Previous research has found multiplex (MPX) children have an advantage in cognition compared to simplex (SPX) children. However, MPX parent's previous experience with older diagnosed siblings has not been considered. We used a large database sample to investigate the MPX advantage and contribution of birth order. Children from the Autism Genetic Resource Exchange (AGRE) were stratified into first- (MPX1, n = 152) and second-affected MPX (MPX2, n = 143), SPX (n = 111), and only-child SPX (SPXOC, n = 23) groups. Both MPX groups had higher cognitive scores compared to SPX groups, with no differences between MPX1 and MPX2 groups. No differences were found for autism symptoms or adaptive behaviour. These results suggest parent experience due to birth order is an unlikely contributor to the MPX cognitive advantage.