How come mere opinions started to affect real life...?

http://www.banksy.co.uk/

"Wake up, you idiots! Whatever made you think paper was so valuable?

There was still plenty of food and fuel and so on for all the human beings on the planet, as numerous as they had become, but millions upon millions of them were starting to starve to death now. The healthiest of them could go without food for only about forty days, and then death would come.

And this famine was as purely a product of oversize brains as Beethoven's Ninth Symphony.

It was all in people's heads. People had simply changed their opinions of paper wealth, but, for practical purposes, the planet might as well have been knocked out of orbit by a meteor the size of Luxembourg." (Galápagos - a novel by Kurt Vonnegut, 1985).