  A multipole spin-down equation based on a monopolar term is derived from the
general expression $\dot{\nu} = -f(\nu,t)$ and used to study pulsar evolution.
We show that the time-independent version of such equation cannot reproduce the
observed properties of pulsars and conclude that there is no equation of the
form $\dot{\nu} = -f(\nu)$ consistent with the \pp diagram and braking index
measurements. We explore the time-dependent model under the hypothesis of
decaying magnetic fields, showing that an inverse linear decay gives reasonable
evolutionary trajectories. This model distinguishes the evolution of Vela from
that of the other three young pulsars considered. We discuss the origin of the
monopolar term, which cannot be attributed to radiative processes, pointing to
the importance of particle acceleration and/or mass loss processes in the
dynamical evolution of pulsars.
