  Pulsars are rotating neutron stars that are seen to slow down, and the
spin-down rate is thought to be due to magnetic dipole radiation. This leads to
a prediction for the braking index n, which is a combination of spin period and
its first and second time derivatives. However, all observed values of n are
below the predicted value of 3. Here we provide a simple model that can explain
the rotational evolution of young pulsars, including the n=2.51 of the
958-year-old pulsar in the Crab nebula. The model is based on a decrease in
effective moment of inertia due to an increase in the fraction of the stellar
core that becomes superfluid as the star cools via neutrino emission. The
results suggest that future large radio monitoring campaigns of pulsars will
yield measurements of the neutron star mass, nuclear equation of state, and
superfluid properties.
