  We use a compilation of disc galaxy rotation curves to assess the role of the
luminous component ("baryons") in the rotation curve diversity problem. As in
earlier work, we find that rotation curve shape correlates with baryonic
surface density: high surface density galaxies have rapidly-rising rotation
curves consistent with cuspy cold dark matter halos; slowly-rising rotation
curves (characteristic of galaxies with inner mass deficits or "cores") occur
only in low surface density galaxies. The correlation, however, seems too weak
to be the main driver of the diversity. In addition, dwarf galaxies exhibit a
clear trend, from "cuspy" systems where baryons are unimportant in the inner
mass budget to "cored" galaxies where baryons actually dominate. This trend
constrains the various scenarios proposed to explain the diversity, such as (i)
baryonic inflows and outflows during galaxy formation; (ii) dark matter
self-interactions; (iii) variations in the baryonic mass structure coupled to
rotation velocities through the "mass discrepancy-acceleration relation"
(MDAR); or (iv) non-circular motions in gaseous discs. Together with analytical
modeling and cosmological hydrodynamical simulations, our analysis shows that
each of these scenarios has promising features, but none seems to fully account
for the observed diversity. The MDAR, in particular, is inconsistent with the
observed trend between rotation curve shape and baryonic importance; either the
trend is caused by systematic errors in the data or the MDAR does not apply.
The origin of the dwarf galaxy rotation curve diversity and its relation to the
structure of cold dark matter halos remains an open issue.
