The representative isodesmic reactions shown in the Abstract graphic for (CH)(4)X(6) hexaheteroadamantane derivatives reveal energetic nonadditivity to remarkably different extents: while the electropositive element stabilizations are exceptionally large and the pnictide and hexaoxaadamantane stabilizations are more modest, the sulfur and selenium analogues are destabilized. Similar behavior is exhibited by (CH(2))(6)X(4) tetraheteroadamantanes. Analysis shows that aromaticity is not involved; the sign and magnitude of the nonadditivity depends on the interplay of hyperconjugation, electrostatic, and steric (lone pair repulsion) effects.