Based on the multi-band multi-shell diffusion MRI data acquisition and fixel-based analysis, the purpose of our study is to provide more insights into neurobiological accounts of autism, inclusive of understudied low functioning individuals. Preprocessed DWI data of 38 TD and 65 autistic participants were analysed. We found reduced fibre density in the splenium of the corpus callosum in autisitic individuals. This autism-associated alteration appears to be mainly driven by those autistic individuals with developmental disabilities. Our findings suggest studies inclusive of autistic people comorbid with developmental disabilities using the novel models may bridge some heterogeneity noted in the literature.