  The levels of heavy elements in stars are the product of enhancement by
previous stellar generations, and the distribution of this metallicity among
the population contains clues to the process by which a galaxy formed. Most
famously, the "G-dwarf problem" highlighted the small number of low-metallicity
G-dwarf stars in the Milky Way, which is inconsistent with the simplest picture
of a galaxy formed from a "closed box" of gas. It can be resolved by treating
the Galaxy as an open system that accretes gas throughout its life. This
observation has classically only been made in the Milky Way, but the
availability of high-quality spectral data from SDSS-IV MaNGA and the
development of new analysis techniques mean that we can now make equivalent
measurements for a large sample of spiral galaxies. Our analysis shows that
high-mass spirals generically show a similar deficit of low-metallicity stars,
implying that the Milky Way's history of gas accretion is common. By contrast,
low-mass spirals show little sign of a G-dwarf problem, presenting the
metallicity distribution that would be expected if such systems evolved as
pretty much closed boxes. This distinction can be understood from the differing
timescales for star formation in galaxies of differing masses.
