What can break the Wandzura-Wilczek relation?

We analyze the breaking of the Wandzura-Wilczek relation for the g(2) structure function, emphasizing its connection with transverse momentum dependent parton distribution functions. We find that the relation is broken by two distinct twist-3 terms, and clarify how these can be separated in measurements of double-spin asymmetries in semi-inclusive deep inelastic scattering. Through a quantitative analysis of available g(2) data we also show that the breaking of the Wandzura-Wilczek relation can be as large as 15-40% of the size of g(2).