# What's the term for this logical fallacy? **URL:** \ **Category:** Factual Questions\ **Created:** [April 10, 2016, 9:51pm UTC](https://boards.straightdope.com/t/whats-the-term-for-this-logical-fallacy/751831 "2016-04-10T21:51:23Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![dstarfire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dstarfire/32/5762_2.png) [@dstarfire](https://boards.straightdope.com/u/dstarfire)\ **Post date:** [April 10, 2016, 9:51pm UTC](https://boards.straightdope.com/t/whats-the-term-for-this-logical-fallacy/751831/1 "2016-04-10T21:51:23Z")
Most people, when they hear, read, or whatever, that all members of group A are members of group B, will tend to assume (wrongly) that all members of group B are also members of group A. For example, many years ago it was true that most criminals in American prisons were black. A lot of people thought this meant that most blacks were criminals. This fallacy is easily countered simply by creating a Venn diagram of the two groups. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [April 10, 2016, 10:08pm UTC](https://boards.straightdope.com/t/whats-the-term-for-this-logical-fallacy/751831/2 "2016-04-10T22:08:39Z")
I don’t know that it has a specific name. Broadly it’s just an inaccurate generalisation. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [April 10, 2016, 10:22pm UTC](https://boards.straightdope.com/t/whats-the-term-for-this-logical-fallacy/751831/3 "2016-04-10T22:22:33Z")
Affirming the Consequent. ---
**Author:** ![Sage\_Rat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sage_rat/32/399_2.png) [@Sage\_Rat](https://boards.straightdope.com/u/Sage_Rat)\ **Post date:** [April 10, 2016, 10:23pm UTC](https://boards.straightdope.com/t/whats-the-term-for-this-logical-fallacy/751831/4 "2016-04-10T22:23:53Z")
> **[Anchor](https://en.wikipedia.org/wiki/Anchoring)** > > An anchor is a device, normally made of metal, used to secure a vessel to the bed of a body of water to prevent the craft from drifting due to wind or current. The word derives from Latin ancoracode: lat promoted to code: la , which itself comes from the Greek ἄγκυραcode: ell promoted to code: el (ankȳracode: ell promoted to code: el ). > Anchors can either be temporary or permanent. Permanent anchors are used in the creation of a mooring, and are rarely moved; a specialist service is normally ... > **[Essentialism](https://en.wikipedia.org/wiki/Essentialism)** > > Essentialism is the view that objects have a set of attributes that are necessary to their identity. In early Western thought, Plato's idealism held that all things have such an "essence"—an "idea" or "form". In Categories, Aristotle similarly proposed that all objects have a substance that, as George Lakoff put it, "make the thing what it is, and without which it would be not that kind of thing". The contrary view—non-essentialism—denies the need to posit such an "essence'". > Essentialism has ... > **[Illusory correlation](https://en.wikipedia.org/wiki/Illusory_correlation)** > > In psychology, illusory correlation is the phenomenon of perceiving a relationship between variables (typically people, events, or behaviors) even when no such relationship exists. A false association may be formed because rare or novel occurrences are more salient and therefore tend to capture one's attention. This phenomenon is one way stereotypes form and endure. Hamilton & Rose (1980) found that stereotypes can lead people to expect certain groups and traits to fit together, and then "Illusor... > **[Stereotype](https://en.wikipedia.org/wiki/Stereotype)** > > In social psychology, a stereotype is a generalized belief about a particular category of people. It is an expectation that people might have about every person of a particular group. The type of expectation can vary; it can be, for example, an expectation about the group's personality, preferences, appearance or ability. Stereotypes are sometimes overgeneralized, inaccurate, and resistant to new information, but can sometimes be accurate. > While such generalizations about groups of people may... > **[Zero-risk bias](https://en.wikipedia.org/wiki/Zero-risk_bias)** > > Zero-risk bias is a tendency to prefer the complete elimination of risk in a sub-part over alternatives with greater overall risk reduction. It often manifests in cases where decision makers address problems concerning health, safety, and the environment. Its effect on decision making has been observed in surveys presenting hypothetical scenarios. > Zero-risk bias is based on the way people feel better if a risk is eliminated instead of being merely mitigated. Scientists identified a zero-risk bia... ---
**Author:** ![TATG](https://avatars.discourse-cdn.com/v4/letter/t/50afbb/32.png) [@TATG](https://boards.straightdope.com/u/TATG)\ **Post date:** [April 11, 2016, 2:29am UTC](https://boards.straightdope.com/t/whats-the-term-for-this-logical-fallacy/751831/5 "2016-04-11T02:29:26Z")
Illict Conversion; see [Hurley](https://books.google.co.uk/books?id=dTx-BAAAQBAJ&pg=PT179&lpg=PT179&dq=%22all+a+are+b+therefore+all+b+are+a%22&source=bl&ots=aIj-N4ByI7&sig=oF1HmcLUaj9L0QoBhHW391yOV4w&hl=en&sa=X&ved=0ahUKEwjE5Jefw4XMAhXF5xoKHS3fCvAQ6AEIIjAC#v=onepage&q=%22all%20a%20are%20b%20therefore%20all%20b%20are%20a%22&f=false) and the [fallacy files](http://www.fallacyfiles.org/illiconv.html) for more info. Well, that is the name of the fallacy you give in your first sentence. The second sentence switches to using “most” instead of “all”. ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [April 11, 2016, 7:34am UTC](https://boards.straightdope.com/t/whats-the-term-for-this-logical-fallacy/751831/6 "2016-04-11T07:34:50Z")
> [@Bear\_Nenno](#): > > Affirming the Consequent. > [@TATG](#): > > Illict Conversion These are the same, or the first is a subset of the second but either applies in the OP’s cases. I think. ---
**Author:** ![TATG](https://avatars.discourse-cdn.com/v4/letter/t/50afbb/32.png) [@TATG](https://boards.straightdope.com/u/TATG)\ **Post date:** [April 11, 2016, 8:10am UTC](https://boards.straightdope.com/t/whats-the-term-for-this-logical-fallacy/751831/7 "2016-04-11T08:10:20Z")
> [@MrDibble](#): > > These are the same, or the first is a subset of the second but either applies in the OP’s cases. I think. No, they aren’t. (Nor is the first a subset of the second. I also don’t think this is an instance of affirming the consequent.) ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [April 11, 2016, 9:27am UTC](https://boards.straightdope.com/t/whats-the-term-for-this-logical-fallacy/751831/8 "2016-04-11T09:27:29Z")
> [@TATG](#): > > No, they aren’t. (Nor is the first a subset of the second. I also don’t think this is an instance of affirming the consequent.) You might be right. ---
**Author:** ![DrDeth](https://avatars.discourse-cdn.com/v4/letter/d/b487fb/32.png) [@DrDeth](https://boards.straightdope.com/u/DrDeth)\ **Post date:** [April 11, 2016, 6:04pm UTC](https://boards.straightdope.com/t/whats-the-term-for-this-logical-fallacy/751831/9 "2016-04-11T18:04:24Z")
Remember to keep a true or\* formal\* fallacy separated from a\* informal\* or debating-club 'fallacy" . For example, “appeal to authority” is perfectly OK (outside of a high school debating club) assuming said authority really is a expert on the subject. Einstein on physics is one thing, Einstein on Capital Punishment is another. Many "informal fallacies’ are not fallacies at all, just weak arguments. However, in the OP case,\* Affirming the consequent\* is a\* formal logical fallacy. \* ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [April 11, 2016, 6:23pm UTC](https://boards.straightdope.com/t/whats-the-term-for-this-logical-fallacy/751831/10 "2016-04-11T18:23:16Z")
> [@Bear\_Nenno](#): > > Affirming the Consequent. Agreed: it’s “[Affirming the Consequent](https://en.wikipedia.org/wiki/Affirming_the_consequent),” a.k.a. the “fallacy of the converse.” ETA: This is in response to the OP’s first sentence. The OP’s second paragraph, which substitutes “most” for “all,” isn’t quite the same thing. ---
**Author:** ![TATG](https://avatars.discourse-cdn.com/v4/letter/t/50afbb/32.png) [@TATG](https://boards.straightdope.com/u/TATG)\ **Post date:** [April 11, 2016, 6:42pm UTC](https://boards.straightdope.com/t/whats-the-term-for-this-logical-fallacy/751831/11 "2016-04-11T18:42:20Z")
> [@Thudlow\_Boink](#): > > Agreed: it’s “[Affirming the Consequent](https://en.wikipedia.org/wiki/Affirming_the_consequent),” a.k.a. the “fallacy of the converse.” Do you think “all A are B, thus all B are A” is an instance of affirming the consequent? The same question goes out to all the other supporting this position. ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [April 11, 2016, 7:00pm UTC](https://boards.straightdope.com/t/whats-the-term-for-this-logical-fallacy/751831/12 "2016-04-11T19:00:31Z")
> [@TATG](#): > > Do you think “all A are B, thus all B are A” is an instance of affirming the consequent? Yes I do, since “All A are B” is equivalent to “If it’s A, then it’s B.” ---
**Author:** ![TATG](https://avatars.discourse-cdn.com/v4/letter/t/50afbb/32.png) [@TATG](https://boards.straightdope.com/u/TATG)\ **Post date:** [April 11, 2016, 7:07pm UTC](https://boards.straightdope.com/t/whats-the-term-for-this-logical-fallacy/751831/13 "2016-04-11T19:07:55Z")
> [@Thudlow\_Boink](#): > > Yes I do, since “All A are B” is equivalent to “If it’s A, then it’s B.” OK, so would I be right in thinking that you think that “if p then q, thus if q then p” is an instance of affirming the consequent? I don’t, and as far as I can tell, your citation doesn’t say that it is. ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [April 11, 2016, 8:01pm UTC](https://boards.straightdope.com/t/whats-the-term-for-this-logical-fallacy/751831/14 "2016-04-11T20:01:50Z")
> [@TATG](#): > > OK, so would I be right in thinking that you think that “if p then q, thus if q then p” is an instance of affirming the consequent? I don’t, and as far as I can tell, your citation doesn’t say that it is. Does [this cite](http://changingminds.org/disciplines/argument/fallacies/affirming_consequent.htm) make it any clearer? ---
**Author:** ![TATG](https://avatars.discourse-cdn.com/v4/letter/t/50afbb/32.png) [@TATG](https://boards.straightdope.com/u/TATG)\ **Post date:** [April 11, 2016, 8:13pm UTC](https://boards.straightdope.com/t/whats-the-term-for-this-logical-fallacy/751831/15 "2016-04-11T20:13:24Z")
“If p then q, q, thus p” is not the same thing as “if p then q, thus if q then p”. I see why people think they are related, but they really aren’t the same (mistaken) form of argument. ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [April 11, 2016, 8:28pm UTC](https://boards.straightdope.com/t/whats-the-term-for-this-logical-fallacy/751831/16 "2016-04-11T20:28:04Z")
> [@TATG](#): > > “If p then q, q, thus p” is not the same thing as “if p then q, thus if q then p”. I see why people think they are related, but they really aren’t the same (mistaken) form of argument. They’re certainly logically equivalent. I’m assuming that “If p then q, q, thus p” means “((p -\> q) ^ q) -\> p” and that “if p then q, thus if q then p” means “(p -\> q) -\> (q -\> p)” To me, they’re the same logical fallacy: assuming (wrongly) that the converse of a true statement must be true. Anybody else want to weigh in? ---
**Author:** ![TATG](https://avatars.discourse-cdn.com/v4/letter/t/50afbb/32.png) [@TATG](https://boards.straightdope.com/u/TATG)\ **Post date:** [April 11, 2016, 8:41pm UTC](https://boards.straightdope.com/t/whats-the-term-for-this-logical-fallacy/751831/17 "2016-04-11T20:41:06Z")
> [@Thudlow\_Boink](#): > > They’re certainly logically equivalent. They are logically equivalent to a whole lot of things. > [@Thudlow\_Boink](#): > > I’m assuming that “If p then q, q, thus p” means “((p -\> q) ^ q) -\> p” > and that “if p then q, thus if q then p” means “(p -\> q) -\> (q -\> p)” In this context I don’t think one should be invoking the deduction theorem to equate “if,then” with “therefore”. Compare, modus tollens “if p then q, not-q, thus not-p” to one direction of contraposition “if p then q, then, if not-q then not-p”. By the deduction theorem, modus tollens is equivalent to “if p then q, thus if not-q then not-p”. Again by the deduction theorem, this is equivalent to “if p then q, then, if not-q then not p”. So by this reasoning modus tollens is just one direction of contraposition. To equate forms by invoking strings of reasoning in this manner seems to me a mistake. Affirming the consequent is a formal fallacy, and that means the precise form matters. If we want to explain a mistake one may point out the relationship to affirming the consequent. In this case this is useful. But as a formal fallacy, they are not the same thing (after all, they aren’t the same form!) ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [April 11, 2016, 9:01pm UTC](https://boards.straightdope.com/t/whats-the-term-for-this-logical-fallacy/751831/18 "2016-04-11T21:01:54Z")
> [@TATG](#): > > Affirming the consequent is a formal fallacy, and that means the precise form matters. If we want to explain a mistake one may point out the relationship to affirming the consequent. In this case this is useful. But as a formal fallacy, they are not the same thing (after all, they aren’t the same form!) Okay, I’m inclined to concede that you’re correct. > [@dstarfire](#): > > For example, many years ago it was true that most criminals in American prisons were black. A lot of people thought this meant that most blacks were criminals. Now this, the OP’s second paragraph, looks to me like [Confusion of the Inverse](https://en.wikipedia.org/wiki/Confusion_of_the_inverse). That’s described in terms of conditional probabilities (specifically, confusing “the probability of A, given B” with “the probability of B, given A”). Is confusing “the proportion of A’s who are B” with “the proportion of B’s who are A” essentially the same thing?