Members of a jury may have knowledge about many different fields but probably not about the topic of a case. This may be by design as courts do not want juries making decisions about cases based on pre-existing knowledge but upon the basis of the evidence alone. To provide background information relevant to the case, attorneys may bring in expert witnesses to testify. 

The rules of evidence prevent most witnesses from basing testimony on analysis and opinions, requiring that they stick to the facts of the case. However, an exception is made for expert witnesses, whose education and experience are brought to bear when forming opinions as to the likelihood of a certain outcome based on the evidence. There are many criminal and civil cases in which an expert witness can provide valuable information, and therefore, many different types of witnesses who may be called to testify. 

1. Financial Experts

In a civil case involving money, people with extensive knowledge of finance may be called in to testify through expert witness services. Examples in which financial experts may be called to testify include the following: 

• A personal injury or workers’ compensation case in which it is necessary to know the value of wages the victim would have earned if not able to work due to catastrophic injury. 
• A probate case in which the true value of a decedent’s property is in question. 
• A divorce case in which one of the spouses may be hiding assets from the other. 

A specific type of financial expert called a securities expert may be asked to testify in a criminal case of white-collar fraud to determine whether there was an intention to deceive. 

2. Medical Experts

Medical experts may be called in both criminal and civil cases. In a criminal case, they can help to make distinctions between significant but sometimes esoteric categories such as “substantial bodily harm” and “great bodily harm.” In a civil case, medical experts can explain limitations resulting from an injury, the pain and suffering an injured patient has endured, and the likelihood of recovery. 

3. Forensic Experts

Forensic experts are scientists who apply their investigations to legal practices. As you can imagine, this is therefore a broad category of experts who have studied many different topics. Blood spatter analysts, ballistic experts, and criminal behavior experts are all examples of forensic experts who may be called to testify in a criminal case. 

Forensic experts are often not individual practitioners. Rather, they are in the employ of law enforcement agencies to first analyze the evidence and then testify to their findings in court.