1) The most interesting thing was that two gorillas Michael and Koko could lie and gave wrong feedback at doctor's questions. Next interesting point was research about ability to recognize a lying.
2) I did not found in the text answer at this question. I think the most are people lying about sensitive things: love, their failures etc.
3) The two main reasons from lying are pretend pain and make some advantages for you.
4) Of course there are justified lies, because some lies could protect somebody or you. And it is necessarily to lie if you need to keep a secret.