Suicidal behavior, like so much else in psychiatry, tends to cluster in families. Clinical studies show that a family history of suicide is associated with a raised risk of both attempts at suicide and completed suicide. Twin studies show that monozygotic twins have a greater concordance for suicidal behavior than dizygotic twins. Adoption studies also suggest that there may be genetic factors in suicide. Most recently, molecular genetic studies report that polymorphisms of the tryptophan hydroxylase gene are associated with suicidal behavior.