  Within the QCD-improved parton model and assuming the factorization theorem
to hold in the helicity basis and for higher twist contributions, we show how
non zero single spin asymmetries in hadron-hadron high energy and moderately
large $p_T$ inclusive processes can be obtained, even in massless perturbative
QCD, 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 asymmetry observed in inclusive pion production in $p\,p$ collisions.
