Family Composition in Psychosis Risk and Belief Implausibility Judgements: A Systematic Review and Empirical Investigation

Delusional beliefs are sometimes experienced by people with a diagnosis of psychosis or schizophrenia. These beliefs are typically unshared and often persist despite strong evidence against them. Understanding what increases psychosis risk, and how it affects thinking, is important for improving care.

Previous research shows family units and home relationships can shape personality, brain development, and relationships. Sibling number and birth order have also been linked to personality traits, yet little recent research has examined how family units affect psychosis risk. This study conducted a systematic literature review to explore this. The review searched three databases, identifying 31 papers on family units and schizophrenia. A quality assessment revealed some limitations across these studies. Despite this, the review found cultural differences: people are more likely to develop schizophrenia if they are the youngest sibling in Western countries, or eldest in Eastern countries. More siblings, particularly close in age, and the mother's physical health during pregnancy may also increase risk.

The second part explored new territory: whether people experiencing delusional beliefs judge others' beliefs as true or false differently depending on whether a stranger or trusted person shares them. It also examined conspiracy mentality, the worldview that powerful groups secretly orchestrate world events, and whether this relates to difficulty judging plausibility. Thirty-three participants were recruited via social media and mental health support groups, completing questionnaires on demographics, mental health, conspiracy mentality, and belief judgement. Findings showed people experiencing delusional beliefs were more likely to rate threatening beliefs as true than the general population, regardless of who presented the belief.

Both parts have clinical relevance: discussing family units during assessment, considering cultural differences in sibling roles, and exploring how people judge belief truth. Therapy should focus on people's relationship with beliefs rather than their origin. Further cross-cultural research is needed to improve generalisability.