The relationship between schizoid personality disorder (SPD) and avoidant personality disorder (AvPD) has been a subject of controversy for decades. Today it is still unclear and remains to be seen if these two personality disorders are genuinely distinct, but overlapping, personality disorders, or if they are merely two different phenotypic expressions of the same underlying disorder. Both have been associated with a shared genetic risk factor and the same polymorphism within the ANKK1 gene. There is also some evidence that AvPD (like SPD) is a personality disorder of the schizophrenia spectrum.