Kelleher et al ^{Reference Kelleher, Jenner and Cannon1} note the significant prevalence of non-clinical psychotic symptoms in the general population and discuss some hypotheses regarding its evolutionary survival. One theory not mentioned by them or those who have so far responded is a trait known as schizotypy. While accepting that to some degree the whole topic is rich with speculation, I suggest that schizotypy may be the missing piece in the puzzle. What follows is necessarily a brief summary of some of the relevant literature.

Differing from both schizotypal and schizoid personality disorders, schizotypy ^{Reference Claridge2} is a heritable trait associated with an increased likelihood of creativity and of religious or mystical experiences. Importantly for this discussion, schizotypy also appears to be necessary but not sufficient for the development of schizophrenia, although not all those with schizotypy develop psychotic illnesses.

The four key dimensions of schizotypy are unusual experiences (which may be considered to be related to positive symptoms), cognitive disorganisation (related to thought disorder), introverted anhedonia (related to social withdrawal and depression) and impulsive non-conformity. This last is related to some of the disturbed behaviour, such as aggression and self-harm, seen in a range of psychiatric illnesses, including psychosis.

Regarding creativity, additional research by Nettle ^{Reference Nettle3} suggests that different dimensions of schizotypy are associated with different types of creativity. Nettle & Clegg further find that schizotypy is associated with increased ‘evolutionary fitness’ due to a greater number of sexual partners (and therefore offspring) in those with the unusual experience and impulsive non-conformity dimensions of the trait. ^{Reference Nettle and Clegg4} In those with the former but not the latter dimension, the relationship with mating success is mediated by creativity. Nettle & Clegg have proposed that schizotypal traits, which in this case may be a proxy for some non-clinical psychotic symptoms, have therefore persisted because their potential negative effects are offset by enhanced mating success.

Regardless of the outcome of the search to understand the persistence of psychotic symptoms in human beings and of possible future research involving those who have the non-clinical psychosis phenotype, it is important for people working in mental health services to remember that not all those they encounter with symptoms are ill. For those that are unwell, there will be other aspects of their existence that are positive and that may be life-enhancing for them and those around them. They should be encouraged to develop these aspects of themselves as part of their long-term recovery, in addition to the treatment and support they receive from health services, carers and friends.