Twin study supports psychosis continuum

A study of adolescent twins suggests that similar factors influence psychotic experiences regardless of their severity, consistent with a psychosis continuum.

“A recognized challenge is to identify individuals at high risk of developing psychotic disorders before disease onset”, say researcher Angelica Ronald (Birkbeck College, London, UK) and colleagues.

“To the extent that severe, frequent [psychotic experiences] are indicators of risk of psychosis, these findings reveal their etiological architecture and can be used to guide investigation of molecular genetic and environmental risk factors.”

The study showed that genetic factors, as well as shared and nonshared environmental factors, play a role in psychotic experiences, but the extent of influence varies according to the specific type of experience. The team assessed six types of psychotic experience plus parent-reported negative symptoms in 4743 twin pairs, using the Specific Psychotic Experiences Questionnaire (SPEQ).

Shared environmental factors played a role only in hallucinations (17% of liability in girls and 20% in boys) and in negative symptoms (24%). However, nonshared environmental factors had a far larger influence, with liability estimates ranging from 17% for negative symptoms to 64% for hallucinations in boys.

“This is consistent with previous research, which has shown a number of environmental risk factors for psychosis that may be specific to the individual, such as stressful life events, cannabis use, and childhood trauma”, write the researchers in JAMA Psychiatry.

They say their findings suggest “that environmental influences, particularly nonshared environment, play an important role and appear to have a more prominent role than suggested from twin studies of the liability of schizophrenia.”

Correlation between psychotic experiences within the twin pairs was stronger for monozygotic than dizygotic twins, indicating additive genetic effects. Liability estimates for these effects ranged from 15% for hallucinations in boys to 59% for negative symptoms.

The team then analysed the aetiology of the most extreme psychotic experiences, defined as the most extreme 5%, 10% or 15% of scores, with the prevalence of 5% cutoff being similar to that of the at-risk mental state. The results of this analysis did not markedly differ from the main analysis, with, for example, shared environmental factors contributing only to hallucinations and negative symptoms. Heritability was similar across the three extreme groups and the main analysis.

These results “support the hypothesis that the same genes that influence symptoms within psychotic disorders also influence variation in [psychotic experiences] in the general population”, say the researchers.

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