Conspiracy theories exist because the human brain is hardwired (a) to seek patterns, and (b) to seek simplistic patterns. Hence we have religious mythologies, various superstitions, and conspiracy theories. Most are clearly absurd and can be believed only by people of extremely modest intellect and little or no understanding of the world around them.

Others - a tiny minority - can be regarded as credible if, and only if, there is sufficient reliable supporting evidence. When it comes to misbehavior by US federal and state institutions, there's a depressingly long history of gross abuse of power and so conspiracy theories concerning official actions start with more credibility than fanciful tales of rectally-obsessed space aliens. But again, we need to apply coherent standards of assessment. The size of the conspiracy group matters - large numbers of people are unlikely to remain silent for extended periods, whereas a small group of insiders may remain silent for decades. The complexity of the conspiracy matters - most plans fail because exogenous factors invalidate plans that have too many variables. And finally the power of the conspirators versus the power of the victims matters - powerful people and institutions can more readily escape consquences when preying on powerless people than when attempting to prey on those who may have some means of recourse.

In the end, we need to remember that conspiracy theories are for the most part mere fantasies; the fact that a tiny percentage are sometimes partially or wholly true does not alter this fact.