From Walster et al. (1973)
 

                                                        Rated Liking
                                                       (Highest = 12)
 
 

Control                                                  8.58  a
 

"easy to get"                                           8.53 a
 

"hard to get"                                            7.90 a
 

selectively                                                9.41 b
hard to get