Modern psychologists refer to the first part of the law of effect as and the second part as

Reinforcement and punishment are the correct answers.

In Psychology, the Law of Effect states that responses that produce a satisfying effect tend to occur again and responses that result in a discomforting effect are less likely to re-occur. Thus, when people receive rewards for acting a certain way, they are most likely going to keep it up. On the other hand, if people are punished for acting a certain way, they are more likely to avoid acting that way again. Modern psychologists refer to the first part of the Law of Effect as reinforcement and the second part as punishment.  

answer; decipher the motives for why people behave the way they do;