'The processes behind the typical S-shaped growth
curve serve as a simple example of shifting loop dominance.
Consider a population expanding toward an upper limit to the
carrying capacity of its environment. When the population is
well below the limit, population expands exponentially, driven
by a linear positive feedback loop in which additions to
population increase in proportion to population itself. The
positive feedback loop produces the initial upward-sweeping
section of S-shaped growth. But as the limit to population is
approached, a previously dormant linear negative feedback loop
becomes active, interacts nonlinearly with the positive loop,
reduces the growth rate of the positive feedback loop toward
zero, and eventually takes full control to adjust population
toward the limit whenever population deviates in either
direction from the limit. The two loops come into operation at
different times. First, the positive feedback loop of growth is in
control during the early exponential growth phase. Later, the
negative feedback loop exerts increasing control to neutralize
the positive loop and convert the system to a goal-seeking search
for an equilibrium at the population limit. Biological and social
systems contain numerous structures that move in and out of
dominance as forces shift.'
from J. W. Forrester, 'Nonlinearity in High-Order Models of
Social Systems', paper number D-3691-1, March 1985, System
Dynamics Group, MIT, Cambridge, MA 02139.