Incest may be defined as sexual relations between close blood relatives. Legally, incest and sexual aggression toward minors are classified as a criminal behaviour. Tunisia is among the countries from which incest cases are rarely reported.

The aim of this study is to investigate the relationship between the psychotic structure and incest, and to describe the individual, clinical, and criminal traits of the incestuous father through clinical observation.

Mr T.G is 46 years old. He is married and has six daughters. His wife appears to be passive, and largely dependent on her husband. Mr T.G has had incestuous relationships, initially, with his two eldest daughters. The acts were followed by the mother's complicit silence and the non-denunciation of the daughters. Two years later, he starts an incestuous behavior with his third daughter. Incest took place in the context of delusion. The patient was convinced that he is responsible of his daughters’ sexuality education. He develops an incoherent theory of purification with a tendency towards morbid rationalism. It is only after four years of insufferable paternal incestuous relationships that the third daughter filed a complaint to the police. A psychiatric expertise concluded that the accused is exempt from criminal responsibility.

Incest is a multi-faceted phenomenon, which makes its approach, comprehension, and treatment quite complex. For a psychotic patient incest is a means to deny alterity by crushing other. It also allows him to find, in this complete power, control over his annihilation anxiety.

The authors have not supplied their declaration of competing interest.