The closer the relationship, the more contagious a yawn is

A study was done on what makes yawning contagious. They found that the closer the bond between two people (friends, acquaintances, loved ones, etc.), the more likely a person is to yawn. The closeness of the relationship also influences how long it takes for the resulting yawn to occur; the time it takes for a person to yawn in response to another person's yawn is shorter for loved ones and friends than it is for strangers.

http://www.sciencedaily.com/releases/2011/12/111212092649.htm