How do you debunk the "I've never seen.. therefore..." fallacy?

I know that logical fallacies are something which are despised on online forums (especially here) and in real life discussions.

I was having a discussion about psychiatric disorders, illnesses, syndromes with a friend and autism came up. The gender (sex) divide came up and we were discussing why there seems to be a lower rate of autism among women. But then, the conversation went downhill from there. He made a claim about autism causes that it manifested itself in facial asymmetry like down syndrome. His claim was that, autistic people both male and female usually look ‘off’ in their facial features and that since less women look ‘autistic’ there are less women with autism.

This was stunning from someone who I thought had a sense of intelligence and after a while we slowly stopped the conversation, but I wanted a rebuttal for him because I knew that was bollocks. It sounded like a logical fallacy which I couldn’t put my name on. His observation in my opinion is incorrect because:

  1. Autistic women show more covert signs as opposed to their male counterparts.

  2. Females tend to have more social contact and are less isolated.

  3. Average/above average women taking into account the two former factors are going to be more approachable towards others and are always most likely going to have a small group of friends, therefore they aren’t going to be diagnosed as easily as less attractive males or less attractive women. It’s only those who are perhaps below average that might not be seen as approachable and have great difficulty forming friends.

Aside from this example of autism, what’s the name of this fallacy? It’s similar to people who say ‘I’ve never seen a transgender woman who passes for a woman’ or ‘I’ve never heard of a female pedophile’?

“Absence of evidence is not evidence of absence”

Probably combined with some amount of confirmation bias and Dunning-Kruger.

You could always point out that you’ve never seen this individual void their bowels but you’re reasonably sure that happens on a regular basis.

The fallacy in your title is “argument from ignorance”.

But the fallacy in your example is not the same. It’s “anecdotal fallacy”.

That’s a variant of the argument I would have used. But in this case, the OP has seen it - the person dumped a big deuce right in the middle of a good discussion.

Have you asked this dude how he feels about eugenics?

I’ve also heard the term “jealous phenomena”, although this isn’t quite that. I think a jealous phenomenon is used to describe pseudo-sciences like ESP (where the presence of a skeptic attempting to investigate the ESP makes it not work, somehow), or UFO landings (where the aliens would deliberately choose an isolated area to land in, rather than someplace like Central Park). I don’t think that’s quite what the OP is after.

I had a conversation with a co-worker some years ago that I still remember. He was, and still is, very knowledgeable about music. I said he knew everything about music. He said he didn’t know everything, so I challenged him to name one single band that he’d never heard of, and he couldn’t. That seems to be what the OP is getting at; why don’t we ever notice things which, by their very nature, go un-noticed? “Self-defeating hypothesis” maybe?

How about just showing them what it is that they’ve never seen, so it is no longer an excuse?

And if evidence doesn’t work there is no helping these people.

I’ve never seen New York City…

I don’t see how you can. If they’re over 5 years old and insist that if they haven’t experienced it, it hasn’t happened, they are stupid beyond redemption.

Well, I can’t afford a plane ticket, but I can at least show you evidence of New York City.

Aw, we know how easy it is to fake photographs. I just watched Doctor Strange, and watched New York fold up like an accordion! :wink:

(One of my favorites is the creationists who say “No one has ever seen a dog evolve into a cat.” Yeah, and no one has ever seen a seedling grow into a full-sized coast redwood, either. Guess it doesn’t happen!)

(Let alone to glaring miscomprehension of how evolution works. But creationists are an easy target.)

femmejean, it looks like your question has been answered, but I’d caution you that just being able to correctly name the fallacy committed is unlikely to convince someone that you’ve won the argument. Perhaps if you were engaging in a collegiate debate, but in a one-on-one conversation with someone, they’re probably going to to be unpersuaded.

[hijack]“FEWER,” not “less.” Unless you’re comparing their net collective weight, women are individuals.[/hijack]

The OP should ask her “friend” how many fewer milliHelens the average autistic woman has?

Or do they perhaps average in the negative Helens? But if so I’ve never seen one. :slight_smile:

What I dislike about this fallacy is that it is part of a debate strategy where the argument boils down to the speaker’s word, a sort of ‘witness argument’. Whether the speakers says “I’ve never seen…” or “I was there. I saw it.”, he expects we should take his experience and his logical conclusions for granted. And instead of discussing the issue, we’ll end up either discussing his credibility or accepting his point of view. A lame technique.

A face that pulls ships back to the dock?

Beaches 'em. In Helen’s day most ships came and went via a beach.

Read the linked article. Many creative folks have worked out the various things that happen at various Helen levels; some good, some bad.

I’ve always found mouth-breathers to be somewhat repulsive, so I guess loose lips sink ships.

I’d like to see the basic derived units …

The Panamanian = Helens per Generation
The Swede = Helens per Hour per square train station
The FaceDown = the inverse natural logarithm of Helens
The Fish = Kelvin - Cup Size per Shrew factor
The Sentence = (5 to 15) - (milliHelens per Age (in Years))

Overlay this all with a Ricci topology and we’d have a true Greek tragedy …

I feel like kind of a jerk for pointing this out, but the question in the thread title isn’t consistent with the OP’s description of the friend’s argument. The friend apparently did NOT claim that he’s never seen an autistic woman and therefore autistic women do not exist, but rather that he knows there are more autistic men than women because he’s seen more “autistic looking” men than women:

If the quoted text is an accurate summary of the friend’s position then he may be guilty of the informal fallacy of hasty generalization (has he really encountered enough autistic people to accurately judge the gender ratio?), but I’d say the bigger problem is his apparent belief that he can reliably identify autistic people on sight. A quick Google indicates that there is some evidence that certain types of facial features may be more common in people with autism, but these are subtle. I do not consider it plausible that your friend can spot autistic people just by looking at their faces.

That said, the reasons you listed for why you thought your friend was incorrect don’t seem to address his claims about a supposedly obvious “autistic look”, and it’s easy to find reputable cites supporting his belief that autism is more common in men than women. According to the CDC, “ASD [Autism Spectrum Disorder] is about 4.5 times more common among boys (1 in 42) than among girls (1 in 189).”