"There is no evidence..."

I'm growing tired of arguments and pronouncements that don the mantle of science but then proceed to make the most embarrassingly false claims. I often see it said that there is "no evidence" or "zero evidence" for some claim. This can't be literally true. 

Anything that can potentially shift your priors is evidence, even if it doesn't shift them by much. If I tell you I saw a leprechaun, that's "evidence" that there was a leprechaun, even if it's not very persuasive. There are very strong priors against there being leprechauns, given the laws of physics and biology. Suppose I claim to have seen one, and you've known me for a while. Maybe I've shown tendencies in the past to be overly credulous, or perhaps I'm a known bullshitter. Still, my isolated claim is a type of evidence. If I told you some other claim, say about my kids or about what was on TV the other day, you would believe me. So my word is evidence. Any claim I make has some degree of intrinsic plausibility, even if there is an overwhelming mountain of counter-evidence. My point here is that there are weak forms of evidence that still count. Observational studies aren't as good as randomized controlled trials, but they are still evidence. Logical and theoretical arguments aren't physical proof, but they are still evidence. The insights you gain from introspection aren't physical or objective, but they are still evidence. Without someone at least temporarily believing these weaker forms of evidence, we'd never get to the stage of generating new hypotheses and testing them. We'd never get around to generating the physical, empirical evidence that ultimately turns a hypothesis into a working theory. 

I'm hearing "no evidence" as shorthand for one of the following:

"There are some plausible theoretical arguments for the claim, but no direct factual evidence."

"There are theoretical arguments for the claim, but there isn't yet any direct factual evidence because this specific question hasn't been studied yet."

"There are strong theoretical arguments for the claim, but the empirical evidence is mixed."

"The claim in question has been studied thoroughly, and all of the factual evidence so far points to 'No.'" 

"There is evidence for the claim in question and there is also countervailing evidence against it. All things considered, I have adjudicated against the claim."

These are all fine responses when someone makes an implausible or disputed claim, but let's just be more honest about which thing we're saying. I'm seeing still-open scientific questions getting short-circuited. 

See this example linked to by a recent Astral Codex Ten links roundup:

The American Academy of Pediatrics states that there are "no studies" to support the concern about young children being unable to learn facial cues due to widespread masking. Mason's response is brilliant, basically saying that a hypothetical proposed study to determine the effect would be so obviously unethical that no Institutional Review Board would approve it.  (I don't know Mason's politics, but this is a pretty basic libertarian point that I have often made. Policy intervention, which we'd all agree would be unethical if we did them as experiments, are done all the fucking time, on the entire population, without bothering to gather information to determine safety or efficacy. The mandatory face masking thing is one such example.) The tweet by the AAP is extremely dishonest if it's implying that this particular question has been studied and evidence for it is lacking. Suppose the tweet instead said, "This question hasn't been studied, but we don't think it's plausible for the following reasons..." Maybe that wouldn't have been as punchy, but it would be far less misleading and much fairer to people concerned about this issue. 

Pretend for a moment that you're a member of an IRB and someone proposes this intervention to study the effects of hiding facial cues from young children for extended periods of time. Really imagine yourself in this person's shoes. Are you filled with unease that you might be harming some of the children in the proposed study? Perhaps not mere unease, but actual horror? Imagine green-lighting this and seeing the final paper. Picturing, say, Figure 3 on page 22 of the published paper showing that the treatment group acquired language less efficiently, would you feel bad that you allowed harm to come to some of these children? Would you feel dread that these effects might be permanent? Introspection is also a kind of evidence. Powerful evidence at that, and too often dismissed. There is some intrinsic plausibility to the notion that hiding facial cues from young children could harm their development, whether or not some particular question has actually been studied. I'm not saying that anyone who prefers to wear a mask should stop because of this. (Imagine yourself being on the IRB for another hypothetical study where people were discouraged from wearing masks during a pandemic. Isn't the world just full of trade-offs and uncertainty?) But we shouldn't be so dismissive when someone points to a plausible cost of a new behavior. Certainly we shouldn't dismiss it in the language of scientific certainty. This should get at least some decision-weight when considering policies (public and private) such as the masking of children or advisements to mask at home around young children. 

Here is a Cafe Hayek post from a few years ago, in which Don Boudreaux takes Paul Krugman to task for making a "no evidence" argument. Krugman absurdly claims:

There’s just no evidence that raising the minimum wage costs jobs, at least when the starting point is as low as it is in modern America.

This is nonsense. There are many studies that find substantial disemployment effects due to minimum wages. (The very best evidence for a strong disemployment effect comes out of the Seattle studies by Jardim et. al., though those weren't out at the time Krugman wrote his piece.) What Krugman is actually saying is that he's done the hard work of weighing the evidence for us and reached a conclusion based on the preponderance of that evidence. Even assuming Krugman had done this literature review (by no means a certainty), he would not then be allowed to say that the contrary evidence doesn't exist. Just that he personally finds it unconvincing, and he should say why.