The measurements of pulsar frequency second derivatives have shown that they are 10 ^ { 2 } -10 ^ { 6 } times larger than expected for standard pulsar spin-down law , and are even negative for about half of pulsars .
We explain these paradoxical results on the basis of the statistical analysis of the rotational parameters \nu , \dot { \nu } and \ddot { \nu } of the subset of 295 pulsars taken mostly from the ATNF database .
We have found a strong correlation between \ddot { \nu } and \dot { \nu } for both \ddot { \nu } > 0 and \ddot { \nu } < 0 , as well as between \nu and \dot { \nu } .
We interpret these dependencies as evolutionary ones due to \dot { \nu } being nearly proportional to the pulsars ’ age .
The derived statistical relations as well as “ anomalous ” values of \ddot { \nu } are well described by assuming the long-time variations of the spin-down rate .
The pulsar frequency evolution , therefore , consists of secular change of \nu _ { ev } ( t ) , \dot { \nu } _ { ev } ( t ) and \ddot { \nu } _ { ev } ( t ) according to the power law with n \approx 5 , the irregularities , observed within a timespan as a timing noise , and the variations on the timescale larger than that timespan – several tens of years .

\PACS 97.60.Jd 97.60.Gb 97.10.Kc 98.62.Ve