I am having a very hard time buying the FAQ's logic that everyone will be motivated to do work in a socialist society.

well historically we can begin by admitting that there have existed "socialist" (there are various disputes about when and to what degree they were and whether they remained so) countries, such as the USSR, Cuba, China, that operated in a socialistic manner and had high productivity (the Russians overthrew a monarchy and put a probe on Venus in 53 years) and see that it is in a sense functional. As for "how", well, a large number of the same factors that drive productivity in capitalist societies. Rational self interest under capitalism would attempt to accumulate private capital, yes, but with the means of production collectivized, the improvement of the modes of production would better not only the individual but the whole of society. By performing labor, you would benefit yourself and improve your own conditions and simultaneously those of others. In a sense this is also what people look to under the capitalist system, to work not only for personal profit (such as maybe enormous wealth from some new invention) but also to help and better their loved ones, friends, and community. Ask yourself, do people really become doctors or surgeons for personal gain? Wouldn't medical school and building a practice be an incredibly inefficient method of personal gain when finance could potentially return higher profit tenfold? Why do people study cancer when they can be stockbrokers? If personal monetary interest was the only motivating factor behind career choices, why are there even school teachers? artists? musicians? Ultimately I think discussions of "human nature" are too speculative stumbles constantly into psychologisms. To "go read a book" is most without a doubt the most important thing you can do when looking for a practical concrete perspective (which you won't find about any political system in just Q&A format) otherwise your conceptions of what they constitute shall largely be anecdotal, conversational banter.

/r/socialism Thread