A method of detecting abnormal serotonergic function in an impulsive human subject is disclosed. The method comprises detecting an L allele of a gene encoding tryptophan hydroxylase. The invention also includes an isolated nucleic acid having a sequence specific to an L allele of a gene encoding tryptophan hydroxylase. The nucleic acid may migrate in a denaturing gel at the same rate as a second nucleic acid specific to a corresponding region of a U allele of a gene encoding tryptophan hydroxylase, and migrate in a nondenaturing gel at a rate about 1.02 times the migration rate of the second nucleic acid.