It has often been said that eyewitness testimony outweighs circumstantial evidence, when, in fact, the reverse is probably true. Circumstantial evidence such as motive, opportunity and means of committing the crime, plus fingerprint or DNA evidence that the defendant was involved, can far outweigh testimony given by an eyewitness. Eyewitnesses are notoriously inaccurate, and it is troubling that many defendants are convicted on eyewitness testimony alone.
External factors affecting eyewitness testimony would include the environment (noise, distractions, lighting, weather, crowds, distractions, danger, etc.)
Internal factors of importance would be the innate capacity of the witness to accurately observe, such as the quality of their vision and hearing, plus their mental capacity (retarded, normal or elderly with dementia). A major internal factor is stress.
Other internal witness issues of which the field officer must be aware are witness bias, truthfulness, motives, and possible involvement in the crime.
It is the job of the field officer to evaluate the witness in terms of the above factors, and then tailor the interrogation appropriately. The officer should assess the witness reliability and indicate this in the police report.
At trial, it is the job of the prosecution and defense councils to weigh the credibility of the witness and demonstrate for the jury the witness’ reliability or lack thereof.
The reference describes the numerous concerns about eyewitness testimony, and outlines the use of expert witnesses to testify about the unreliability of such testimony.
Eyewitness testimony is subject to marked variation in accuracy and reliability. Circumstantial evidence is often more reliable than eyewitness testimony.