  Early analyses of exoplanet statistics from the Kepler Mission revealed that
a model population of multiple-planet systems with low mutual inclinations
(${\sim1^{\circ}-2^{\circ}}$) adequately describes the multiple-transiting
systems but underpredicts the number of single-transiting systems. This
so-called "Kepler dichotomy" signals the existence of a sub-population of
multi-planet systems possessing larger mutual inclinations. However, the
details of these inclinations remain uncertain. In this work, we derive
constraints on the intrinsic mutual inclination distribution by statistically
exploiting Transit Duration Variations (TDVs) of the Kepler planet population.
When planetary orbits are mutually inclined, planet-planet interactions cause
orbital precession, which can lead to detectable long-term changes in transit
durations. These TDV signals are inclination-sensitive and have been detected
for roughly two dozen Kepler planets. We compare the properties of the Kepler
observed TDV detections to TDV detections of simulated planetary systems
constructed from two population models with differing assumptions about the
mutual inclination distribution. We find strong evidence for a continuous
distribution of relatively low mutual inclinations that is well-characterized
by a power law relationship between the median mutual inclination
($\tilde{\mu}_{i,n}$) and the intrinsic multiplicity ($n$): $\tilde{\mu}_{i,n}
= \tilde{\mu}_{i,5}(n/5)^{\alpha}$, where $\tilde{\mu}_{i,5} =
1.10^{+0.15}_{-0.11}$ and $\alpha = -1.73^{+0.09}_{-0.08}$. These results
suggest that late-stage planet assembly and possibly stellar oblateness are the
dominant physical origins for the excitation of Kepler planet mutual
inclinations.
