  We study the effect that uncertainties in the nuclear spin-dependent
structure functions have in the determination of the dark matter (DM)
parameters in a direct detection experiment. We show that different nuclear
models that describe the spin-dependent structure function of specific target
nuclei can lead to variations in the reconstructed values of the DM mass and
scattering cross-section. We propose a parametrization of the spin structure
functions that allows us to treat these uncertainties as variations of three
parameters, with a central value and deviation that depend on the specific
nucleus. The method is illustrated for germanium and xenon detectors with an
exposure of 300 kg yr, assuming a hypothetical detection of DM and studying a
series of benchmark points for the DM properties. We find that the effect of
these uncertainties can be similar in amplitude to that of astrophysical
uncertainties, especially in those cases where the spin-dependent contribution
to the elastic scattering cross-section is sizable.
