  The density profile of simulated dark matter structures is fairly
well-established, and several explanations for its characteristics have been
put forward. In contrast, the radial variation of the velocity anisotropy has
still not been explained. We suggest a very simple origin, based on the shapes
of the velocity distributions functions, which are shown to differ between the
radial and tangential directions. This allows us to derive a radial variation
of the anisotropy profile which is in good agreement with both simulations and
observations. One of the consequences of this suggestion is that the velocity
anisotropy is entirely determined once the density profile is known. We
demonstrate how this explains the origin of the \gamma-\beta relation, which is
the connection between the slope of the density profile and the velocity
anisotropy. These findings provide us with a powerful tool, which allows us to
close the Jeans equations.
