  In this letter we discuss two possible reasons which cause the observed
braking indices n of young radio pulsars to be smaller than 3: (a) the evolving
spin-down model of the magnetic field component $B_{\perp}$ increases with
time; (b) the extrinsic braking torque model in which the tidal torques exerted
on the pulsar by the fallback disk, and carries away the spin angular momentum
from the pulsar. Based on some simple assumptions, we derive the expression of
the braking indices, and calculate the spin-down evolutionary tracks of pulsars
for different input parameters.
