  Dark matter halos are the building blocks of the universe as they host
galaxies and clusters. The knowledge of the clustering properties of halos is
therefore essential for the understanding of the galaxy statistical properties.
We derive an effective halo Boltzmann equation which can be used to describe
the halo clustering statistics. In particular, we show how the halo Boltzmann
equation encodes a statistically biased gravitational force which generates a
bias in the peculiar velocities of virialized halos with respect to the
underlying dark matter, as recently observed in N-body simulations.
