ADHD is the most common childhood psychiatric disorder. Scientists do not know what causes it, but genetics play a clear role. Other risk factors have also been identified, including premature birth, low birth weight, a mother's use of alcohol or tobacco during pregnancy, and environmental exposures to toxins like lead. Regions with high sunlight intensity have a lower prevalence of ADHD, suggesting that high sunlight intensity may exert a 'protective' effect for ADHD.