Conspiracy theories circulate about all manner of things - the world is run by ten rich men (or their wives), by Masons, by the Mafia, select Japanese businessmen and so on.

A conspiracy involves the people in the group acting in the interests of the group, usually in a way which would be disapproved of by those outside the conspiracy, and are usually illegal.

Conspiracy theories are attractive because

• they explain things in the world around us;
• they justify the vilification of certain groups; and
• they give an excuse for the believers to not be successful.

And have the added strength that

• they are generally formulated in such a way that they cannot be disproven. It is effectively impossible to disprove that an unknown group of powerful people met in secret at some stage and formulated a common immoral plan.

The common belief in witches centuries ago provided the same need.

There are two types of conspiracies - natural, and artificial.

In a natural conspiracy, everyone is acting in their own interests regardless of the behavior of the others. For example, two people who have committed a crime have a conspiracy not to tell - neither of them is likely to break the secret because they will go to jail.

An artificial conspiracy is like the prisoner's dilemma. People act against their own interests and in the interests of the group. They do well if they are the only defector, or if they cooperate for the greater good of the group.

While some conspiracy theories are doubtless true (yes there are conspiracies), most are not. Ultimately people act in their own selfish interests. The reason that people would set up such a conspiracy in the first place is to satisfy their own greeds above the benefit of the larger group. These are precisely the type of people who will break away from a large group if they judge it is in their own selfish interest.

Small conspiracy groups can be stable, but larger groups become unstable very quickly, and it only takes one person to break rank for the group to become exposed. Setting down rules to bring retribution upon defectors will be somewhat helpful (eg Mafia-style pay-backs), but such as system will collapse as soon someone judges that they are invulnerable.

Greedy people are selfish people, and will see a conspiracy as an opportunity. Sometimes this will be to cooperate with it, sometimes to defect. Hence conspiracies are unstable, and large conspiracies do not exist.