  Recent radio pulsar observations have shown that a number of pulsars display
interesting long term periodicities in their spin-down rates. At least some of
these pulsars also undergo sharp changes in pulse profile. This has been
convincingly attributed to the stars abruptly switching between two different
magnetospheric states. The sharpness of these transitions has been taken as
evidence against free precession as the mechanism behind the long term
variations. We argue that such a conclusion is premature. By performing a
simple best-fit analysis to the data, we show that the relationship between the
observed spin and modulation periods is of approximately the correct form to be
accounted for by the free precession of a population of neutron stars with
strained crusts, the level of strain being similar in all of the stars, and
consistent with the star retaining a memory of a former faster rotation rate.
We also provide an argument as to why abrupt magnetospheric changes can occur
in precessing stars, and how such changes would serve to magnify the effect of
precession in the timing data, making the observation of the precession more
likely in those stars where such switching occurs. We describe how future
observations could further test the precession hypothesis advanced here.
