  The general formalism to describe single spin asymmetries in hadron-hadron
high energy and large $p_T$ inclusive production within the QCD-improved parton
model and assuming the factorization theorem to hold for higher twist
contributions is discussed. Non zero values of the single spin asymmetries
originate from and reveal non perturbative universal properties of quarks: the
quark distribution and fragmentation analysing powers, which need not be zero,
provided the quark intrinsic motion is taken into account. A simple model is
constructed which reproduces the main features of the data on the single spin
asymmetries observed in inclusive pion production in $pp$ collisions.
