  A substantial fraction of the warps in spiral galaxies may result from
bending instabilities if the disks are essentially self-gravitating. With
N-body simulations, we show that galaxies with self-gravitating disks as thick
as HI disks are subject to bending instabilities generating S-shaped, U-shaped
or asymmetric warps. S-shaped warps persist during several rotations and keep
the line of node straight. The warp amplitudes generated by bending
instabilities remain however modest. Other factors must be invoked for extreme
warped disks. However, bending instabilities can account for most of the cases
reported in optical surveys, where the warp angle is generally less than
$5^\circ$. This mode of warping is very sensitive to the disk flattening. It
also constrains the fraction of dark matter distributed in the disk and in the
dark halo.
