  We show how to avoid unnecessary and uncontrolled assumptions usually made in
the literature about soft SU(3) flavor symmetry breaking in determining the
two-flavor nucleon matrix elements relevant for direct detection of WIMPs.
Based on SU(2) Chiral Perturbation Theory, we provide expressions for the
proton and neutron scalar couplings $f_u^{p,n}$ and $f_d^{p,n}$ with the
pion-nucleon sigma-term as the only free parameter, which should be used in the
analysis of direct detection experiments. This approach for the first time
allows for an accurate assessment of hadronic uncertainties in spin-independent
WIMP-nucleon scattering and for a reliable calculation of isospin-violating
effects. We find that the traditional determinations of $f_u^p-f_u^n$ and
$f_d^p-f_d^n$ are off by a factor of 2.
