The extent to which an expert witness’ controversial beliefs may be used to impeach the expert, when the beliefs are unrelated to the expert’s testimony, was spotlighted in a ProPublica report concerning Dr. David Ayoub. Dr. Ayoub is a radiologist who often testifies as an expert in child abuse cases.

Credibility Attacked

Dr. Ayoub’s credibility has been attacked on cross-examination because he believes that efforts to increase vaccination rates in poor countries are part of a conspiracy to control third-world population growth by giving allegedly dangerous vaccines to children. While Dr. Ayoub once accused the Gates Foundation of promoting infanticide, the accusation misconstrues the position taken by those organizations. The Gates Foundation and the World Health Organization, among other entities, hope that vaccinations will reduce infant mortality and thus reduce the incentive for parents to have large families in the hope of some of their children will grow to adulthood.

Dr. Ayoub’s credibility was also challenged because he supported the “anti-vax” movement based on his concern that vaccinations contain mercury and other metals that contribute to autism. While a compound containing mercury has been used as a vaccine preservative, its link to autism has not been clearly established, and has been ruled out by some studies. According to the Centers for Disease Control and Prevention, the single-dose vials that contain vaccines administered in the United States contain no preservatives, and the multiple-dose vials that are often distributed in poor countries are highly unlikely to cause autism.

Dr. Ayoub became the medical director for two anti-vaccination groups. He acknowledges that his views about the link between vaccinations and autism place him on the fringe of medical opinions, and his views about the use of vaccinations to commit infanticide (from which he has at least partially retreated) place him on “the fringe of the fringe.”

Everyone is entitled to an opinion. The question is whether an expert’s credibility should be attacked by fringe opinions that have no relevance to the subject matter of the expert’s testimony.

Relevance and Credibility

There is a difference between controversial opinions about the subject matter of an expert’s testimony and controversial opinions about unrelated matters. For example, an Australian hematologist who has testified as an expert witness in child abuse cases maintains that some alleged cases of shaken-baby syndrome (a controversial condition that has often been misdiagnosed) are vaccine-related. Cross-examination of that opinion can fairly be based on the fact that most experts disagree that vaccinations are at all harmful to children.

But Dr. Ayoub’s testimony relates to medical conditions that make bones fragile to explain how injuries might be unrelated to child abuse. The tendency to mistake the cause of a fracture as child abuse rather than bone disease is well established. Dr. Ayoub’s views on vaccination have nothing to do with that testimony.

The risk of allowing an expert to be cross-examined regarding controversial beliefs (as Dr. Ayoub was in a Maryland trial) is that the jury will discount his expert testimony simply because they disagree with the expert’s beliefs about issues that are not relevant to the expert opinions he expressed. The fact that a witness might hold controversial opinions about an unrelated subject should not be used as proof as the expert cannot testify credibly about any subject. After all, we don’t let lawyers ask witnesses who they voted for in the last election in the hope that half of the jurors will disbelieve the witness because he or she voted for the “wrong” candidate.

Trustworthiness of Expert Opinions

Attacks upon an expert’s credibility generally focus on the expert’s qualifications, bias, methodology, reasoning, or the adequacy of the facts upon which an opinion is based. Those attacks are relevant to the specific opinion about which the expert testifies.

Attacks on credibility should not be confused with attacks on character. As a general rule, the credibility of any witness, including an expert, can be attacked with evidence that the witness is not a trustworthy individual, but only if the evidence is relevant to the witness’ truthfulness.

Under the federal rules, evidence of bad character is not generally relevant to credibility unless it relates to a character for truthfulness. If a white supremacist testifies against a nonwhite party, for example, the witness’ repugnant views about race would be relevant to establish his or her bias, but not to prove that the witness is a bad person and therefore unworthy of belief.

The fact that an expert holds unorthodox beliefs about subjects that are unrelated to the expert’s testimony will not usually be relevant to an expert’s truthfulness or bias. It is difficult to understand how an expert’s beliefs about vaccinations in a case that has nothing to do with vaccinations is relevant to the expert’s trustworthiness.

The lesson to learn from ProPublica’s conflation of Dr. Ayoub’s writings about vaccinations and his expert testimony about bone disease is that lawyers should be prepared to address anything in an expert’s background that may prove to be controversial. If an opposing party might attack an expert’s credibility at trial because of controversial opinions that are unrelated to the expert’s testimony, lawyers may want to raise that issue prior to trial by seeking an order barring cross-examination about irrelevant matters that have no bearing on the expert’s truthfulness.