The developmental aetiology of autism spectrum disorders (ASDs) is poorly understood — partly because of the difficulty in accurately modelling human cortical development. Induced pluripotent stem cells (iPSCs) derived from fibroblasts taken from individuals with severe idiopathic ASDs were cultured as neural cell organoids. Although the genomic alterations varied between individuals with ASDs, organoids showed increased expression of proteins involved in cell proliferation and differentiation and a FOXG1-dependent increase in GABAergic neurons compared with control iPSC-derived neural cells. These findings suggest that the genomic differences among individuals with ASDs converge on common neurodevelopmental mechanisms.