It seems to me that this study is talking about how one can "identify" with the characters in the story which I find to be believable being that I have done it myself on occasion.
I had to read "A Raisin in the Sun" in my ENG 102 class and found myself identifying strongly with the Beneatha character in that I admired the hardships that she had to overcome being an independent black woman wishing to become a doctor, a profession not usually sought out by people of her gender and race back in the 50's. I actually found myself being like her in the sense that I could see certain attributes in her that I saw in me. Not sure if I want to say that I wanted to be Beneatha but that I could see certain parallels.
It seems that the study is saying that the more emotionally charged the story is and the more that the character has to overcome certain obstacles, the more that people will really get themselves involved and invested emotionally into the story.