Cognitive biases are a part of the human condition. No one, not even the most rigorous academic scientists, [is] immune from them. The biases can come in all sorts of forms that are well researched by social scientists. Some of the most prominent, and the ones that are most problematic in the forensic science world, include confirmation bias, role effects, and context bias. The more subjective a judgment is, the more likely it is to be tainted by the kinds of biasing information that leads to these cognitive distortions.

Unfortunately, most of the pattern-matching forensic disciplines are highly subjective. Even in much more objective scientific inquiries, academic and laboratory scientists know that the risk of unintentional cognitive biases demands that they conduct their work in a double-blind manner — so that they are shielded from as much potentially biasing information as possible. But almost uniquely among those who claim to be doing scientific work, forensic scientists do their work in an environment in which little, or often no, attempt is made to shield the examiner from biasing information. Routinely, evidence submissions from law enforcement to the laboratories tell the examiner who the suspect is, what other evidence there is against the suspect and even law enforcement’s desired outcome (to link the evidence to the suspect). Contamination with contextual information is the norm.

Medical examiners, along with child-abuse pediatricians and other physicians called upon to render opinions about cause and manner of death or injury, are particularly vociferous in insisting that they must have all the context information, because it helps inform their determinations about what happened. And, certainly, such information can be useful to physicians when performing their investigative roles as a part of a multidisciplinary child abuse team, or as a medical examiner required by statute to determine cause and manner of death for the death certificate.

But the physician’s role is quite different when she enters the courtroom to render an expert opinion. The jury is the ultimate fact-finder in the courtroom, and the jury has full capacity to consider all the nonmedical evidence in the case. Although most courts today permit medical experts to go beyond this, the medical expert’s role should be just to offer opinions about those things the jury cannot understand on its own — medical findings, like the presence of a subdural hematoma, or the presence of gunshot entry wounds, and any medically validated opinions about the kinds of things that could have caused those medical findings.

The expert goes too far when she offers opinions based on nonmedical evidence (such as the presence of a suicide note, or the fact that the child’s caregiver was not the child’s father and was an unhappy babysitter) related to matters that are not medical questions — like whether some third party inflicted the injuries observed by the physician, and whether that person must have done so with intent to kill or with recklessness. And while courts routinely permit physicians to exceed these limits, some courts and legal authorities are finally beginning to take note that such broad testimony, based on biasing information, on matters that exceed true medical diagnosis, goes too far.