<< Petre and Leo work in a factory making...uhm...tractor tires.
Petre is new on the job and is highly motivated. Today he makes 16 tires.
Leo has been around the block a time or two and only makes 4 tires. At the end of the day they each get 16 rubles.
Petre realizes that for every tire he makes he earns 1 ruble while Leo earns 4 rubles. This is where the lack of motivation sets in. As the (dimly-lit) lightbulb comes on, Petre realizes that the less he produces the more he is compensated per unit. Tomorrow, Petre won't work as hard and he'll get paid 4 rubles per tire. Communism shoots for the lowest common denominator and will bring everyone down to that level.
Interesting example. Only problem, it didn't work that way. Russia had just as many incentive programs as they have (or used to have) here in the US. For example, my dad sells so many insurance policies per month, he can get a bonus of a cruise to the Carribean. In Russia, you accomplish a goal defined by the state, such as developing and producing a new weapons system, being a sucessful team member, and you become employee of the month and spend a summer holiday on the beaches of the Black Sea in the company-owned resort.
Human nature is still human nature no matter what system you have. Ask the Chinese. They have the same sort of thing. >>
If you reward individual accomplishments then it is no longer true communism. Obviously there are external influences, but if you look at it as a closed society (a union, minimum wage, etc.), where there is a fixed compensation spread equally among workers, then the unscrupolous among the group will produce the bare minimum requirement in order to get by. This will influence the rest of the population to work at the same level. There may still be a small percentage that continue to give "110%" as they look out for the benefits of the whole, but in this closed environment the net product will be less than a capitalist system where compensation more readily scales to meet output.