  We report on the long-term evolution of the spin period of the symbiotic
X-ray pulsar GX 1+4 and a possible interpretation within a model of
quasi-spherical accretion. New period measurements from BeppoSAX, INTEGRAL and
Fermi observations have been combined with previously published data from four
decades of observations. During the 1970s GX 1+4 was spinning up with the
fastest rate among the known X-ray pulsars at the time. In the mid 1980s it
underwent a change during a period of low X-ray ux and started to spin down
with a rate similar in magnitude to the previous spin up rate. The spin period
has changed from ~110 s to ~160 s within the last three decades. Our results
demonstrate that the overall spin down trend continues and is stronger than
ever. We compare the observations with predictions from a model assuming
quasi-spherical accretion from the slow wind of the M giant companion.
