  The recently discovered rotationally powered pulsar PSR J1640-4631 is the
first to have a braking index measured, with high enough precision, that is
greater than three. An inclined magnetic rotator in vacuum or plasma would be
subject not only to spin-down but also to an alignment torque. The vacuum model
can address the braking index only for an almost orthogonal rotator that is
incompatible with the single peaked pulse profile. The magnetic dipole model
with the corotating plasma predicts braking indices between $3-3.25$. We find
that the braking index of $3.15$ is consistent with two different inclination
angles, $18.5\pm 3$ degrees and $56 \pm 4$ degrees. The smaller angle is
preferred given the pulse profile has a single peak and the radio output of the
source is weak. We infer the change in the inclination angle to be at the rate
$-0.23$ degrees per century, three times smaller in absolute value than the
rate recently observed for the Crab pulsar.
