The operant conditioning concept of positive reinforcement is illustrated when the next day, her boss gave her a gift certificate to a local coffee shop. As a result, Janice now regularly cleans out the office refrigerator.
Positive reinforcement simply means the process of establishing a pattern of behavior by giving a reward when such behavior is exhibited.
In this case, since Janice cleaned out the office refrigerator without being asked and her boss gave her a gift certificate to a local coffee shop, she'll continually clean it.
The answer is evolutionary psychology. It basically states that women are more selective because they produce a limited number of gametes, whereas men produce sperm their whole lives.