When reading memoirs of terrorists for instance, I have often found that many terrorists fervently believe in what they are doing and think it beneficial for their soul. They have been brought up in a culture -- such as in Korea, or in the former Soviet Russia, and consider themselves the truly knowing person, and the other who has been duped.

This reminds me too of memoirs that I have read of people suffering from bipolar or who have been declared insane. Oftentimes, these patients, particularly the latter consider themselves 'normal' and surrounding people to be abnormal. The question then is one that hinges on the definition of abnormality. A person in one society may be considered 'normal'; in another society this 'well-functioning', 'normal' person may be rated an outcast and considered 'aborning'. The problem it seems to me comes about when the person is following customs and norms that his society considers 'normal" but that are mutilating to mental, psychological, social or physical health of person.,

Many think that this is particular to a so-called fundamentalist or brainwashed country such as North Korea or an Islamic...

This was America in the 1970s. People allowed their daughters to freely explore sex in all its minutes under the influence of contemporaneous scholars such as Spock, Freud, and others. Daughters killed and ruined themselves.
Today, we have others that shape society. These may be voices such as Dr. Phil, Oprah, and Dr. Laura. The thing is that their being celebrities or having names prefaced with doctorates should not influence us. Yet too many are. They inform our thinking and our social beliefs, and we, invariably consider ourselves correct and the other wrong. These thoughts and beliefs shape our decisions and ipso facto our acts. Our acts and decisions are our lives.

This is where I see my-confirmation as being the most major -- since it is the most significant -fallacy.

Source

Flanagan, C. (2012) Girl land. Brown & Co. USA