Unlike the other societies, the Incans had no system of slavery. Instead, the government expected all citizens to put in their fair share of hard work for the empire as a whole. Since there was no tribute (or tax) system, peasants were expected to serve government officials with physical labor instead. This system was called the corvee. It was very important for the expansion of the Incan society because everything was built by peasants fulfilling their labor duties. In fact, when the Spaniards invaded they were shocked to find that the Incans would rather do hard work than give up some of their material goods.