Donella Meadows writes, "Now we're beginning to move from the physical part of the system to the information and control parts, where more leverage can be found."
In the few times I've heard the phrase "negative feedback" spoken, the speakers have usually meant criticism or psychology's negative reinforcement. Meadows is using the phrase to mean that something a system does has a slowing effect on itself. Think of foxes as negative feedback for field mice. The more the mice breed, the more food there is for the foxes, making more foxes, which reduce the mouse population.
What Meadows was saying was that we can get better results from a kind of system jiu jitsu than from concentrating on a problem's hardware.
She uses the thermostat example to illustrate negative feedback. You want a certain temperature, and you set the thermostat. There's a furnace in the basement that comes on when the house temperature drops below the setting. The fire in the furnace would keep heating the house until it became uncomfortable, except for the thermostat's also being an off switch. The negative feedback is that the more heat the furnace puts out, the more likely it is to turn itself off.
Meadow' other examples include:
Emergency cooling in nuclear power plants,
Sweating and shivering,
Markets (provided they get accurate information).
She says that the real leverage related to markets is in ways of getting them accurate, unambiguous information.
One paragraph that I took to be encouraging, in thinking about gun violence, says, "The strength of a negative feedback loop is important relative to the impact it is designed to correct. If the impact increases in strength, the feedbacks have to be strengthened too.
"A thermostat system may work fine on a cold winter day -- but open all the windows and its corrective power will fail."
The reason this is hopeful is that the phenomenon of mass shootings -- even gun violence in total -- is a minor blip in the system of our society. Horrific as it is.
If we can figure out what's happening, we ought to be able to correct for this painful phenomenon without strong negative feedback.