I missed this the first time around...
I am actually going to have to disagree on requiring Communism. They weren't under a Communist society when they wrote this. Rather, it inspired the rise of several Communist societies.
In my game, I've actually started making the later wonders cost more than just shields. I now have wonders with negative happiness values... they have very powerful positive effects, but that comes at a price.
I've created a series of wonders, advances, and citizens intended to represent what historians have termed the "Age of Uncertainty". The orderly Victorian society was coming to an end as early as the 1860s. In the United States, civil war was the beginning of their foundation being gutted from underneath them. In Europe, from the Franco-Prussian War to the beginning of the First World War, fear and tension were on the rise and a new, far more brutal version of war was being born out of a period of uneasy peace. Men like Freud and Nietzsche were tearing the heart out of the Victorian society from the inside of the human mind. When Nietzsche pronounced "God was dead", people heard it. And when the dust had settled from the great Continental war, many people believed it. As technology improved, the rich got richer and the poor... well, they got richer too, but it was a lot harder to see because of the decadence around them. Class struggle reached a peak, and Marx and Engels were there to exploit the break. Charles Darwin had been laughed or condemned in the 1850s for his Theory of Evolution, and yet, not a generation later, those same people who shunned him were using him to justify conquest and genocide in Africa just as a generation after that, the Nazis used it to justify conquest and genocide in Europe. Even men like Einstein were doing their part... destroying the old Newtonian view of the universe with its center and fixed laws. Later Heisenberg would add to this with his Uncertainty Principle. In regular society, Jazz was horrifying the older generation and liberating the younger one. Skirts were getting shorter and women were breaking free of their old bonds, and with their freedom came a virtual sexual explosion in the post-World War I era. Sure, you see those old reels of the Old Guard of Europe marching off to war in 1914, but their war was as much a product of the upheaval as the Communists who denounced the war as Imperialist and capitalist in nature, and the Old Guard would not survive to see the end of it.
For the people of the time, this era must have looked almost like an implosion of society!
So I've reflected that in my mod with techs like Socialism, Atheism, Existentialism, and Psychology, and with wonders like Freud's Psychoanalysis and Marx's Das Kapital, which maybe I'll replace with this lovely graphic here...
I think that the Communist Manifesto would fall into the same vein. A cause of social disorder, but with its own unique boost in Civ3 terms.
But as Marx said, communism will come sooner or later, it's just a matter of time.
