We perform ab initio spin-polarized density functional calculations of Fen aggregates with n <= 17 atoms to reveal the origin of the observed magic numbers, which indicate particularly high stability of clusters with 7, 13 and 15 atoms. Our results clarify the controversy regarding the ground state geometry of clusters such as Fe-5 and indicate that magnetism plays an important role in determining the stability and magic numbers in small iron clusters. (C) 2014 Elsevier B.V. All rights reserved.