Life-experience and specific circumstances impose limits, like a standard deviation, in credibility determinations. When we assess credibility, we are performing a balancing test.
If, for example, you were a juror and a defendant’s explanation for possessing a large amount of cash was that they found it under a tree, how believable would you consider their explanation? I would say it is not very credible. It is possible to find money at random, but in general experience this is rare. We know the saying that if something sounds too good to be true, it probably is. Consciously or unconsciously, we are constantly comparing sensory inputs to our knowledge base in determining if information is true. What I propose is that the more unusual a story, the less truthful it will appear.
Assume you are a judge in a divorce case. Mom offers into evidence a hundred transcribed text messages allegedly sent to her from dad. He acknowledges only the ten most banal, scattered among the others. The first balancing you might employ is across the entire range of human behavior; it involves a general standard where you know definitively that one party is lying. Either dad wrote ninety messages or mom made them up. Is it more likely that a man would deny authorship in an attempt to cover up, or that a woman would fabricate such a large number of vile, vituperative, verbatim transmissions? General experience suggests women usually do not do that, although they could.
Let’s change the facts. Instead of a typewritten transcript, mom offers her actual cell phone. The texts are in time-stamped chronological order, each indicating it originated from dad’s phone. The credibility balance has shifted. Now, for mom to be the wrongdoer she (or her agent) would need one of two things; either the skill set to spoof the source of an incoming text message, or access to dad’s phone for consecutive hours over a number of weeks. How much education and training does mom have in information technology? Would her utilization of such computer wizardry (or access to the father’s portable device) match up with her state of mind; or do the messages align more closely with dad’s angry, misogynist outlook as described by mom’s witnesses? Which party is more credible now?
Balancing alternative probabilities is an innate part of human nature. Judges rely on probability from general experience; or calculate the odds regarding specific people, words and conduct in the context of everything else known about those individuals and events. This is why, among more experienced legal counsel, judicial decision-making often does not have to look like a roulette wheel spinning at a casino. It is the reason why witness credibility is so critical to a judicial outcome.