The saying, "out of sight, out of mind," can easily summarize the social psychological phenomenon known as the rule of interpersonal attraction.

The correct answer should be proximity.
Proximity refers to physical closeness of people. The saying 'out of sight, out of mind' means that we are likely to forget about people who are far away from us than if they were near us all the time. Even if we used to be close, when they are not with us, we tend not to feel the same closeness. 

Communism best describes china's government