Non-genetic, environmental, post-infectious, or psychosocial factors—while not causing Tourette's—can influence its severity.  Autoimmune processes may affect tic onset and exacerbation in some cases. In 1998, a team at the US National Institute of Mental Health proposed a hypothesis based on observation of 50 children that both obsessive–compulsive disorder (OCD) and tic disorders may arise in a subset of children as a result of a poststreptococcal autoimmune process.  Children who meet five diagnostic criteria are classified, according to the hypothesis, as having Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal infections ( PANDAS ).  This contentious hypothesis is the focus of clinical and laboratory research, but remains unproven.