We study the interaction between individuals in a population, where each individual encounters one another at random intervals, and in each encounter the two individuals play one round of the game of prisoner's dilemma. By discounting future reward, and allowing for imperfect memory and mobility of the individuals, we study the evolutionary equilibrium strategy to identify situations where cooperation emerges. We find that cooperation among individuals typically emerges when future reward becomes more important, when individuals in the population have better memory, and when the individuals move in and out of the population less frequently. The findings help explain social loafing and free rider commonly seen in towns, corporations, and military units.