It is well-known in the combustion community that curvature effect in general slows down
ame propagation speeds because it smooths out wrinkled ames. However, such a folklore has never been justied
rigorously. In this paper, as the rst theoretical result in this direction, we prove that the turbulent
ame speed (an eective burning velocity) is decreasing with respect to the curvature diusivity (Markstein
number) for shear flows in the well-known G-equation model. Our proof involves several novel and rather sophisticated inequalities
arising from the nonlinear structure of the equation. On a related fundamental issue, we solve the selection problem of weak solutions or nd
the "physical fluctuations" when the Markstein number goes to zero and solutions approach those of the inviscid G-equation model. The
limiting solution is given by a closed form analytical formula.