  Dynamical Chern-Simons gravity (dCS) is a four-dimensional parity-violating
extension of general relativity. Current models predict the effect of this
extension to be negligible due to large decay constants $f$ close to the scale
of grand unified theories. Here, we present a construction of dCS allowing for
much smaller decay constants, ranging from sub-eV to Planck scales.
Specifically, we show that if there exists a fermion species with strong
self-interactions, the short-wavelength fermion modes form a bound state. This
bound state can then undergo dynamical symmetry breaking and the resulting
pseudoscalar develops Yukawa interactions with the remaining long-wavelength
fermion modes. Due to this new interaction, loop corrections with gravitons
then realize a linear coupling between the pseudoscalar and the gravitational
Chern-Simons term. The strength of this coupling is set by the Yukawa coupling
constant divided by the fermion mass. Therefore, since self-interacting
fermions with small masses are ideal, we identify neutrinos as promising
candidates. For example, if a neutrino has a mass $m_\nu \lesssim {\rm meV}$
and the Yukawa coupling is order unity, the dCS decay constant can be as small
as $f \sim 10^3 m_\nu \lesssim {\rm eV}$. We discuss other potential choices
for fermions.
