Genetics: Genetics play a strong role in the cause of OCD. First-degree relatives of individuals with OCD are much more likely to have OCD than the general population, especially in the case of child-onset OCD. There is also a strong genetic link between individuals with eating disorders and those with OCD. One theory of why these genes survived evolutionarily is that consistent checking made our ancestors environments safer. Environment: Individuals with extreme childhood stress (such as physical abuse, bullying, or the loss of a loved one) are more likely to develop OCD. Stressful situations in adult life can also trigger new-onset OCD. Bacterial infection: Infection with Group A Strep can cause a disorder called Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal Infections, or PANDAS. This disorder can cause rapid-onset OCD or Tourette’s. It is hypothesized that this happens due to the autoimmune response toward the Strep bacteria. Neurotransmitters: OCD is often treated with medications that increase serotonin. The reasoning is that OCD can be caused by a lack of serotonin in the brain. Lack of serotonin is a common culprit in many mental disorders, including anxiety and depression, though the exact connection between serotonin and mental illness is not fully known.