18 Logical Fallacies in Psychology
Kenneth S. Pope, James N. Butcher, & Joyce Seelen
Practicing psychology requires countless decisions. We think through meanings, options, implications in situations that are complex, diverse, and constantly changing.
We’ve tried to pull together some of the most basic ways that this process of thinking through can go wrong. This list of 18 logical fallacies is by no means comprehensive. We’ve tried to choose those fallacies that seem to thrive, that cause much mischief in our field, and that are unlikely to appear on the “endangered species” list in the psychological literature and in psychological discussions. We’ve all probably fallen for them -- and perhaps used them -- from time to time.
The fallacies are denying the antecedent, composition fallacy, affirming the consequent, division fallacy, golden mean fallacy, appeal to ignorance (ad ignorantium), disjunctive fallacy, false dilemma, mistaking deductive validity for truth, post hoc ergo propter hoc (after this, therefore because of this), red herring, ad hominem, straw person, you too (tu quoque), naturalistic fallacy, false analogy, begging the question (petitio principii), and argument to logic (argumentum ad logicam).
“18 Logical Fallacies, we’ve all probably fallen for them, and used them, from time to time”
The name of each fallacy is followed by a brief description and an example -- often in exaggerated form -- from the area of assessment and testimony.
1) Denying the Antecedent
This fallacy takes the form of:
If x, then y.
Not x.
therefore: not y.
Example: “In my experience, when I’m conducting forensic assessments and the people I’m assessing doesn’t want me to see their healthcare records, it usually means they’re hiding something and I can’t trust them. But this new person I’m evaluating let me see all her healthcare records, so I don’t think she’s hiding anything and I can trust her.”
2) Composition Fallacy
This fallacy takes the form of assuming that a group possesses the characteristics of its individual members.
Example: “Each of these standardized psychological tests is an efficient method of assessment with excellent levels of sensitivity and specificity. An assessment battery comprised of these tests must be an efficient method of assessment with excellent sensitivity and specificity.
3) Affirming the Consequent
This fallacy takes the form of:
If x, then y.
y.
therefore: x.
Example: “Forensic psychologists who are smart, well-prepared, articulate, honest, and well-respected are always in demand. The forensic psychologist my attorney hired for my case is always in demand, so he must be smart, well-prepared, articulate, honest, and well-respected.”
Another example: “If this client is competent to stand trial, he will certainly know the answers to at least 80% of the questions on this standardized test. He knows the answers to 87% of the test questions. Therefore he is competent to stand trial.”
4) Division Fallacy
The division fallacy or decomposition fallacy takes the form of assuming that the members of a group posses the characteristics of the group.
Example: “This MMPI-2 scale shows excellent validity and reliability in differentiating these 2 groups of litigants. Each item on the scale must be answered one way by one group of litigants and another way by the other group.”
5) Golden Mean Fallacy
The fallacy of the Golden Mean (or fallacy of compromise, or fallacy of moderation) takes the form of assuming that the most valid conclusion is that which accepts the best compromise between two competing positions.
Example: “On one hand, I believe that administering an MMPI-2 is the best way to assess this particular forensic client for this particular case. On the other hand, I didn’t schedule enough time in the session to administer an MMPI-2. So the best thing for me to do is administer an abbreviated form of the MMPI-2 that will fit into the available time limitation.”
Alternate example: “The defense expert’s test results suggested that the defendant’s I.Q. is around 80. The prosecution expert’s assessment battery suggested that the defendant’s I.Q. was around 120. The best estimate of the defendant’s I.Q. is therefore around 100.”
6) Appeal To Ignorance (Ad Ignorantium)
The appeal to ignorance fallacy takes the form of: There is no (or insufficient) evidence establishing that x is false.
Therefore: x is true.
Example: “The basic instrument in the test battery I use in custody cases is my own version of the MMPI-2, which I call M-Projective: I ask each parent to pick their favorite MMPI-2 item and then draw a picture based on it. In the 12 years I’ve been using it, there has not been one published study showing that it has any weaknesses in reliability, validity, sensitivity, or specificity. My version of the MMPI-2 is clearly one of the best forensic assessment instruments ever devised for custody evaluations.”
7) Disjunctive Fallacy
This fallacy takes the form of:
Either x or y.
x.
Therefore: not y.
Example: “I can see from my review of the records in this workers’ compensation case that either the employee has become disabled due to her work or else she’s exaggerating her condition. It’s clear that she has sometimes been exaggerating. Therefore, she is not disabled.”
8) False Dilemma
Also known as the “either/or” fallacy or the fallacy of false choices, this fallacy takes the form of only acknowledging 2 (one of which is usually extreme) options from a continuum or other array of possibilities.
Example: “Either I’ll be able to remain calm and answer each cross-examination question clearly and persuasively or else I’m just not a good forensic psychologist.”
9) Mistaking Deductive Validity for Truth
This fallacy takes the form of assuming that because an argument is a logical syllogism, therefore the conclusion must be true. It ignores the possibility that the premises of the argument may be false.
Example: “I just read a book that proves that that book’s author knows the best way to identify malingering. He has a chart showing that every other method can fail sometimes but that his always works. That proves his method is best.”
10) Post Hoc, Ergo Propter Hoc (After this, therefore on account of this)
The post hoc, ergo propter hoc fallacy takes the form of confusing correlation with causation and concluding that because Y follows X, then Y must be a result of X.
Example: “I’ll never forget my first forensic assessment. The person was clearly a malingerer, and had been for years. That’s what the jury decided. From the first time he walked into my office, he never could look me straight in the eye. It taught me quite a lesson: When people are malingering, it prevents them from looking you straight in the eye.”
11) Red Herring
This fallacy takes the form of introducing or focusing on irrelevant information to distract from the valid evidence and reasoning. It takes its name from the strategy of dragging a herring or other fish across the path to distract hounds and other tracking dogs and to throw them off the scent of whatever they were searching for.
Example: “Some of you have objected to the new test batteries that I use in my forensic practice, alleging that they have no demonstrable validity, were not adequately normed for the kind of clients we see from various cultures, and are unusable for clients with physically disabilities. What you have conveniently failed to take into account, however, is that they cost less than a third of the price for the other tests I had been using, are much easier to learn, and can be administered and scored in less than half the time of the tests I used to use.”
12) Ad Hominem
The argumentum ad hominem or ad feminam attempts to discredit an argument or position by drawing attention to characteristics of the person who is making the argument or who holds the position.
Example: “That attorney keeps telling me I should wear a suit when I testify as an expert witness, sit up a little straighter, speak in “plainer” language, and all sorts of stuff that’s just not me. But that attorney doesn’t have a doctorate in psychology, isn’t licensed as a psychologist, never testified as an expert witness, and never studied forensic psychology, so who does she think she is trying to tell me how to do my job?”
13) Straw Person
The straw person, or straw man, or straw woman fallacy takes the form mischaracterizing someone else’s position in a way that makes it weaker, false, or ridiculous.
Example: “The idea that I should look up the validity, reliability, sensitivity, specificity, and so on about each test I administer is the kind of notion that assumes you can know everything about everything.”
14) You Too! (tu quoque)
This fallacy takes the form of distracting attention from error or weakness by claiming that an opposing argument, person, or position has the same error or weakness.
Example: “You accuse me of not telling you the complete truth about myself before you hired me as an expert witness. I was hired. But you’re not the most honest person in the world!”
15) Naturalistic Fallacy
The naturalistic fallacy takes the form of logically deducing values (e.g., what is good, best, right, ethical, or moral) based only on statements of fact.
Example: “The Wechsler instruments are the most commonly used to estimate I.Q. in forensic settings, and no tests have more empirical support. It is clear that they are the right way to estimate I.Q. in forensic settings and they should always be used to estimate I.Q. in a forensic assessment.”
16) False Analogy
The false or faulty analogy fallacy takes the form of argument by analogy in which the comparison is misleading in at least one important aspect.
Example: “I know at least 5 very senior expert witnesses who barely look at the manual when administering one of the psychological tests in their standard forensic battery. It seems foolish for me to waste my time paying much attention to manuals, especially now when I’m just starting and have so many other things to think about when doing some initial forensic assessments.”
17) Begging the Question (Petitio Principii)
This fallacy, one of the fallacies of circularity, takes the form of arguments or other statements that simply assume or re-state their own truth rather than providing relevant evidence and logical arguments.
Examples: Sometimes this fallacy literally takes the form of a question, such as, “Have you stopped using that invalid assessment battery yet?” (The question assumes--and a “yes” or “no” response to the question affirms--that your assessment battery lacks validity.) Sometimes this fallacy takes the form of a statement such as “No one can deny that my new psychological test is the only way to assess potential recidivism.”
18) Argument to Logic (Argumentum ad Logicam)
The argument to logic fallacy takes the form of assuming that a proposition must be false because an argument offered in support of that proposition was fallacious.
Example: “I had started to question whether the method I’ve used for years to assess the presence of brain damage using the MMPI-2 might be wrong, but I just found out that the 3 colleagues who told me that it was an invalid method don’t specialize in MMPI-2 assessments, aren’t up on the MMPI-2 literature, and don’t specialize in neuropsychology, so I think my method is sound after all.”
This excerpt is from The MMPI, MMPI-2, & MMPI-A in Court: A Practical Guide for Expert Witnesses & Attorneys, 3rd Edition by Kenneth S. Pope, PhD, ABPP, James N. Butcher, Ph.D., and Joyce Seelen, J.D., published by APA Books (copyright 2006 by the American Psychological Association). Here are excerpts from two reviews that give a sense of the book’s content and approach:
“The third edition of this essential work by three acknowledged giants in the field achieves the contrasting goals of completeness and practicality. Dealing with new rules (HIPAA), new cases (Daubert v. Merrell-Dow) and new research on psychological testing, this classic reference work superbly prepares everyone from novice to seasoned expert witness for the presentation of psychological evidence in court. The icing on this cake includes excellent appendices, a very useful forensic glossary and extensive references.”
—Thomas G. Gutheil, MD
Professor of Psychiatry,
Harvard Medical School
Past President,
American Academy of Psychiatry and Law
Cofounder,
Harvard Program in Psychiatry and the Law“With its clear, compelling descriptions of the MMPI and its progeny, and expert guidance for management of effective cross-examination, this book is essential to the libraries of forensic psychologists and attorneys who deal with MMPI profiles in court. The reader will be brought abreast of potential uses and misuses of the MMPI in forensic settings, advances in forensic malingering research, and emerging legislation and case law that may bear on admissibility issues. Included is a step by step guide to deposing or cross-examining the expert who is presenting MMPI data. Springing from the authors’ experience, ethical grounding, and wisdom, the material is accessible and eminently useful.”
—Mary Connell, Ed.D., ABPP,
Board Certified in Forensic Psychology
Independent Practice, Fort Worth, Texas
For more information, please see http://kspope.com/assess/mmpi.php. Individual copies may be ordered from the American Psychological Association (1-800-374-2721).
