There are many examples of how human behavior is influenced by broader social influences, ranging from the benign to the genocidal. Indeed, the concept of "peer pressure" is precisely that psychological phenomenon in which one's behavior is modified to conform to that of the larger group to which one aspires to be accepted. Many teenagers take up smoking because it is what their friends and people they admire do to exert their independence and as a rejection of broader social mores.
Academic studies of gang membership are replete with instances of "lost" youths joining gangs as a means of gaining acceptance among their peers when no other "family" provides the security and sense of camaraderie that membership entails. Many individuals adopt the customs or behavior of a larger group as a means of gaining social acceptance.
A positive way in which social influences affect human behavior is the movement toward more environmentally friendly practices in how one lives one's life. Many individuals and families have altered their behavior to conform to the standards presented by prominent environmentally-conscious organizations, including buying smaller, more fuel efficient cars, shopping for groceries at stores with more home-grown and organic produce, and so on. Individuals who have conducted their lives in a certain way for years now find themselves altering their behavior because of social influences with regard to environmentalism.
In a less benign context, social influences have been instrumental in transforming human behavior in extremely negative ways. The rise of Adolf Hitler and his National Socialist Party (the Nazis) is a textbook case of how millions of individuals conformed to societal dictates, at the expense of their own humanity and at the cost of tens of millions of lives. As the attached eNotes essay on the psychology of perpetrators notes:
"Ordinary men are usually part of a social and moral network that helps them maintain their humanity toward others and prevents them from becoming involved in inhuman acts. In order to socialize them into becoming murderers, they have to be insulated from their original social network and an alternative network has to be created for the potential killers, composed of men like themselves, led by a genocidal authority. This is not an easy a task to achieve, and therefore careful attention needs to be given to the process that the potential killers are led through."
Nazi Germany could be considered an extreme case, but it is more representative of this particular phenomenon than many realize. Subsequent genocidal policies in Cambodia, Rwanda, and Bosnia-Herzegovina all involved the same pattern of individual behavior being influenced by social influences. The so-called "pack mentality" exists among animals and humans alike, wherein individuals' actions are heavily influenced by larger societal pressures or influences.