'O' blood type is associated with larger grey-matter volumes in the cerebellum. 
Recent evidence indicated higher incidence of cognitive deficits in ABO blood-type system 'AB' individuals.  Since this statistical difference might originate from the lack of protective effects exerted by 'O' alleles on the brain via vascular or non-vascular routes, this study investigated volumetric differences in grey matter between 'O' and non-'O' adults to explore the possibility of a structural endophenotype visible in 'O' adults without cognitive impairment or neurodegeneration.  A large sample of cognitively healthy adults who had previously undergone structural MRI for research purposes were contacted telephonically and enquired about their ABO blood type.  Out of the 189 individuals who were able to retrieve and communicate this information, 'O' (n=76) and 'A' adults (n=65) were included in Model 1.  In Model 2, all non-'O' (n=113) were instead collapsed in a single group.  Voxel-Based Morphometry analyses were carried out on three-dimensional T1-weighted scans, and between-sample t tests were run to compare the maps of grey-matter volumes of the subgroups of interest, controlling for major nuisance variables.  In Model 1, 'O' adults had larger grey-matter volumes in two symmetrical clusters within the posterior ventral portion of the cerebellum.  This was confirmed in Model 2.  Additionally, non-'O' adults showed lower volume values in temporal and limbic regions, including the left hippocampus.  The cerebellar clusters were located in regions previously found to be part of a network responsible for sensorimotor integration.  It is speculated that the structural reductions seen in non-'O' adults might result in a susceptibility to down-regulation of this network.  This occurrence is likely to intensify along the ageing process and may contribute to foster cognitive decline.  Although Model 2 seems to suggest that having a 'O' blood type might play a role in protection against those conditions in which temporal and mediotemporal volumetric loss is observed (Alzheimer's disease), additional supporting evidence is needed.  A number of potential biological processes might sustain these between-group differences, including sensorimotor ontogenesis, hormonal function, and a regional impact of cerebral amyloid angiopathy.  These findings identify the cerebellar tissue as a candidate for further studying ABO function, and support a general association between ABO blood type and variance in the development of the nervous system.