They are several reasons for an eyewitness’s memory to be affected.
One of the reasons is that images in our mind are not lucid enough when we try to bring back to our memory a specific place/location etc. Let’s say for example you are trying to reminisce your living room. The image that you will bring to your mind won’t be as specific and detailed as if you were actually in the room and you were looking around. You will be able to recall certain objects, colors and their location, but this will be just a summarized concept of the actual content of the room.
Our memory’s perspective is better when we are viewing an object or a scene, than what it is when we are trying to describe something out of memory. One good example is the identification of colors. Even though we have the ability to recognize up to millions of present physical colours, studies have shown that our memory can only recognize 17 colors in total. For example, if you are driving and your eye catches a car of a specific brand and after a while you see a car of the same brand but in the color blue, your memory will immediately recognize the difference between the two cars. But if you see a car of a specific brand in grey-black color and a while later you see again the same car brand but with the combination of colors blue-black, your memory will have difficulty distinguishing the differences and you might find yourself searching for other insignias, to make sure if it is a different car or not!
Memory can only be reconstructed and since there are no records, you can go back and check or verify that what’s been said is accurate. It is acceptable for eyewitnesses to have insufficient information regarding the crime scene. In a situation like this, where people are not able to remember the exact scene, they tend to fill in the empty memory space with events that might have occured before, after or during the crime scene and they might be vivid in their memory and could fill in the gaps. Unfortunately though, this may lead to providing false information regarding the crime.
Beyond the tricks our memory can play, other factors that can influence the memory of an eyewitness are the natural factors, such as it could be a foggy or rain night, the lighting might have been really bad, the eyewitness could have been standing significant far from the crime scene, amount of alcohol consumed before he witnessed the scene or even fear. Fear can create a lot of false images in our head and therefore these images will stay in our memory. If the eyewitness is shocked by what he witnessed, then it’s obvious that afterwards he won’t be able to recall the scene as he really witnessed it.
We hear many times of eyewitnesses “altering the facts”, but that doesn’t mean that this always happens deliberately. Memory changes over time and retelling.
So the conclusion is that memory is imperfect and due to that, eyewitnesses befall to errors of identification