HJ Eysenck deserves primary credit for discovering and formulating the personality trait of Psychoticism (P) - but I believe the concept as he left it (an his death in 1997) requires further clarification and development.
One necessary clarification is to distinguish between Psychoticism and Psychopathy - that is, between Psychoticism-proper versus Psychopathy - I attempted that yesterday:
Another necessary clarification is to distinguish between the personality trait of Psychoticism, and the state of illness in which a person can be described as Psychotic.
I believe that Eysenck was wrong to suggest that high-Psychoticism-trait and Psychotic-illness should be seen as points of severity on a single dimension - with the personality trait as a moderately high Psychoticism, and the highest levels of the same trait being characterized by a Psychotic illness.
What follows is my attempt to distinguish clearly between Psychoticism and Psychosis; between Psychoticism-personality and Psychotic-illness (i.e. between the trait and the state) - but also to explain where there is a statistical association between the two (ie. why people high in Psychoticism-trait are more prone to Psychotic-illness).
1. Personality/ character - permanent trait, persists through life
2. Inherited (substantially), genetic
4. Adaptive (on average increases reproductive success of the group)
5. Creative - Uses inner cognitions and perceptions to address external problems
1. Illness - usually temporary state, seldom persists through life
2. Usually caused by environment - e.g. toxicity, trauma, infection
3. Reversible impairment in brain functioning
4. Maladaptive (while operative reduces individual reproductive success)
5. Non-creative - Absorbed-by (turned-in-on) inner cognitions and perceptions
Anyone can have a Psychotic-illness if the environmental cause is sufficiently strong - for example, delirium due to hallucinogenic drugs, or withdrawal of alcohol, will produce Psychosis in everyone; so will some types of dementia.
But those who are born hard-wired for high Psychoticism trait are more prone to 'flip into' the temporary state of a Psychotic-illness - it takes less than average of an environmental 'insult' or brain impairment to make a high Psychoticism person have a Psychotic episode.
This accounts for the often noticed and multiply confirmed observation of a higher frequency of Psychotic illnesses/ episodes among highly creative people (i.e. those people with high Psychoticism-trait).
Therefore, Psychosis (psychotic states) in a person with high Psychoticism (psychoticism trait) may not indicate severe brain malfunction - because a relatively mild degree of brain dysfunction may be sufficient to trigger psychotic symptoms in a person with high trait Psychoticism.
Indeed, for a person with high trait Psychoticism, temporary episodes of Psychotic states may be of some positive, functional value in terms of creativity.
This is most obvious in tribal shamans who deliberately induce Psychotic episodes in order to access non-normal sources of knowledge (conceptualized as coming from the spiritual realm). Shamans usually use sleep deprivation, fatigue, prolonged and rhythmic dancing, music, singing to trigger these Psychotic episodes - and also hallucinogenic or other consciousness-altering agents where these are available (eg. among Amerindians).
Also modern creative thinkers, artists etc. have often been known successfully (albeit hazardously) to use consciousness-altering drugs such as alcohol and opium to stimulate creative insights and states in the context of a strategic Creative Quest.
In sum, high Psychoticism people are hard-wired in such a way as to be more-then-usually prone to brief Psychotic episodes, and may also deliberately trigger such temporary states in pursuit of creativity.
In biological terms, the major difference between high Psychoticism-trait and Psychotic-illness is that high levels of Psychoticism-trait are strategically functional, group-adaptive, evolved; in a nutshell high-P individuals exist for-a-reason; and that reason is essentially to enable creativity.
By contrast, the occurrence of Psychosis caused by the variabilities of existence and strength among the specific circumstantial causes of Psychosis.
And while Psychosis is commoner and more-easily-triggered in people with high Psychoticism trait - Psychoticism and Psychosis are causally distinct; and the relationship between Psychoticism and Psychosis is indirect, multistep, accidental and contingent on circumstances.