Serotonin and vitamin D have been proposed to play a role in autism, however, no causal mechanism has been established. Now, researchers show that serotonin, oxytocin, and vasopressin, three brain hormones that affect social behavior related to autism, are all activated by vitamin D hormone. Supplementation with vitamin D and tryptophan would be a practical and affordable solution to help prevent autism and possibly ameliorate some symptoms of the disorder.