1Department of Human Biology, University of Cape Town, Cape Town, South Africa, 2Department of Statistical Sciences, University of Cape Town, Cape Town, South Africa, 3Department of Paediatrics & Child Health, Stellenbosch University, Stellenbosch, South Africa, 4A.A. Martinos Centre for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, 5Department of Radiology, Harvard Medical School, Boston, MA, United States