The small lines jutting out of the trend lines indicate the time & amount of reward. VR = Variable Ratio, FR = Fixed Ratio, VI = Variable Interval, and FI = Fixed Interval.

Zynga’s success has much to do with their skillfully executed manipulation of the human brain. One such method is known as the Random Reward Schedule, based on the results of a study conducted by psychologist B.F. Skinner. In this study, he found that giving pigeons a consistent food reward lead to the least engagement. They would eventually get bored and only come back when hungry. Skinner then found that randomizing whether the reward was given made the pigeons come back more often, as did randomizing the amount of the reward. Lastly, he found that combining these experiments to randomize both whether the reward would occur and how much the award was for lead to a striking increase in engagement. Zynga and other social games companies have implemented the Random Reward Schedule to great effect in their games to keep players coming back.