  We use the APOSTLE and Auriga cosmological simulations to study the star
formation histories (SFHs) of field and satellite dwarf galaxies. Despite
sizeable galaxy-to-galaxy scatter, the SFHs of APOSTLE and Auriga dwarfs
exhibit robust average trends with galaxy stellar mass: faint field dwarfs
($10^5<M_{\rm star}/M_\odot<10^{6.5}$) have, on average, steadily declining
SFHs, whereas brighter dwarfs ($10^{7.5}<M_{\rm star}/M_\odot<10^{9}$) show the
opposite trend. Intermediate-mass dwarfs have roughly constant SFHs. Satellites
exhibit similar average trends, but with substantially suppressed star
formation in the most recent $\sim 5$ Gyr, likely as a result of gas loss due
to tidal and ram-pressure stripping after entering the haloes of their
primaries. These simple mass and environmental trends are in good agreement
with the derived SFHs of Local Group (LG) dwarfs whose photometry reaches the
oldest main sequence turnoff. SFHs of galaxies with less deep data show
deviations from these trends, but this may be explained, at least in part, by
the large galaxy-to-galaxy scatter, the limited sample size, and the large
uncertainties of the inferred SFHs. Confirming the predicted mass and
environmental trends will require deeper photometric data than currently
available, especially for isolated dwarfs.
