I read the short version but I agree. Ideology, religion, nationalism etc are responsible for the majority of institutionalised mass murder.

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An exhaustive review of more than 600,000 Iraqi documents that were captured after the 2003 U.S. invasion has found no evidence that Saddam Hussein's regime had any operational links with Osama bin Laden's al Qaida terrorist network.

Jim Jones was a freak. If there is a hell there is probably a special place in it for him. About 900 people died that day. The babies were injected with the poison.
I watched a doco about it on netflix. I always wondered how those people could just drink the stuff down like sheep at the slaughter.
What I read about it and learned from the doco was that these people had been under Jone's leadership for years.
They were isolated even before they went to Gauyana. Living together and cut off from their families. SLeep deprivation, group confessions, beatings, sexual abuse by Jones himself(men and women). Also, Jones had made them drink the stuff several times before, only it wasn't poisoned. He had mass suicide drills to test their faith.

At the same time the nature of ideologies change over time. Not everybody inherited Marxism from their parents for example particularly at the time it was created. Nazism, was to some extent, a new idea that gained wind with Hitler's rise to power. Gender and racial equality movements gained wind over a relatively short period of time. The peace movement and the environmental movements also formed in a similar way. While parental influence plays a role in the formation of these ideologies it does not determine the shape of the scope of them.

I think we have not addressed two important questions; what are the consequences of forming ideologies and why do people adhere to them?

lostinacause

There appear to be many types of ideologies and I think that the Internet provides a convenient venue for the ‘ad hoc ideology’.

In his book “Group Psychology and the Analysis of the Ego” Freud writes about the characteristics of ideological groups in general and extensively on large artificial groups such as the Church and the Army.

What is striking is that members of these ideological entities often undergo a major change in behavior just by being members of such entities. Under certain conditions individuals who become members of these groups behave differently than they would as individuals. These individuals acquire the characteristics of a ‘psychological group’.

What is the nature of the ‘group mind’, i.e. the mental changes such individuals undergo as a result of becoming a group?

"Marx's Theory of Ideology" by Bhikhu Parekh is a very good book for understanding the fundamental aspects of ideology.

I don't know that the "psychological group" is the best way to characterize ideological thought at least as it is characterized in your quote. People who adhere to ideologies usually have some degree of consistent behavior even when they are not with members of the said group. While their opinions are stronger when they are with members of the ideology this is, in my opinion, nothing special about ideology. With the exception of people who like to debate, most people's opinions come out stronger around like-minded people. At the same time spending time with a group that has the same beliefs as you, particularly in a cult like atmosphere, aids the formation of beliefs that appear to be consistent. It creates an atmosphere belief reinforcement.

I find it to be quite expected that ideologies often center around difficult questions. Most fall into the categories of religion and spirituality ideologies and political and social ideologies. Religion can be seen as a guess at the state of the world. To the best of my knowledge no one can with any authority say that they truly know whether a god exists and if so what religion, if any, appropriately worships him. Given a lack of information people generally settle on some incomplete heuristic belief formation process and ideologies provide an avenue for belief formation.

A similar idea applies to political issues. People only have a basic understanding of the complexities of political decisions. I study economics and I could not tell you the affect that a 1% decline in the the marginal tax rate would have on tax revenue. Very few people have a good idea of the best way to deal with North Korea. Other political choices have the same inherent complexity. Ideologies provide a way for people to form beliefs on these issues that connect with the preferences. The beliefs are consistent enough not to create major problems of cognitive dissonance and others who share ones beliefs provide a mechanism for belief reinforcement.

This answers the main reasons why I believe ideologies form in stable atmospheres. In the case of Communism and Nazism ideologies provided a simple solution to poor conditions. The spread of these ideologies was largely because of people's desire for change. While I can think of instances in developed countries where people adhered to ideologies because of a desire for change from the status quo, I don't think it plays the same role in day to day activities as other types of ideologies.