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Only a fraction of psychological tests used in court may be valid

One of the biggest problems facing criminal defendants may be the use of bad expert testimony. Courts are supposed to be gatekeepers for scientific claims. They're supposed to exclude any technical or scientific evidence that isn't "the product of reliable principles and methods" or that isn't supported by the majority of scientists in the given field.

Unfortunately, judges aren't experts in every field and often don't keep out questionable evidence if it involves science. They're supposed to sift out the junk from the reliable science and only allow the reliable evidence, but that doesn't always happen. This allows people to be convicted based in part on junk science.

Recently, a group of researchers published a study in the journal Psychological Science in the Public Interest. They examined 876 American court cases that took place between 2016 and 2018 which included psychological tests as evidence. They considered hundreds of different psychological tests used in these cases and looked into how reliable they were considered by the psychological community.

Researchers: Courts flooded with pseudo psychology

There was some good news. The most common psychological test used in those cases was the Minnesota Multiphasic Personality Inventory, which enjoys wide acceptance in the field.

Unfortunately, the second most common was the Rorschach "ink blot" test developed in 1921. That's troubling because, while the test has some defenders, most psychologists consider it unscientifically ambiguous and subjective.

Of the tests used in the 876 cases the researchers reviewed, a third had never been reviewed in professional literature at all and therefore could not live up to the basic standard required for admission as evidence. Of the two-thirds that had been reviewed, only 40% were found to be generally accepted in the field as shown by literature reviews. Almost 1/4 of the tests that had been reviewed were considered unreliable.

That translates to a huge number of psychological tests that have been admitted as evidence but that do not meet the legal requirements for admission as evidence.

Assuming those 876 court cases are representative of general practice, many crucial decisions are being made based on faulty psychological testimony. This needs to stop.

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