The idea behind the warranted rate of 
growth is that the system of advanced capitalism
(mature capitalism!) is in some way 
adapted to conditions of very rapid growth
which obtained in earlier phases of its development.
This is to say that the savings ?
of this community is high in relation to the
investment required for growth of capacity. ??
This is due not so much because of
high savings propensities of individual ? firms or persons,
but because of the distribution of income between
profits and wages, on which the saving of the community
mainly depends.