Elevated blood serotonin may be inherited. It occurs in 30-40% of individuals with Autistic Disorder. It is also present in unaffected first and second-degree relatives [Abramson, 1989, Cook, 1992, Kuperman, 1985, Perry, 1991]
In the general population, patients with Depression have been found to have significantly lower blood serotonin (5HT) levels [Banki, 1978, Aberg-Wistedt. 1981]