The human mind is difficult to understand in normal circumstances; add stress and it becomes even more difficult. The judicial system tries to seat a panel of impartial jurors, but every individual brings opinions and beliefs. Facts are critical to a legal case, but how they are presented, especially by a medical expert witness makes a difference.
In general, people do their best to be forthright and accurate in their dealings with other people and with officials. But as has been demonstrated many times, the mind is fallible and our powers of observation are not as good as we would like to think. There are numerous word games that demonstrate how thinking patterns can be tricked or confused.
Safety professionals have long known that if you ask someone who watched an aircraft crash if they saw a fire before the plane fell, they will say yes. To get accurate information, instead of question which may lead them to an answer, it is best to tell them to explain what they saw. In our attempt to get things right, our mind will fill in gaps or draw the most logical conclusion we can in the absence of fact.
Anyone who has taken an introductory psychology class has probably had the instructor demonstrate how weak the human power of observation really is. Typically the professor will show a short video clip about a murder or a car crash and then ask questions like who noticed the cat in the corner or the lady pushing the baby in a stroller. Invariably a large portion of the class will raise their hands when in fact neither of those events was in the clip.
These anomalies in human cognition allow trial lawyers to successfully manipulate testimony. If possible, lawyers confuse or distract the individual to have them testify for their side. If that does not work they stress the person and then attack their character, highlighting misspoken responses as an indication of untruth.
The frailties of our powers of reasoning are frightening when observed from a distance and dispassionately. People also tend to attach a special reverence to those who have been awarded a degree or certificate of some kind. So when someone appears in front of jury with a degree clearly announced by the trial lawyer, their words carry extra weight.
In fact, testimony from a medical doctor or psychologist can actually be entered in court records as fact. Often these kinds of statements will be stipulated to, meaning the opposing lawyer allows the information to be entered into the record without challenge. The concern is that not all degree laden individuals will agree on what is truly fact.
Trials are a complicated process of gathering data from a crime scene, then passing that information to legal teams. The presentation to this group includes fact and form, with the trial lawyers intensely aware of the value of the presentation. Added to the detailed specific instructions of the judge on the requirements of the law, it is easy to see how an medical expert witness can save or sink a case, despite all other evidence.