In You are Not So Smart, what is McRaney asking you to believe about cognition and decision-making? How does this challenge your assumptions about your thinking? Are heuristics helpful, or harmful or both? What are some factors that influence decision-making?

McRaney is primarily asking you to understand the various ways in which your brain limits your perspective and thus your decision-making abilities. The very heuristics and other cognitive processes that allow your brain to function efficiently can also lead to perceptual weaknesses. McRaney hopes to challenge your assumption that you can trust your own thoughts. By understanding the limitations of your thoughts, you can overcome them through more conscious thought efforts and hone your ability to see things objectively.
Heuristics can be very helpful when it comes to processing events and experiences. The human brain can only process so much information at once, and heuristics categorize information and allow the mind to make reasonable predictions that save time and effort. For example, the availability heuristic allows your brain to solve problems more efficiently by calling to mind other events with similar themes. By drawing upon these experiences, you benefit from the accumulation of your life experiences rather than approaching each problem from scratch.
On the other hand, heuristics can also be harmful. Because heuristics simplify things, they make us vulnerable to stereotypes, assumptions, and oversimplification of complex ideas. An example of the harmful effects of the availability heuristic is the fact that most people fear flying more than driving an automobile even though flying is statistically much safer than driving. Because plane crashes are so widely reported in the news, the availability heuristic leads people to believe they are more common and thus that planes are more dangerous.
Past experience, cognitive biases, individual differences, age, the escalation of commitment, and belief in personal relevance are some of the most common factors that influence decision-making. Past experience provides a repository of knowledge your brain uses to draw from in reference to current problems and decisions. Cognitive biases limit your ability to see a decision in a fully objective light. Individual differences allow two people to see the same situation in significantly different ways. Age affects the maturity of the brain which, in turn, affects the way you perceive things and the extent of your decision-making abilities. The escalation of commitment refers to the tendency to hold onto existing ideas and values because you have become invested in them. The degree to which you believe you have personal relevance refers to the fact that you will make decisions with greater frequency and confidence if you perceive yourself as someone who has the authority to make decisions.
https://www.csus.edu/indiv/s/sablynskic/ch5obe150.htm

https://www8.gsb.columbia.edu/researcharchive/articles/406