In studies that assess perceptions of autistic people by non-autistic people, researchers often ask participants to review vignettes depicting fictional autistic characters. However, few studies have investigated whether non-autistic peers accurately identify these hypothetical individuals as being on the autism spectrum. Accurately ascribing autism as a cause of depicted behaviors likely influences perceptions about autistic peers. In this study, 469 college students (M[subscript age] = 18.62; 79.3% female) ascribed cause(s) of an autistic peers' behaviors as depicted in a written vignette. We reviewed and categorized open-ended responses into 16 categories. Non-autistic college students primarily attributed an autistic vignette character's behavior to non-autistic origins. The most commonly ascribed causes of behavior were: attention-deficit/hyperactivity disorder (55.4%), inattention symptoms (20.9%), autism (12.8%), generalized anxiety disorder (11.7%), hyperactivity (11.3%), an unspecified diagnosis (10.7%), an environmental influence (9.6), anxiety or insecurity (8.3%), irritability or anger or annoyance (6.0%), social anxiety disorder (5.3%), and learning disorder (5.1%). Additional ascribed causes include other mental health diagnoses; environmental stressors; and cognitive, emotional, behavioral, biological, or personality characteristics/etiologies. Non-autistic young adults may not always recognize their autistic peers as autistic, which may affect acceptance and inclusion. Future anti-stigma interventions should assess the impact of helping non-autistic peers to accurately identify and better understand behaviors associated with autism. Additionally, autism-focused researchers using vignettes should assess participants' awareness of the character as autistic and interpret their findings with this in mind.