Normal is defined by the actions and beliefs of the group. For example, there will always be some serial killers in the human population. However, we don’t consider their behavior to be normal and society makes it clear that such behavior is wrong and will be punished. As to Margaret Mead and her study of the Samoans, Derek Freemans work seems to strongly suggest that she was either decieved by her sources or simply published a fraudulent work. http://www.stpt.usf.edu/~jsokolov/314mead1.htm
That depends. Some groups have no beliefs---animals, of course. Then, in human beings, it depends upon what group---those who strive to change human behavior, those who defy the controlling group, but themselves constitute a group. Take homosexuality, for an example, which group defines normality? Take cigarette smoking---which group defines normality?