As already noted by Kahn & Woltjer (1959), any non-spherical mass distribution will cause a differential precession of a warp shape, and thus leads to an increasing corrugation of the outer disk. The observed statistics of warps are such that at least 50% of all spirals are thought to be warped (cf. Bosma 1991). To explain the frequency of warped HI disks, it is thus necessary to have recourse to a mechanism which can keep them going. Several proposals have been made, none of them entirely satisfactory (cf. Binney 1992).
Bosma (1991) also shows that the frequency of warps depends on the ratio of halo core radius to optical radius of the galaxy : galaxies for which this ratio is small do not have warped HI disks. This is usually attributed to dynamical friction between a misaligned disk and a dark halo. If the dark halo is strongly concentrated, such misalignments are short lived, as is shown also by numerical simulations (Dubinski & Kuijken 1995).