Genealogical fallacies in logic

When I was in high school I took a course called “Theory of Knowledge.” I may have enjoyed this course more than any other course I had ever taken up to that point and since (right up until I took Course 4 at the Institute of Genealogy and Historical Research).

One of the course units dealt with classical logic, and logical fallacies. I recently found a handout on “Logical Fallacies” that I received in the course. Sadly I cannot produce a “full and accurate” citation to this handout, as I can’t remember details from nearly twenty years ago.

Much of scholarly genealogical research involves the use of logic–deductive and inductive reasoning. Many of the errors that occur in research similarly result in fallacies of logic. I would like to discuss the most common fallacies, as described on this twenty-year-old high school handout, here.

First the definition of a fallacy:

Fallacies are mistakes in reasoning.

A fallacy is an error in reasoning or in an argument. As such they are not rational in the true sense of the word. Most people who commit fallacies do so out of carelessness or haste. Many people who use fallacies are trying to side-step pure reasoning and manipulate others through psychological techniques. There are hundreds of different fallacies, but a few are listed below:

Not only does this definition address fallacies in a simplified way, it also demonstrates that many of the most common genealogical mistakes are directly caused by fallacies. The small selection of fallacies that I have extracted from the handout for this blog post below further demonstrate this connection between genealogical error and poor reasoning:

1. Hasty generalization – Drawing a conclusion (inference) on insufficient evidence.

2. Dicto simpliciter – Not taking genuine exceptions into account.

. . .

5. Circular reasoning – Going around in a circle – using the premise as the conclusion and vice versa – sometimes called “begging the question.”

6. False cause – supposing that two things that are connected by coincidence somehow cause each other.

. . .

8. Ad ignorantium – drawing a conclusion with no evidence one way or the other.

. . .

11. Appeal to authority – trying to get your position accepted because an authority says so when the authority is irrelevant to the issue.

12. Composition – Assuming what is true of the parts is true of the whole and vice – versa – what is true of the whole is true of the parts.

To demonstrate some of these fallacies, let us examine a few scenarios a genealogist might come across:

1. Hasty Generalization. Susan Smith, age 12 years, appears in the 1860 federal census in the household of Richard Smith, age 36 years. The fallacious conclusion would be that Susan Smith is the daughter of Richard Smith, with no further evidence to support it. The 1860 federal census does not provide family relationships, so any conclusion involving family relationships based on this evidence constitutes a fallacious argument.

2. Dicto simpliciter. Alexander Brown, an African American man, appears in the 1870 federal census in Mississippi. A genealogist falling prey to this fallacy would not even look for Alexander in the 1860 federal census, assuming that he was enslaved until the Civil War. The presence of free people of color in Mississippi (though relatively few) constitute “genuine exceptions.” The possibility that Alexander Brown was free in 1860–perhaps in another locality, migrating as a soldier during the War itself–exists.

8. Ad ignorantium. This fallacy is probably the most rampant in genealogy. Suppose family tradition says that your great-grandfather James Lee was related to Robert E. Lee. No connection is found. Or you great-great-grandmother Beatrice Jones was a “full-blooded American Indian.” Again, no evidence suggests this conclusion. Believing what you want to believe despite a lack of evidence is this fallacy in a nutshell.

11. Appeal to authority. A published family history says that William Mulliken came from  West Virginia. The fallacious conclusion would accept this statement as gospel, without further research. Just because it’s in print–even if the author is a well-respected authority–doesn’t mean its true.

We are all guilty of nearly all of these fallacies at one point or another in our career.

Do you have any other examples that immediately come to mind?

If you would like to cite this post:

Michael Hait, CG, “Genealogical fallacies in logic,”Planting the Seeds: Genealogy as a Profession blog, posted 19 December 2012 ( : accessed [access date]). [Please also feel free to include a hyperlink to the specific article if you are citing this post in an online forum.]


12 thoughts on “Genealogical fallacies in logic

  1. Michael,

    Great topic, but I have some thoughts on some of the fallacies, just because I find rhetoric very interesting.

    1. Hasty Generalization is a jump from “things usually work this way” to “they always work this way” or “therefore it must be so in this case”. In your case, it probably is a hasty generalization but you should identify what is generalized. I would say “the head of household is usually the father of young children with the same last name, so Susan must be Richard’s daughter” would be a hasty generalization. I say this because the key is to understand where the generalization comes in.

    2. Dicto Simpliciter is an odd one and I don’t think your example is of this kind. In fact, I think it’s a hasty generalization. Dicto simpliciter is a fallacy of generalizing from a specific kind to everything. It typically involves applying the assumed rule to things not at all covered, such as failing to look for a record of an African American living in London in 1860 since African American records don’t exist prior to 1865. For the fallacy to be Dicto simpliciter you need to over-generalize to all cases something meant to apply to only some.

    8. Ad Ignorantium can support or reject an hypothesis, but it is essentially proving a point by saying it can’t be disproved. It’s more than belief despite lack of evidence, it is that the very lack of evidence is taken to be proof. This is not the same as “negative evidence”. It isn’t not finding evidence where you expect it. In your case you could argue that nothing has ever been found that says James Lee wasn’t related to Robt E Lee, therefore he must be. Or equivalently, nothing has ever been found connecting James Lee to Robt E Lee, therefore he can’t be related. These are ad ignorantium fallacies.

    Let me add my favorite to the list. I like “Post hoc, ergo propter hoc”, or “After this, therefore because of this”. This is the fallacy that if two events occur in order, the first caused the second. In family history you could find that an ancestor emingrated from Ireland to the United States in the late 1890’s. A fallacy of this type would be saying that he left because of the potato famine. While possible, the famous famine was between 1845 and 1852. I doubt that it would trigger an emigration 40 years later. It may not even have triggered a famine at the time, but there is a tacit assumption that if it was after 1850 it was due to the famine.

    Love your blog!

  2. Michael,

    This is a nice topic, and an important part of identifying and categorizing genealogical errors.

    Wikipedia has a nice article on logical fallacies which can help suggest examples:

    Often there is more than one fallacy at work in an individual example as well.

    Another type of appeal to authority is “conventional propriety” (a form of irrelevant conclusion), i.e. assuming that “people back then just wouldn’t have done that”, when in fact we often see just as many actions back then as now that would be regarded as immoral or unethical according to mainstream religious or civil standards. This can be confused with the tenet of prosopography (as I understand it), that says that the things that people in general did, an individual likely did, but where that mainly applies to the routines and customs of daily life.

    Using your example of incorrectly assuming all the children in an 1850 census record are children of the head of household and his wife, we often find the wife in her mid-50s, and a child of 2 or 3 years of age. While possible, of course the burden is on the genealogist to prove an exception to the general rule of thumb regarding a woman’s child-bearing years. And the likely true explanation is that one of the other daughters over the age of puberty, no matter how tender her years, is actually the mother of that small child.

    A particular series of books I won’t name, and which covers the early settlers of a certain county, is regarded as the “gospel” by many genealogists for that area. And somewhat rightly so because of its very broad scope and the fact that it does contain much useful and true information. Nevertheless the errors it contains are many. The authors interviewed a great many modern descendents, and examined a huge number of original sources at the local courthouse, and of such a wide variety that would normally be admirable (i.e. vital data, land records, court records, etc.).

    But often they will recite a number of deeds and court records that really do not prove anything, and indeed the authors often made no proof argument. So that amounts to a formal fallacy of irrelevant conclusion (the stated relationship/identity), combined with the informal fallacy of proof by verbosity, which leads naive users to commit the fallacy of appeal to authority by taking the unproven assertions in those books as “gospel”.

    The ironic thing is that many “genealogists” who commit fallacies of appeal to authority by relying on old error-ridden genealogies also often will not accept more modern valid proofs involving correlated indirect evidence where no one record states a relationship, especially if that contradicts an older genealogy that utterly failed to prove its assertions.

    • “Proof by verbosity”–I get it, and I love it! Just keep writing stuff til the reader gives up & accepts it just so she can move on, past all the obfuscating verbiage!

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