The Ethics of Being Expert Witness

ASLO Ethics Forum

This series of articles is published in the Limnology and Oceanography Bulletin and on the ASLO website. The ASLO Professional Ethics Committee receives and deals with inquiries regarding ethical dilemmas and issues, and selected topics will be published here.

Do you have a question? Contact the Professional Ethics Committee.


An expert witness is a person who testifies at a trial because he or she has specialized knowledge of a particular subject area. Even though the expert witness was not present at the event in question, his or her testimony may be useful in determining the truth. For example, an expert in ocean currents might be able to estimate where a body was dumped into the water based on where the body was found and current speed and direction. An expert in fish taxonomy might be able to determine if a species being sold in the supermarket as locally caught is indeed a species found locally. An expert in habitat might testify as to whether or not a tract of waterfront land slated for development is important for an endangered or commercially important species.

Ethical issues arise whenever expert witnesses are involved in a trial because the experts are paid by one side, but are expected to tell the whole truth, not just the parts of the truth that will help their clients (Kadane 2005). Furthermore, there are inherent conflicts between the goals of attorneys and the goals of expert witnesses (Murphy 2000). Attorneys participate in an adversarial system, attempting to sway a judge or jury to their point of view, whereas scientists seek the truth, regardless of the goals of the interested parties. Should the expert witness appear as an impartial party, attempting to explain a difficult topic to the judge and jury, or should the expert act as a responsible advocate for the party that hired him or her? If the expert witness chooses the latter path, he or she must present research fairly, without distorting or misrepresenting the evidence (Murphy 2000).

Each country has its own policies governing the use of expert witnesses. The American Bar Association (ABA) Model Code of Professional Responsibility (ABA 1983) and Model Rules of Professional Conduct (ABA 2007) contain some guidelines regarding expert witnesses in the United States. An expert cannot be paid contingent upon the outcome of a case, and should be free from financial incentives that would tempt him or her to testify untruthfully. The ABA rules also state that lawyers may not persuade experts to testify in areas outside their expertise. The ABA makes clear that unlike attorneys, expert witnesses do not owe loyalty to their clients, and must refrain from advocating for the client. This is also the case in the United Kingdom, where the Civil Procedure Rules specify that an expert's duty is to the court and not to either of the parties involved in the trial (U.K. Ministry of Justice 2007). For a review of policies in several other European countries, as well as Hong Kong, please see Bird and Bird (2006).

Many professional societies, including the American Medical Association (AMA) and the American Psychological Association (APA), have explicit rules governing the behavior of expert witnesses, going beyond the ABA requirements. These rules cover such things as the preparations that an expert must make, the need to use current methods, and the obligation to make clear any limitations of or reservations about the methods used (APA 2002, AMA 2006). Although ASLO has no specific rules regarding expert witnesses, the ASLO Code of Professional Conduct (ASLO 1994) states that members have the responsibility to "accurately represent expertise and identify as such factual knowledge and interpretation(s) based on that knowledge." This rule may be interpreted to mean that ASLO members must not testify outside their area of expertise. For example, although a biological oceanographer may know more about physical oceanography than the average person, he or she would not be considered an expert in physical oceanography.

To avoid the conflict of interest inherent in expert witness testimony, Murphy (2000) "proposes the creation of a permanent organization to provide courts with experienced, respected, and impartial experts in various specialized fields." The expert witnesses would be hired directly by the courts, not by an interested party. It is unlikely that this solution will be implemented in the United States any time soon. Until then, expert witnesses are advised to adhere to the guidelines set forth in the codes of ethics set forth by their professional societies and the ABA.


American Bar Association. 2007. Model Rules of Professional Conduct. Available online.

American Bar Association. 1983. Model Code of Professional Responsibility. Available online.

American Medical Association. 2006. Code of Medical Ethics of the American Medical Association, 2006-2007 edition. American Medical Association, Chicago. 350 pp.

American Psychological Association. 2002. Ethical Principles of Psychologists and Code of Conduct. Available online.

American Society of Limnology and Oceanography. 1994. Code of Professional Ethics. Available online.

Bird and Bird Law Firm. 2006. Bird and Bird's International Dispute Resolution Newsletter, No. 16. Available online.

Kadane, J.B. 2005. Ethical issues in being an expert witness. Law, Probability and Risk 4: 21-23. Murphy, J.P. 2000. Expert witnesses at trial: Where are the ethics? Georgetown Journal of Legal Ethics 14: 217 –240.

United Kingdom Ministry of Justice. 2007. Civil Procedure Rules, Part 35. Available online.

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