Psychotic Spectrum Symptoms in Adults with Autism Spectrum Disorder and in Their First-Degree Relatives

<b>Objectives</b>: Autism Spectrum Disorder (ASD) and psychotic disorders have long been considered separate diagnostic entities, yet increasing evidence highlights shared neurodevelopmental mechanisms and symptom overlap. Psychotic-like experiences have been frequently reported in individuals with ASD, while subthreshold autistic traits (ATs) in first-degree relatives may also confer vulnerability to psychotic symptoms. This cross-sectional study aimed to compare psychotic spectrum manifestations among adults with ASD, their first-degree relatives (BAP), and controls (HCs), to explore associations between psychotic and ATs, and to evaluate whether psychotic symptoms predict diagnostic group membership. <b>Methods</b>: 22 adults with ASD, 22 BAP, and 24 HCs were evaluated with the Psychotic Spectrum-Self Report (PSY-SR) and the Adult Autism Subthreshold Spectrum (AdAS Spectrum). <b>Results</b>: ASD participants scored significantly higher on the PSY-SR. BAP individuals showed higher PSY-SR total scores compared to HCs, though less severe than in ASD. All PSY-SR domains positively correlated with all AdAS Spectrum domains, with few exceptions. Multinomial regressions showed that higher PSY-SR total scores significantly predicted ASD and BAP membership, and that the PSY-SR <i>Paranoid</i> domain score specifically predicted inclusion in both groups in relation to HCs. <b>Conclusions</b>: Psychotic spectrum symptoms are elevated not only in individuals with ASD but also among first-degree relatives, supporting a continuum linking autistic and psychotic vulnerabilities. The strong association between paranoid symptoms and ATs highlights a dimension of potential clinical relevance for early identification and assessment. These findings reinforce shared neurodevelopmental pathways between the autism and psychosis spectra and underscore the importance of dimensional approaches across diagnostic categories.