Motivated reasoning is one of the easiest traps for an engineer to fall into. This pleasantly oxymoronic term can be considered as an extreme case of confirmation bias.
So what is it? I see motivated reasoning as the practice of allowing emotion to creep into the engineering process — usually through the emotion-based evaluation of data or calculations leading to an emotion-based decision. It is a matter of putting more credence into your feelings than in the data at hand.
Now this doesn’t mean you always have to be Spock rather than Kirk, but one needs to understand that all decisions are, to a certain extent, emotional, but that one can’t dismiss engineering facts just because they don’t fit your emotional needs.
A case came up recently that illustrated this tendency perfectly. A report generated by the best of the field offices completely disregarded a previous report from another office. Why? Because if that older report was true, it would mean more work and expense. By disregarding the old data, they could avoid a lot of work and cost. Trouble was that there was nothing really wrong with the old data. Certainly nothing that would support tossing out the whole report. To rationalize their position, they harped on some small errors and inconsistencies. Their position became an emotion-driven one based on external pressures to reduce cost and schedule impact. The correct, data-driven position was overridden.
We corrected this situation but it illustrates how easy it is, even for first-rate engineers, to fall into this trap. Experienced engineers can sometimes go with their gut (emotions) and succeed, but when data and calculations are available, go with the math every time. Sort of like flying in the clouds: trust your instruments and not the seat of your pants.