  Observations of several protostellar jets show systematic differences in
radial velocity transverse to the jet propagation direction, which have been
interpreted as evidence of rotation in the jets. In this paper we discuss the
origin of these velocity shifts, and show that they could be originated by
rotation in the flow, or by side to side asymmetries in the shock velocity,
which could be due to asymmetries in the jet ejection velocity/density or in
the ambient medium. For typical poloidal jet velocities (~ 100-200 km/s), an
asymmetry >~ 10% can produce velocity shifts comparable to those observed. We
also present three dimensional numerical simulations of rotating, precessing
and asymmetric jets, and show that, even though for a given jet there is a
clear degeneracy between these effects, a statistical analysis of jets with
different inclination angles can help to distinguish between the alternative
origins of transverse velocity shifts. Our analysis indicate that side to side
velocities asymmetries could represent an important contribution to transverse
velocity shifts, being the most important contributor for large jet inclination
angles (with respect the the plane of the sky), and can not be neglected when
interpreting the observations.
