  The t<0 branch of pre-big bang cosmological scenarios is subject to a
gravitational wave instability. The unstable behaviour of tensor perturbations
is derived in a very simple way in Hwang's covariant and gauge-invariant
formalism developed for extended theories of gravity. A simple interpretation
of this instability as the effect of an "antifriction" is given, and it is
argued that a universe must eventually enter the expanding phase.
