - V ) ( or , prior to 2014 , the Fourth Edition [ DSM - IV ] ) . 38 C . F . R . § 4 . 125 ( 2015 ) . What constitutes credible supporting evidence that a claimed in - service stressor occurred depends on the nature of the claimed stressor . If the evidence establishes that the veteran engaged in combat with the enemy and the claimed stressor is related to that combat , in the absence of clear and convincing evidence to the contrary , and provided that the claimed stressor is consistent with the circumstances , conditions , or hardships of the veteran ' s service , the