The first week of the trial of Derek Chauvin – the former Minneapolis police officer charged with the murder and manslaughter of George Floyd – was largely taken up with the evidence [1] of eyewitnesses to the incident. As we saw in a previous post [2], the State called several eyewitnesses, or bystanders, who gave evidence about what they had seen and heard during the incident.

As we also saw, there are rules which apply in relation to witness evidence. The primary purpose of evidence is to prove facts – facts which are relevant to any issue in the case. In the case of Derek Chauvin, the State – the prosecution – must prove several facts if they hope to secure a conviction of either murder or manslaughter. These include, for example, facts about what Derek Chauvin actually did during the incident. For example, did he place his knee on George Floyd’s neck? How long was his knee on George Floyd’s neck?

In this case, the eyewitness evidence in relation to these kinds of matters is probably not as crucial as it often is in criminal trials. Why? Because the entire incident – before, during, and after - was recorded on a number of devices – including mobile phones, street surveillance cameras, and police body worn cameras. Much of the video evidence is of very high quality.

It is important to remember that the general rule in a criminal trial is that witnesses should only give evidence about facts that they saw and heard with their own eyes and ears. What a witness should not do is give an opinion about what they saw and heard or speculate about things.

However, there are sometimes facts that the prosecution in a criminal trial like this must prove which require expertise. For example, in the Derek Chauvin trial, the prosecution alleges that Derek Chauvin’s actions caused the death of Mr Floyd. In this case, as we know, the defence claims that the cause of Mr Floyd’s death was connected with the fact he had taken illegal drugs. The prosecution alleges that it was the action of Derek Chauvin in placing his knee on the neck of Mr Floyd which caused asphyxiation (a lack of oxygen).

In this situation, eyewitness evidence about the incident does not help. In order to establish the cause of death, expert evidence is required. Expert evidence is one exception to the general rule that opinion evidence is not permissible in a criminal trial. Expert evidence is an expression of opinion, but one based on expertise, skill, and knowledge in a particular area. In the Derek Chauvin trial, for example, the prosecution called Dr Andrew Baker, the medical examiner who performed the autopsy [3] on Mr Floyd. They also called expert evidence in relation to the force used by Derek Chauvin during the restraint of Mr Floyd, and whether it was 'excessive'. As defence counsel Eric Nelson pointed out, sometimes the police have to undertake procedures which are 'awful but lawful' (meaning that to a bystander they may look bad but which, in reality, must be undertaken). Whether the jury feels Derek Chauvin's actions fall into this category ultimately is a matter for them to decide.

In English law, there are several considerations before a court will permit expert evidence on an issue in the case. These include:

· Does the issue need an ‘expert’ at all? If a fact or matter is one within the knowledge or understanding of the jury, an expert will usually not be required.

· Is the person who a party wishes to call really ‘an expert’ in this area? In other words, are they sufficiently qualified in an area to be able to express an expert opinion about it?

· What is the basis for the expert opinion? This will include possibly difficult evidential issues in an area called ‘hearsay’, which we will look at in a future post.

There is good reason to be cautious before a jury relies on expert evidence, however compelling it may seem to be. In the past, several miscarriages of justice have occurred on the basis of expert evidence. In recent years, convictions have been overturned, for example, in cases involving ‘sudden infant death syndrome’, in which expert evidence was called and relied on by a jury.

Text Notes

[1] In the United States, this is called ‘testimony’. In England, we would normally say ‘evidence’, and witnesses give evidence.


[3] An autopsy is a medical examination of a person after death, with the aim of trying to establish the cause of death.

© Cambridge Legal English Academy 2021

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