Arguments from personal incredulity versus inductive arguments from sampling
April 16, 2013
All I want to do in this post is make people aware that there is a difference between these two things, and occasionally that affects those of us who work in natural history.
In one of his books or essays, Stephen Jay Gould made the point that in natural history we are usually not dealing with whether phenomena are possible or not, but rather trying to determine their frequency. If we find that in a particular population of quail most of the birds eat ants but some avoid them, then we know some things: that quail can tolerate eating ants, that quail are not required to eat ants, and that both strategies can persist in a single population.
This idea has obvious repercussions for paleoart, especially when it comes to “long-tail” behaviors. I dealt with that in this post, and also in the comment thread to this one. But that’s not what I want to talk about today.
Sometimes it is useful to talk about things that never happen, or that have at least never occurred in the sample of things we know of. Obviously how certain you can be in these cases depends on the intensity of sampling and the inherent likelihood of a surprising result, which can be hard to judge. If you argued right now that T. rex lacked feathers because no T. rex specimens have been found with feathers, you’d most likely be wrong; it is almost certainly just a matter of time before someone finds direct evidence of feathers in T. rex, given the number of T. rex specimens waiting to be found and the strength of the indirect evidence (e.g., phylogenetic inference, analogy: ornithomimids are known to be feathered even though most specimens are found without feather impressions). If you argue that sauropods are unique among terrestrial animals in having necks more than five meters long, you’re most likely right; being wrong would imply the existence of some as-yet undiscovered land animal of sauropod size, or with seriously wacky proportions (or both), and our sampling of terrestrial vertebrates is good enough to make that extremely unlikely.
The reason for this post is that sometimes people confuse that last argument, which is about sampling and induction, with the argument from personal incredulity.
For example, in our no-necks-for-sex paper (Taylor et al. 2011), we included this passage:
Sauropoda also had a long evolutionary history, originating about 210 million years ago in the Carnian or Norian Age of the Late Triassic, and persisting until the end-Cretaceous extinction of all non-avian dinosaurs about 65 millions years ago. Thus the ‘necks-for-sex’ hypothesis requires that this clade continued to sexually select for exaggeration of the same organ for nearly 150 million years, a scenario without precedent in tetrapod evolutionary history.
One of the reviewers argued that we couldn’t include that section, because it was just the argument from personal incredulity writ large, like so:
There are no other known cases of X in tetrapod evolutionary history, and therefore we don’t believe that the case in question is the sole exception.
…with the second part of that unstated (by us) but implied. But we disagreed, and argued (successfully) that it was an argument based on sampling, like so:
There are no other known cases of X in tetrapod evolutionary history, and therefore it is unlikely that the case in question is the sole exception.
Now, it is perfectly fair to criticize arguments like that based on the thoroughness of the sampling and the likelihood of exceptions, as discussed above for T. rex feathers. Just don’t mistake arguments like that for arguments from personal incredulity.* On the flip side, if someone makes an argument from personal incredulity, see if the same thing can be restated as an argument about sampling. Maybe they’re correct but just expressing themselves poorly (“I refuse to believe that the moon is made out of cheese”), and maybe they’re wrong and restating things in terms of sampling will help you understand why.
* If you want to get super pedantic about it, they’re both arguments from ignorance. But one of them is at least potentially justifiable by reference to sampling. Absence of evidence is not necessarily evidence of absence, but it may get to be that way as the sampling improves (e.g., there is no evidence of planets closer to the sun than Mercury, and at this point, that is pretty persuasive evidence that no such planets exist).
Parting shot: one thing that has always stuck in my head from Simberloff (1983) is the bit about imagining a large enough universe of possible outcomes. And I’ve always had a perverse fascination with Larry Niven’s “Down in Flames”, in which he pretty much demolished his Known Space universe by assuming that every basic postulate of that universe was false. Neither of these follow directly on from the main point of the post, but they’re not completely unrelated, either. Because I think that they yield a pretty good heuristic for how to do science: imagine what it would take for you to be wrong–imagine a universe in which you are wrong–and then go see if the thing that makes you wrong, whatever it is, can be shown to exist or to work. If not, it doesn’t mean you’re right, but it means you’re maybe less wrong, which, if we get right down to it, is the best that we can hope for.
The photos have nothing to do with the post, they’re just pretty pictures from the LACM to liven things up a little.
References
- Simberloff, D. (1983). Competition theory, hypothesis-testing, and other community ecological buzzwords. The American Naturalist, 122(5), 626-635.
- Taylor, M. P., Hone, D. W., Wedel, M. J., & Naish, D. (2011). The long necks of sauropods did not evolve primarily through sexual selection. Journal of Zoology, 285(2), 150-161.
Sauropod Vertebra Picture of the Week 
April 16, 2013 at 10:52 pm
We do know that tyrannosaurids had scaley skin, because skin impressions have been found. Therefore, T. rex had scales not feathers.
April 16, 2013 at 11:16 pm
Ooh, ooh, let me play!
IOW, you fail at logic.
To put it another way: tyrannosaurid skin impressions do not cover the entire animal, nor do we have a complete set of skin suits covering every ontogenetic stage. I never said that T. rex had feathers over every square mm of its body for its entire life. The circumstantial evidence that T. rex had at least some feathers somewhere on its body at some point in its life is freakin’ phenomenal.
But do feel free to rebut.
April 17, 2013 at 5:43 am
Pedantic ecologist moment:
“If we find that in a particular population of quail most of the birds eat ants but some avoid them, then we know some things: that quail can tolerate eating ants, that quail are not required to eat ants, and that both strategies can persist in a single population.”
The most we can say without long-term monitoring is that quail sometimes do not eat ants. It doesn’t say anything about whether that behavior is sustainable in the long run.
As you’re well aware, animals like to experiment!
April 17, 2013 at 5:51 am
The most we can say without long-term monitoring is that quail sometimes do not eat ants. It doesn’t say anything about whether that behavior is sustainable in the long run.
Surely we can’t say whether either eating ants or or not eating ants is sustainable. Maybe they have to eat ants now and then to get some otherwise unobtainable nutrient, and maybe they have to lay off now and then to avoid overloading on some toxin carried by the ants.
Anyway, you’re right, I should have said, “quail can tolerate eating ants for a while, but they’re not required to eat ants all the time”, or something to that effect.
April 17, 2013 at 6:56 am
Nice post, widely applicable. Tracy was really quick to confuse there-exists with for-all in the comments: there exists T. rex skin for which the skin has scales; therefore for all T. rex skin, the skin has scales. Nope.
One quibble and one clarification:
I think you can make a much stronger statement than that. The induction argument is not from ignorance, it’s from observations. In other words, we have data (which is about our failure to observe a thing despite having had opportunities) and from that data we draw our conclusion.
And then there’s this:
I hate this quote. It’s snappy, but it’s just plain wrong. Absence of evidence is evidence of absence. It just isn’t proof, and like all evidence can be countered and overwhelmed by contrary evidence. Each additional fossilised patch of T. rex skin that we find is more evidence for its not having had feathers. But no amount of such specimens would constitute proof, and contrary evidence (phylogenetic and analogous) stands against our absence-of-evidence evidence. As always, we have to weigh up both sides of the case. That’s what we do.
April 17, 2013 at 7:06 am
And then there’s this:
I hate this quote. It’s snappy, but it’s just plain wrong.
Umm…that’s why I didn’t put it in the post. Or are you ranting about its normal usage outside the post? Because what I said is pretty close to what you just said.
April 17, 2013 at 7:15 am
I think you can make a much stronger statement than that. The induction argument is not from ignorance, it’s from observations.
When I said super pendantic, I really meant SUPER pedantic, as in, philosphers consider what we’re both talking about to be a species of the argument from ignorance. As explained here. To be clear, I don’t disagree with you at all on the observation-based-ness of the induction argument. Most of the time, induction is what we have to work with.
April 17, 2013 at 10:23 am
Dud, it was no part of my goal to criticise your post. I was taking the opportunity to kick out tangentially against this all-too-common mantra.
April 18, 2013 at 2:43 pm
Interesting post. Arguments from personal incredulity are often the starting point for many people when confronted with new ideas and sometimes we can never progress past it. Their first reaction to a new (or different) idea is – No! “I just don’t believe it” – to borrow a saying from a well-known TV show. Sometimes we can get pass this initial emotional response by asking for some logical reasoning but for some people even this is too much. It’s very interesting why people believe what they do and why they seem to have difficulty thinking logically about new ideas and progressing to the next level of understanding. Perhaps the most interesting aspect is that individuals who always follow the crowd will never encounter any problems and so they can argue that there isn’t a problem. Unfortunately if everyone did that we wouldn’t have had the revolutions of Copernicus, Galileo, Newton, Hutton, Wegener etc, etc who all encountered this same predicament.
April 20, 2013 at 1:36 am
Matt,
In the example, when one claims that scales in a patch of tyrannosaur skin disproves the presence of filamentous integument, we can simply say that all that can be said is that THAT ANIMAL lacks filamentous integument on that body part at the time of impression. We DEDUCE the likelihood that this integumental morphology is true at other stages of its life by reference to multiple animals, multiple ontogenetic stages, multiple regions of preservation, but also reference to skin patches from other animals that suggest a generalization of this condition; but in the absence of this, we can say nothing more can, should, or is likely to be true.
Bonaparte noted the presence of skin in a specimen of Carnotaurus sastrei, extensive and apparently quite knobbly and wrinkly, not like it was covered in filaments. Skin patches assumed to belong to Tyrannosaur rex, or associated with bones belonging to that taxon, have been suggested, but not described, whereas we have filamentous integument preserved as a halo around the skeleton of a purported basal tyrannosauroid, another less-basal tyrannosauroid, and then further around a purported basal carnosaur, a so-called basal megalosaur (I dislike assuming phylogeny for juvenile animals), and of course a basal coelurosaur. We have argued this suggests a phylogenetic initiation of megalosaur and onward (at least) of filamentous integument throughout Theropoda, but which larger animals may not preserve well — or at all.
April 20, 2013 at 5:49 am
I agree with all of that.
I also note that once a tetrapod lineage evolves sophisticated integumentary structures like hair or feathers, they’re almost never completely lost. There are no birds that lack feathers entirely, and if there are any truly hairless mammals, they’d have to be among the cetaceans. Among the big terrestrial mammals that are relevant to the T. rex discussion, elephants and rhinos still have hair, they’re just not hairy.
I wonder if this discussion got off on the wrong foot because people thought I was arguing that T. rex was feathery, rather than arguing that it had feathers. Because I’m virtually certain of the latter, and not at all certain of the former (I’m not against it, either).
What would a fossilized integumentary trace from the flank of a horse look like? I know it wouldn’t look like pebbly skin or anything, but I wonder if the hairs would even show up, or if it would look more like smooth skin.
April 21, 2013 at 4:02 am
Unsure if this is the proper thread for this, but given your interests in ‘unexpected behaviors’ and how they relate to paleoart, you might appreciate this:
Account of a leopard dying due to a failed predation attempt on a domestic cat. Leopard kills the cat, and cat kills the leopard.