A hierarchical organization of instinctive actions has already been postulated by comparative behavioural research. However, these views require a corresponding hierarchical structural correlate in the brain. The field of evolutionary neuroethology tries to fathom these connections between specific behaviour and brain structure. According to Paul MacLean, the human brain is from an evolutionary point of view a “triune brain”, i.e. an evolutionary three-part neural system, which represents the phylogenetic heritage of reptiles as well as of lower- and higher-developed mammals. These associations form a unit, corresponding to a conglomerate of three brains, although they differ significantly in structure and neurochemistry, and—in evolutionary terms—are aeons apart. Although the associations are highly interconnected, there is also evidence of independent functioning. The findings of evolutionary neuroethology are also reflected in the ideas of evolutionary psychiatry, especially with regard to the genesis of psychopathological symptoms.