  Gravitational instability plays a substantial role in the evolution of
galaxies. Various schemes to include it in galaxy evolution models exist,
generally assuming that the Toomre $Q$ parameter is self-regulated to
$Q_\mathrm{crit}$, the critical $Q$ dividing stable from unstable conditions in
a linear stability analysis. This assumption is in tension with observational
estimates of $Q$ that find values far below any plausible value of
$Q_\mathrm{crit}$. While the observations are subject to some uncertainty, this
tension can more easily be relieved on the theoretical side by relaxing the
common assumption that $Q\ge Q_\mathrm{crit}$. Based on observations of both
$z\sim 2$ disks and local face-on galaxies, we estimate the effect of
gravitational instability necessary to balance out every other physical process
that affects $Q$. In particular we find that the disk's response to low $Q$
values can be described by simple functions that depend only on $Q$. These
response functions allow galaxies to maintain $Q$ values below
$Q_\mathrm{crit}$ in equilibrium over a wide range of parameters. Extremely low
values of $Q$ are predicted when the gas surface density is greater than $\sim
10^3$ M$_\odot$ pc$^{-2}$, the rotation curve provides minimal shear, the
orbital time becomes long, and/or when the gas is much more unstable than the
stellar component. We suggest that these response functions should be used in
place of the $Q\ge Q_\mathrm{crit}$ ansatz.
