  The shear-current effect (SCE) of mean-field dynamo theory refers to the
combination of a shear flow and a turbulent coefficient $\beta_{21}$ with a
favorable negative sign for exponential mean-field growth, rather than positive
for diffusion. There have been long standing disagreements among theoretical
calculations and comparisons of theory with numerical experiments as to the
sign of kinetic ($\beta^u_{21}$) and magnetic ($\beta^b_{21}$) contributions.
To resolve these discrepancies, we combine an analytical approach with
simulations, and show that unlike $\beta^b_{21}$, the kinetic SCE
$\beta^u_{21}$ has a strong dependence on the kinetic energy spectral index and
can transit from positive to negative values at $\mathcal{O}(10)$ Reynolds
numbers if the spectrum is not too steep. Conversely, $\beta^b_{21}$ is always
negative regardless of the spectral index and Reynolds numbers. For very steep
energy spectra, the positive $\beta^u_{21}$ can dominate even at energy
equipartition $u_\text{rms}\simeq b_\text{rms}$, resulting in a positive total
$\beta_{21}$ even though $\beta^b_{21}<0$. Our findings bridge the gap between
the seemingly contradictory results from the second-order-correlation
approximation (SOCA) versus the spectral-$\tau$ closure (STC), for which
opposite signs for $\beta^u_{21}$ have been reported, with the same sign for
$\beta^b_{21}<0$. The results also offer an explanation for the simulations
that find $\beta^u_{21}>0$ and an inconclusive overall sign of $\beta_{21}$ for
$\mathcal{O}(10)$ Reynolds numbers. The transient behavior of $\beta^u_{21}$ is
demonstrated using the kinematic test-field method. We compute dynamo growth
rates for cases with or without rotation, and discuss opportunities for further
work.
