  Statistical physics cannot explain why a thermodynamic arrow of time exists,
unless one postulates very special and unnatural initial conditions. Yet, we
argue that statistical physics can explain why the thermodynamic arrow of time
is universal, i.e., why the arrow points in the same direction everywhere.
Namely, if two subsystems have opposite arrow-directions at a particular time,
the interaction between them makes the configuration statistically unstable and
causes a decay towards a system with a universal direction of the arrow of
time. We present general qualitative arguments for that claim and support them
by a detailed analysis of a toy model based on the baker's map.
