Normal course of cortical maturation has been evidenced for many years through gray matter volume measurements and abnormal gray matter volumes have frequently been reported as a part of a pathological brain development in neurogenetic or psychiatric disorders. The pathogenic processes underlying abnormal gray matter volume are not yet well understood, as either altered cortical thickness or a defect in cortical expansion may decrease cortical volume. Resolving this issue may provide clues for localizing the timing of the alteration. Indeed, expansion of the cortical surface is allowed by both increase in brain perimeter and folding of the cortical sheet (gyrification) during early brain development. Conversely, alteration of cortical thickness may occur at different developmental stage. Here, we aimed to identify the respective contribution of these processes to cortical volume in the parietal lobe, in a cohort of typically developing young individuals compared to patients affected by 22q11 Deletion Syndrome (22q11DS). 22q11DS is a common neurogenetic condition in which parietal gray matter (Eliez, 2000) and gyrification (Schaer, in press) are specifically altered early in development.