True communism has not existed yet, and will not for quite some time. I believe Marx theorized that true communism (workers control the means of production) will only exist after socialism ceases to exist, which in turn comes in to play after capitalism falls (give it another <50 years).
And human nature has not been proven to exist. Only our upbringings determine our thoughts (and therefore actions) for the future.
Communism WILL work in another <100 years or so (assuming we don't destroy ourselves before that time).