  We test stability against probabilistic evolution of sum rules for
transverse-momentum-dependent distribution and fragmentation functions. We find
that preservation of the Burkardt sum rule for Sivers distribution functions is
similar to the conservation of longitudinal momentum related to spin-averaged
parton distributions. At the same time, preservation of the Schaefer-Teryaev
sum rule for Collins functions is similar to preservation of the
Burkhardt-Cottingham sum rule for the spin-dependent g_2 structure function.
