Fermions with no fundamental charges call for extra dimensions
  We call attention to that if assuming no conserved charges in the fundamental theory with fermions, which carry only a spin and interact with only the gravity, the dimensions $4, 12, 20,.....,$ as well as all odd dimensions, are excluded under the requirement of mass protection. If more than one family is required, than only dimensions d=2 (mod 4) remains as acceptable, since then other by 4 devisable dimensions are excluded. 
Norma Susana Mankoc Borstnik, Holger Bech Nielsen