The Americans with Disabilities Act (ADA) is a law passed in 1990 that prohibits discrimination against people with disabilities in employment, transportation, public accommodation, communications, and governmental activities. The ADA pertains to a wide range of disabilities, including both physical and mental conditions.
The federal agency tasked with enforcing the ADA within the context of employment law is the Equal Employment Opportunity Commission (EEOC). In the simplest terms, the ADA prohibits employers from discriminating against job candidates on the basis of their disabilities when making hiring decisions. Employers may not use hiring criteria that seek information about an applicant's mental or physical health, as these sorts of criteria may be construed as a "medical exam." These regulations apply to every type of hiring criteria, including background checks, interviews, and pre-employment tests.
Most of the more common types of commercially prominent employment tests, including aptitude, personality, and skills tests, do not measure mental health or medical conditions of any kind, and therefore would be unlikely to violate the ADA. In general, the ADA prohibits employers from inquiring about an applicant's mental or physical health unless doing so is job-related and consistent with business necessity. In other words, employers may evaluate a candidate's mental or physical health if it is relevant to the successful performance of that job.
For instance, law enforcement officers are frequently required to take mental health assessments as part of the hiring process because of the high-stress nature of their work. Similarly, in positions as varied as machine operators and firefighters, applicants might be asked to pass a physical test, such as the ability to carry a certain amount of weight, because doing so is a job requirement. In general, however, tests that assess mental or physical health should not be given prior to an offer of employment, as they may run afoul of the ADA.