The Primary Rational Category Error
A category error is the fancy way of saying 'comparing apples to oranges.' The most significant trap that we fall into in our reasoning and decision making is a simple category error that can be avoided once we are made aware of it. I used to make this error all the time when I was younger, and it wasn't until college when my worldview crystallized that I realized that this category error was responsible for many of my mistakes and had caused me to misjudge many situations.
Emotional problems cannot be overcome by rational solutions alone.
If you find yourself facing a problem or difficult decision a good first question to ask is always 'what kind of problem is this?' Most people don't bother to ask this question and will jump in and go with their gut instinct. When I was younger my gut instinct was to default to the rational, scientific approach I had been taught and had grown comfortable with. I would find myself frustrated when that process failed to achieve results.
In hindsight, the knowledge that I had been looking for was there all along. The answer lies in observing how other people make decisions, by acting instinctively on their immediate reaction, a feeling or a hunch. The goal of proper decision making technique is to improve the process to the point where instinct can be trusted and hunches can be relied upon, but that comes only after the necessary emotional intelligence skills have been developed.
Therefore, empathy is a vital characteristic of an effective decision maker.
Our language has grown sloppy as a society, so I'll point out that empathy and sympathy are distinct concepts. In this context I mean empathy to be the ability to put oneself in another's shoes and identify their feelings. Individuals with a high capacity for empathy can literally feel the emotions of others and can use that information to make better decisions based on anticipating how others will respond to the decision.
Hint: If you are dealing with a problem or a decision and it involves other people in any way then there is typically going to be some emotional component to the decision, however minor it might be.