The professional standards of industrial-organizational psychologists (I-O psychologists) require that any selection system be based on a job analysis to ensure that the selection criteria are job-related. The requirements for a selection system are characteristics known as KSAOs – knowledge, skills, ability, and other characteristics. US law also recognizes "bona fide occupational qualifications" (BFOQs), which are requirements for a job which would be considered discriminatory if not necessary – such as only employing men as wardens of maximum-security male prisons, enforcing a mandatory retirement age for airline pilots, a religious college only employing professors of its religion to teach its theology, or a modeling agency only hiring women to model women's clothing.