Obsessive-compulsive disorder (OCD) and diabetes mellitus type 2 (DM2) show compulsive behaviour1 and share genetic vulnerability2. Using Diffusion Tensor Imaging as a translational approach, we investigated differences in corpus callosum (CC) body white matter microstructure in a paediatric human OCD cohort and juvenile animal models for OCD and DM2. In all three groups, fractional anisotropy increased in the CC body compared to controls, which correlated with increasing compulsive behaviour. This was coupled with a decrease in CC mean diffusivity in the animal models. Our results underline the importance of compulsive behaviour as a possible trans-diagnostic trait across OCD and DM2.