An expert witness is a person with special knowledge and experience who is invited to give evidence in a court of law on a matter that requires his or her expertise.
One such witness is a forensic pathologist, who will testify on the cause of death, the time of death and the circumstances of death. Courts usually attach great importance to, and believe, expert evidence, especially if the expert appears to have spectacular credentials.
Expert evidence is often complex and difficult for the court or the layman to comprehend. And because of this, experts may mislead court by taking advantage of the ignorance of the stakeholders. This vulnerability is one of the main reasons why educational programmes are important to enhance judicial scientific literacy.
It is, however, doubtful that such programmes will cover technical details applicable to specific cases before courts of law. In addition, since science is continuously evolving, judges require up-to-date knowledge of the latest advances in the area in question. Judges who aim to fully engage in the technical aspects of evidence would, therefore, of necessity conduct their own research to understand some of the issues before them.
In one particular case, a pathologist reported that a victim was manually asphyxiated and concluded that the attack took place between 8pm and 10pm. There was only one person with the victim at the time of the alleged attack. This evidence appeared to be highly persuasive.
Other expert witnesses testified that the physical evidence, such as a bruise in the neck of the deceased and pinpoint bleeding in the eyelids, pointed to manual strangulation.
An independent reviewer later noted that the information given to court at the time of trial contradicted the then-existing knowledge already documented in textbooks.
Forensic science relies on three key indicators to determine the time of death, and these involve the distribution of blood in the body after death has occurred, the stiffening of the body and the rate at which the temperature of the body drops after death. These methods are well documented in the most basic textbooks of Forensic Pathology.
When death occurs, the heart stops pumping blood to the body, and the circulation of blood subsequently stops.
The blood then settles and accumulates in blood vessels due to gravitational forces. The accumulation of blood under the skin causes the skin to take on a purple or red-purple colour.
This phenomenon is known as post-mortem staining. If the body is lying on the back, the settling of blood will give a purple or dark colour on the back of the body, except in areas where the body is pressed on a supporting surface such as the shoulder blades and buttocks. Similarly, if a body was lying facing the ground, post-mortem staining will be present on the front of the body.
Post-mortem staining has been documented to be apparent anywhere from 20 minutes to several hours after death and becomes complete (also known as becoming fixed) anywhere from six to 12 hours. The colouring may shift in the early stages, and this means if the body was moved, the discolouration would move to other areas of the skin. The discolouration becomes permanent after eight to 12 hours. In addition, tiny blood vessels may burst, giving rise to pinpoint bleeds.
The stiffening of the muscles and joints is another time-dependent process that occurs after death. When a person dies, initially the muscles become flaccid.
However, with time, the muscles then stiffen, which then makes the joints immobile. The time of onset of this varies from within half an hour to four hours after death. The process becomes complete within 12 hours and then gradually wanes off, although the time for this to occur also varies, anywhere from 12 hours to 60 hours.
It is clear that the time frames in which the processes of post-mortem staining and stiffening of the body occur vary widely and are affected by a number of factors. Nevertheless, the fact that these processes are time-dependent makes it tempting for pathologists to use these observations to estimate the time of death.
There is also the use of post-mortem body temperature as a method of estimating the time of death. The theory is that the body of the deceased cools down and comes to an equilibrium (the same) temperature as its surroundings.
This process has been termed post-mortem cooling. If one assumes that the temperature of the deceased was the normal body temperature at the time of death, one can theoretically perform a retrograde estimation of the time of death, given the temperature of the deceased at the time of discovery of the body. As intuitive and appealing as this may be, academic literature has observed numerous factors that can affect the rate of cooling of a body, such as environmental temperature, clothing of the deceased, size of the body and position of the body. Rates of cooling of bodies have been quoted to be anywhere from 1 to 2.5 degrees Fahrenheit per hour.
The expert witness, a pathologist, in this particular case, told court that the deceased must have died between 8pm and 10pm, the night before the body was discovered.
He based his conclusion on his observation that the stiffening of the body was beginning to wear off in the face and neck and that this usually occurs 12 hours after death. He did not give any reason as to why he chose the particular time interval of 15 to 17 hours before the discovery of the body as the range for the time of death.
He told court that post-mortem staining begins normally around two hours and becomes fixed around 12 hours from the time of death. He told court that the post-mortem staining had become fixed at the front of the body and that the body of the deceased must have been in that position in excess of 12 hours.
The expert, however, contradicted himself when he also observed that the post-mortem staining had shifted when he saw the body the day after the post-mortem examination.
Other experts, however, told court that these methods used to estimate the time of death were highly unreliable, as the time these changes take to occur after death is variable and prone to subjective interpretation.
A professor of Forensic Pathology was to tell court that these benchmarks all suffer from extreme variability, where post-mortem staining of the body can become fixed anywhere between one to 20 hours, and the timelines for stiffening of the body vary widely.