If they ain't just a little hungry, they cease to exert themselves. Human beings are HARD-WIRED in this respect. They cannot be trained out of this aspect of their nature.
This is especially true when the work we do is not the work we were designed to do.
The work we were hard-wired to do is fun to do: hunting, fishing, and gathering.
But as population increased, the natural environment was not productive enough to supply enough people with their wants.
So people had to invent agriculture and turn from the fun and healthy life of hunting and gathering to the back breaking toil of farming, to be followed by the work of industrial man in a few more centuries.
Now most people will work only until their most dire and immediate needs are met--beyond that, they balk.
So it is in the interest of the powerful to always keep the general population of workers hungry.
This they do by encouraging immigration to flood the country with workers, to prevent the average worker from having it too easy.
But to accomodate the population explosion from immigration, technology must be developed even further, and the lives of people are removed even further from the lives they were by nature designed to live.
Unhappiness, discontent, perversion, irritability and anger result.