Studies suggest that non-genetic, environmental, infectious, or psychosocial factors—while not causing Tourette's—can influence the severity of the disorder.[5] Twin studies have shown that the twin with lower birth weight is more likely to have more noticeable symptoms.[22] Other perinatal events, such as maternal stress,[23] maternal smoking,[24] and obstetric complications,[25] while not causing tics, may be risk factors for increased severity of tics. Autoimmune processes may affect tic severity in some cases: the unproven and contentious hypothesis that Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal infections, PANDAS, plays a role in the onset of tic disorders and OCD is a current focus of research.[26][27]