BACKGROUND: The association between attachment style and psychotic symptomatology is well established. However, little is known about the mechanisms underlying this relationship. In the present study, we aimed to investigate whether the sensitivity of the behavioural inhibition and activation system mediates the association between attachment style and positive or negative symptoms, in patients with psychotic disorders and unaffected siblings. METHODS: In a subsample of the Genetic Risk and Outcome of Psychosis (GROUP) study (87 patients and 94 siblings), attachment style was assessed using the Psychotic Attachment Measure, behavioural inhibition or activation with the Behavioral Inhibition System (BIS) and Behavioral Activation System (BAS) Scale and positive and negative symptomatology with the Community Assessment of Psychic Experiences. Linear regression models and mediation models were used, while correcting for age and gender and adjusting for multiple testing. RESULTS: We found that anxious and avoidant attachment were associated with negative symptoms across both groups. Additionally, in siblings, anxious and avoidant attachment were associated with positive symptoms. In the patient group, BIS sensitivity mediated the association between anxious attachment and negative symptoms. In the sibling group, BIS sensitivity mediated the association between avoidant attachment and both positive and negative symptoms, and between anxious attachment and positive symptoms. CONCLUSIONS: Findings indicate that BIS sensitivity, but not BAS sensitivity, is a possible mediating mechanism contributing to the association between insecure attachment and psychotic symptoms. Longitudinal studies are required to confirm the associations between attachment style, BIS and BAS sensitivity, and psychotic symptoms, and explore causality.