  The star formation activity of the host galaxies of active galactic nuclei
(AGNs) provides valuable insights into the complex interconnections between
black hole growth and galaxy evolution. A major obstacle arises from the
difficulty of estimating accurate star formation rates in the presence of a
strong AGN. Analyzing the $1-500\, \mu m$ spectral energy distributions and
high-resolution mid-infrared spectra of low-redshift ($z < 0.5$) Palomar-Green
quasars with bolometric luminosity $\sim 10^{44.5}-10^{47.5}\rm\,erg\,s^{-1}$,
we find, from comparison with an independent star formation rate indicator
based on [Ne II] 12.81$\, \mu m$ and [Ne III] 15.56$\, \mu m$, that the
torus-subtracted, total infrared ($8-1000\, \mu m$) emission yields robust star
formation rates in the range $\sim 1-250\,M_\odot\,{\rm yr^{-1}}$. Combined
with available stellar mass estimates, the vast majority ($\sim 75\%-90\%$) of
the quasars lie on or above the main sequence of local star-forming galaxies,
including a significant fraction ($\sim 50\%-70\%$) that would qualify as
starburst systems. This is further supported by the high star formation
efficiencies derived from the gas content inferred from the dust masses.
Inspection of high-resolution Hubble Space Telescope images reveals a wide
diversity of morphological types, including a number of starbursting hosts that
have not experienced significant recent dynamical perturbations. The origin of
the high star formation efficiency is unknown.
