  Using numerical simulations of forced turbulence, it is shown that for
magnetic Reynolds numbers larger than unity, i.e., beyond the regime of
quasilinear theory, the turbulent magnetic diffusivity attains an additional
negative contribution that is quadratic in the kinetic helicity. In particular,
for large magnetic Reynolds numbers, the turbulent magnetic diffusivity without
helicity is about twice the value with helicity. Such a contribution was not
previously anticipated, but, as we discuss, it turns out to be important when
accurate estimates of the turbulent magnetic diffusivity are needed.
