Although spiral structure in grand design galaxies is generally present in the young and old stellar components and also in both atomic and molecular gas, the issue of whether the spiral structure is merely "organized" by a density wave, or whether star formation is triggered preferentially in the arms is still an open question. Here we present evidence which appears to favor triggering, or more correctly nonlinear dependence of star formation efficiency on the underlying gas density. Starting with a detailed Hα map of NGC 3992, a well-ordered four-armed spiral, of type 9 on Elmegreen's scale (where 1 is flocculent and 12 highly ordered), we define an efficiency index for massive star formation based on the ratio of the density of ionizing photons from these stars to the underlying H I density. We show that the arm/interarm ratio of this index, on length scales of 1 kpc, which would take values close to unity if star formation rate depend linearly on the H I density, rises to values of well over 10, peaking at ˜25, which each arm showing comparable behavior. There is, in addition, a striking dip to unity in the ratio, for all arms, at a common galactocentric radius, which we suggest is that of corotation. The arm class 9 galaxy NGC 628 shows highly comparable behavior. Given that star formation rates almost certainly depend on H2 and not H I densities, that recent evidence for M51 (Vogel et al.) shows strong H2 }clumping in the arms, but that average arm/interarm H2 densities are generally not greater than ˜3, on scales of t kpc, and shown no significant changes at corotation, the present evidence points to a nonlinear star formation rate enhancement mechanism, functioning via preferential formation of dense H2 clouds in arms, at sites other than the corotation and the Lindblad resonance radii. The observed absence of high arm/interarm ratios of the efficiency index in the flocculent galaxy M33 tends to confirm this reasoning.