Associated to a compact Lie group G is the abelian group P(G) of total Stiefel-Whitney classes of representations. In certain cases the rank of P(G) is equal to the number of conjugacy classes of involutions in G. For the symmetric groups Sn, the total Stiefel-Whitney class of the regular representation is highly divisible in P(Sn) and this implies the existence of 'global' Dickson invariants i...