Absence of evidence is evidence of absence; it just isn’t proof
August 24, 2025
One often hears it said that “absence of evidence is not evidence of absence”. For example, if you excavate some fossil sauropods and they don’t have preserved feathers, that not evidence that sauropods didn’t have feathers.
Oh yes it is.
This is an example of a mantra that’s short, catchy, and wrong. Every time you see absence of evidence, you are accumulating evidence of absence. What you’re not doing is proving absence — not with any single observation, at least.
Here’s an example. You toss a coin, and it comes down heads. You now have evidence of the presence of a head side (hurrah!) but only absence of evidence for a tails side. It’s perfectly likely that the coin has a tails side but merely happened to come up heads. So the absence of evidence for a tails side is certainly not proof that there is not a tails side.
But now suppose you toss the coin ten times, and it comes up heads every time. For a fair coin, there is less than one chance in a thousand of that happening (2^10 = 1024). Now let’s toss it 20 times. If you still see heads every time, you’re seeing a one-in-a-million event (2^20 = 1048576), and if you’re still seeing only heads after 100 tosses, that is a 1-in-1267650600228229401496703205376 event.
By this point, I’m hoping you’d be concluding with reasonable certainly that you had a two-headed coin. What’s happening is that each observation of absence (no tails side) is increasingly your confidence that there is no such side.
Absence of evidence is evidence of absence.
Sauropod Vertebra Picture of the Week 
August 25, 2025 at 6:15 am
This comes across as wrongly pitched. If you have no fossilised sauropod skin then that is an absence of evidence. But you do have fossilised skin and if it does not show evidence of feathers then that is the situation you are describing. There is no real way it can be honestly called absence of evidence.
August 25, 2025 at 10:43 am
Fossilised skin without feathers tells you only that that particular part of the skin didn’t have feathers — just as the scaly legs of chickens don’t tell you that the whole chicken was unfeathered.
August 25, 2025 at 11:07 am
what is the expected prior probability of feathers preserving in situations where sauropods are found?
(I fear nobody really knows, and surely it varies depending on specifics of the formation, the locality, and sometimes even across different parts of the same specimen.)
To the best of my knowledge, there are only a few formations which are known to have yielded both sauropods, and feathers (never (yet) associated with sauropods), which seems to imply the expected prior probability of preserved feathers is very low, so the accumulation of evidence for the absence of feathers on sauropods would be very slow. It’s also very uneven; the handful of partially articulated sauropods from the Yixian, where feathers exceptionally common, would seem to contribute more than sauropods from other formations.
I just glanced through Zhou et al 2018 , a redescription of Liaoningotitan, which is from the Yixian, a Barremian Early Cretaceous formation famous for preservation of feathers. It says there are 3 (including Liaoningotitan) sauropods known from the Yixian. I couldn’t find any mention in it of feathers, scales, filaments, skin, or any kind integument. How much evidence for absence of sauropod feathers does this represent?
ref: https://peerj.com/articles/19154/
August 25, 2025 at 11:31 am
That’s the question. In general, to preserve a sauropod you need a lot of sediment delivered very quickly, and that almost always means coarse-grained sediment of the kind that’s not good for preserving integument. By contrast, all those tiny feathered theropods from Liaoning were small enough to be quickly covered by even a light fall of fine-grained ash, preserving details that we don’t get from most sauropods.
So if we’re going to find the kind of preservation we want in sauropods, our best bet might be something like a baby individual from the Yixian Formation — one that might have had down-like protofeathers and which is preserved well enough to show them. I have heard rumours that a lot more sauropods are known from the Yixian than have been described, because the Chinese scientists with access tend instead to zoom into the beautifully preserved theropods that are more likely to get them the ravenously desired Nature paper that they get directly financially rewarded for. So I think it’s possible that some collection somewhere already holds a “feathered sauropod” (for some value of “feathered”) but it’s never had any attention.
With all that said, yes: Liaoningotitan, Dongbeitian and Ruixinia do represent some non-trivial negative evidence for feathers in sauropods.
August 25, 2025 at 11:45 am
(It’s kind of weird that none of the Liaoningotitan, Dongbeitian or Ruixinia descriptions so much as mentions integument, skin or feathers, even in a negative sense.)
August 25, 2025 at 12:25 pm
Thank you, Mike. That’s the kind of reply I was hoping for.
August 25, 2025 at 5:08 pm
I couldn’t have put it better.
This reminds me of an argument I had with someone who at first opportunity introduced themselves as a paleobiologist or something along those lines. They kept repeating that phrase so much that I abandoned the conversation. I belive they were more of a politician than a scientist. That phrase is mostly used to defend ideas that aren’t supported by any evidence, what politicians do.
The argument was about the abundance of taxon A versus taxon B. 150 individuals of taxon A have been discovered, but only a dozen of taxon B, all in the same formation, often at the same sites. In floodplains, taxon A is very common, taxon B is completely absent. In river channels, taxon A is common, taxon B is also present, although not in the same numbers as taxon A. This is repeated every time. There is no site where more individuals of taxon B have been discovered than of A. Therefore, it could be said that taxon A is much more common than taxon B, and that taxon B is limited to river channels. The “politician” simply said that absence of evidence is not evidence of absence, no other counter argument. Taxon B is also completely absent from the upper strata of the formation, while taxon A and other taxa are just as abundant as in the lower ones. Therefore, it could be said that taxon B went extinct before the other taxa and the end of the formation. This person repeated again: absence of evidence is not evidence of absence, no further arguments. They were denying the evidence (evidence of absence of arguments), that there is no reason to think that the absence of taxon B in upper strata and floodplains is due to some taphonomic factor that magically does not affect the other taxa (taxa A and B have similar size, similar morphology, overlaping geographic range etc).
With the damned phrase: absence of evidence is not evidence of absence, one could argue almost everything, even that non-avian dinosaurs survived the K-PG extinction, we are just not finding them!
Sometimes is just a waste of time
August 26, 2025 at 1:42 pm
The problem is that people have historically used the phrase stupidly, but the phrase is in itself still logically true. Take the source where most people have heard it from, Carl Sagan (1977), who said “A few students of the subject seem to have concluded that, because they have been unable to isolate and localize all higher brain functions, no future generation of neuroanatomists will be able to achieve this objective. But absence of evidence is not evidence of absence. The entire recent history of biology shows that we are, to a remarkable degree, the results of the interactions of an extremely complex array of molecules…” But gee, what he just said about the history of biology (and science in general) showing things previously thought to be incomprehensible become increasingly comprehensible as we study them more IS evidence. As llewelly would put it, the prior probability of human neurology being solvable by humans is better than chance.
Now, the actual absence of evidence is different than the absence of positive evidence for something. But we have evidence for so many relatable subjects that you have to go pretty far astray into unknowns or fictional worlds. Like “the millionth favorite number of the first species in the universe to come up with the concept of favorite numbers was even.” That’s probably evidence-less enough considering the range of potential biologies and minds, and there our absence of evidence for that statement being true is NOT evidence of its falsity.
So it’s true “the absence of evidence is not the evidence of absence”, it’s just that we pretty much always have some evidence when it comes to topics people use the phrase for.
August 26, 2025 at 1:49 pm
Sorry, Mickey, I don’t agree. In the case you cite (which I admit I did not know was the source of the phrase), Sagan is wrong. People who went looking higher brain functions and didn’t find them certainly did not prove there they’re not there to be found. But they provided some amount of evidence for the hypothesis that they’re not there, whether or not we like or agree with that conclusion.
I think Sagan made a straight-up mistake. He knew, and should have said, that “absence of evidence is not proof of absence”, which is unambiguously true. But either through carelessness or through affection for the more lyrical cadence of the now-popular version, he instead said something stronger — which is not true.
August 26, 2025 at 2:42 pm
Well Sagan’s not the source of the phrase, but that’s where most modern people probably first heard it. There’s a page at https://quoteinvestigator.com/2019/09/17/absence/ that takes a version of the phrase back to 1887, but they’re all basically wrong in the same way.
And you and I don’t actually disagree on the relevant point, since I only claimed Sagan had evidence in one direction, while you say you have evidence in another. But the whole question is still laden with evidence, of varying strengths and directions. And we agree Sagan was mistaken (“used [the phrase] stupidly” in my words) in what he said, as were the prior examples cited in the page I linked to, being scientists and historians instead of philosophers.
Basically what Sagan and the other examples were getting at is that the absence of a certain kind of specified direct evidence does not mean that the premise is false, because other kinds of evidence can often exist (often a probability-based defense that the specified direct evidence you want is unlikely to have been found yet even if it did exist). Which is true, but doesn’t roll off the tongue…
August 27, 2025 at 5:52 pm
Thank you, Mickey, for the quote investigator link. Among other interesting variants, it mentions my favorite: “The absence of evidence is not necessarily evidence of absence” apparently from 1939. (It’s also imperfect.)
Now I’m going to crowbar Matt Wedel’s interview into this thread, because it doesn’t have its own post. (Yet?)
In Matt Wedel’s interview interview with Terrible Lizards podcast, he and Dave Hone briefly discuss two things tangentially related to this thread: aquatic dinosaurs, and upland dinosaurs. Together, I think they represent two end members of this whole discussion. On the end of much evidence of absence, aquatic dinosaurs ought to be among the most frequently preserved of dinosaurs, but, famously, there are exceptionally few non-bird examples; the Halszkaraptorines, the Spinosaurids, … and that’s about it.
Upland dinosaurs, on the other hand, would not be expected to preserve, because most upland habitats are eroding. Worse, many living upland animals (such as goats and their close relatives) rely on adaptations that would probably not be easily identified in the often fragmentary skeletons of dinosaurs. And in that sense, they represent an opposite example.
https://terriblelizards.libsyn.com/tls11e08-sauropodcast-1
August 27, 2025 at 6:42 pm
oops, there’s an error in my previous comment; Halszkaraptorines and Spinosaurids are only semi-aquatic, (and arguably, barely semi-aquatic at that), but it’s about the best there is for non-bird dinosaurs.
September 30, 2025 at 8:36 pm
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