Trial by expert: when science comes into the courtroom, only the experts know what to make of it

Mr. Wilson is a professor at UCLA, and the author of Moral Judgment, to be published in May by Basic Books, a division of HarperCollins, from which this article is adapted. Copyright (symbol) 1997 by James Q. Wilson.

For several decades the courts have increased dramatically the extent to which they admit testimony by people claiming to be experts -- claiming, that is, that they can make statements that are not easily disproved about the sources of human behavior. Much of this new testimony has been the inevitable result of matters entering the courtroom -- such as the effect of a drug on a patient or the precision of DNA testing in identifying blood -- that once were not matters of scientific clarity at all.

Under the rules governing federal courts a judge may allow testimony from an "expert" -- defined (quite loosely) as anyone "qualified" by "knowledge, skill, experience, training, or education" -- if the judge believes that testimony will assist the judge or jury in understanding the evidence. Lay witnesses in a trial can testify only about what they saw or heard. But when experts testify, they may give their opinion on a matter.

Presumably the expert opinion reflects real knowledge and not mere opinion. At one time the federal courts followed a rule -- known as the Frye rule, after the case in which it was announced in 1923 --that required an expert to be someone who could show that what he or she was saying had survived the test of "general acceptance in the particular field in which it belongs." Many state courts continue to use some version of the Frye test.

"General acceptance" is a useful test, helpful in distinguishing real experts from fanciful quacks. It has governed many state and federal appellate court decisions. But even courts that use this test may use it in a way that lets in questionable expert evidence. The California Supreme Court in 1989 decided that, though it was bound by the "general acceptance" rule, it would let a psychologist testify in support of the innocence of persons charged with repeated acts of sexual perversion against children. The psychologist, who had been barred from the trial on the basis of the Frye test, wanted to say that after having given two written tests to a defendant in her jail cell and talking to her for about two hours, he was confident she had a "normal personality" and is "is falsely charged in this matter." Based on his tests, he found her innocent. In reaching this conclusion, he could cite no studies that proved that the written tests could tell the difference between child abusers and normal people. The test -- the Minnesota Multiphasic Personality Inventory (MMPI) -- is a widely used and scientifically valuable test, but it cannot possibly ascertain the guilt or innocence of a person, and was never designed to do so. It estimates, but cannot fully depict, someone's personality. And even if it estimates it accurately, an individual's behavior is driven by many forces -- an emotional crisis, the influence of others, the consumption of drugs, the prospect of easy gains, a belief in secrecy -- in addition to personality. But the Court ordered a new trial in which this "data" would be discussed.

Despite the liberties taken even under the Frye test, the Supreme Court has frequently looked for an even more generous rule. Since the mid-1970s the Court has relied on its own Federal Rules of Evidence which offer much broader support for alleged experts. By Rule 702, an expert is anyone who is qualified by "knowledge, skill, experience, training, or education" to offer a useful opinion. Read literally, this could qualify almost anyone who claims to have knowledge and can produce some bits of personal history to show that they have acquired it by some means.

This rule, along with the laxity with which some states have applied the Frye test, has contributed to the explosion of expert testimony and made it far easier for attorneys to enlist their own advocates whose qualification may be little more than having an advanced degree. There is no lawyer worth his hire who cannot find an expert to testify in a way that will help his client, even though the opposing lawyer has his own expert testifying with equal assurance on the other side.

In 1993 the Supreme Court tried to set some boundaries around this proliferation by issuing an opinion that seemed to establish some principles that would define expertise beyond the expert's claimed education or training. These limits were drawn from the standards of scientific discourse. The court rejected the old Frye test, and instead has urged courts to apply the usual standards that scientists use in deciding what statements to believe: Has it withstood falsification? Does the claim have a low rate of error? Was it published in a scientific journal?

These are admirable sentiments, but they are stated in rather general terms. How do you apply these scientific customs to the reality of legal arguments in real courts? The Court was not very clear on this except to urge judges and lawyers to argue about it: "Vigorous cross-examination, presentation of contrary evidence, and careful instruction on the burden of proof" will tell the jury whom to believe and whom to ignore. In short, lawyers, guided by judges, can settle expert arguments.