A fun fact that has been revealed by functional MRI machine research: any narrative explanation a person gives about their motivation for doing something ALWAYS occurs after the decision to do the thing. Your explanation for why your ate the chocolate bar will almost always follow your decision to eat the chocolate bar. Put another way, we can say we ate the chocolate bar because we were hungry, or because there was nothing else to eat, or because we felt like it, but this justification will always come after the decision to eat the chocolate bar.
This makes even less sense with decisions that the individual should have made differently. Choosing to eat a doughnut, when salad is available, makes less sense for someone who says they want to drop a few pounds immediately before eating the doughnut. Their explanation will, however, be logical and consistent. This doesn’t make sense if we are actually in charge of our bodies and our decisions; although the apparent contradiction rarely becomes evident to the person who is wanting to be leaner while continuing to eat the food they know they should not be eating.
If we were in charge, we would simply do what we have set out to do. Saying we want to eat better would be sufficient for us to eat better. Saying we want to stop smoking would be all that is required to stop smoking. That isn’t how it works. Almost everyone knows they should eat better yet most people don’t do it. Almost everyone who smokes wants to stop, yet they continue. When they are asked why they choose the food they do or why they keep smoking, they are able to explain why in a way that doesn’t seem at odds with their desire for things to be different. The logical incongruence does not seem to exist, or at least it isn’t experienced as incongruent, when explanations are given after the fact.
All explanations that are given after the fact are post hoc and therefore worthy of suspicion. Any dissonant characteristics are explained away and any logical inconsistencies are accounted for and rationalized so that what happened seemed like it was always going to be what happened.
This is why I don’t trust the reasons people give when they answer questions. I do not believe that they are deliberately being dishonest, and I have no reason to believe that their memory of historic events is wrong, I just know that the human brain does not handle dissonance effectively and will sooner make up a story that it immediately believes is true than it will acknowledge and examine the inconsistencies.
And again, I don’t blame people for this. The brain is unbelievably complex and so much is going on below the level of conscious awareness that to assume that anyone understands their motivation beyond simply staying alive is to give them undue credit. Eating the doughnut does help them stay alive, going without the doughnut denies the body high energy calories which could be interpreted as a life risking decision. Having a cigarette supplies nicotine that eliminates the negative physical symptoms of nicotine withdrawal, allowing these physical symptoms to continue and grow could be interpreted as a serious sign of unwellness.
If we really want to uncover a persons motivations or if we want to help them understand what is going on in their brain (or shape their decision making) we will ask the question before the action is taken. While the answer here is about as accurate as the answer after, it has one thing going for it that post hoc explanations will not, it is speculation about a future. The person is talking about their motivation for a thing that has not yet happened and is therefore free from any of the need to eliminate cognitive dissonance. The question “under what circumstances would you choose to eat a doughnut given that you have committed to eating better?” is rather different than “why did you eat the doughnut given that you have committed to eating better?”
One thing we know with certainty, the brain will supply a coherent narrative to rationalize and justify any decision that it has made, which is why we cannot consider our post hoc explanations to be the reasons we think they are.