A large scale numerical study was conducted in collaboration with the Center for Devices and Radiological Health at the Food and Drug Administration (FDA), Washington DC. The Specific Anthropomorphic Mannequin (SAM), which is used for the compliance testing of mobile phones, was compared to anatomically correct head models of adults and children. Since several previous studies on the same topic concluded with conflicting results, a total of 14 anatomical head models have been considered in the study. The results clearly show that SAM gives a conservative estimate of the exposure in anatomically correct head models for head only tissue. The variations observed for the different heads depend on individual anatomical features. A correlation between head size and/or SAR could not be observed.