Suppose language A and language B belong to the same language family. And suppose the speakers of language A understand language B a lot better than the speakers of language B understand language A. How can linguists explain this phenomenon?

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    Well that's actually a part of the question. Is it mutual intelligibility if asymmetrical? As far as I'm concerned the notion of mutual intelligibility (for defining a common language) is a lot more accepted in the Anglosphere compared to Continental Literature. And the further you go away from the West the more normal asymmetrical intelligibility becomes. Commented Aug 16, 2022 at 11:51
  • I think @leftaroundabout 's answer gets across most of what I have to say, by challenging the assumption of mutual intelligibility itself instead of appealing to "obstacles" and "noise". Commented Aug 16, 2022 at 11:54
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    Possibly related: linguistics.stackexchange.com/a/44163/27599
    – Roger V.
    Commented Aug 17, 2022 at 9:47

4 Answers 4


The previous answers gave a lot of possible factors, including external ones. I'll give a single example where it clearly has a lot to do with quirks of the harder-understandable language: Danish.

The Scandinavian languages Swedish, Danish and Norwegian are very similar, so that speakers of any of them can read the written forms of all the others with little effort. For the same reason, Danish and Norwegian people can also understand spoken Swedish without much trouble.

The Danish language however has undergone some pretty strong phonetical mangling, which makes it much more challenging for Norwegians and Swedes to understand spoken Danish. Many syllables that are shared in the written languages aren't directly spoken at all in Danish, but only surface via subtle changes in e.g. the preceding vowel, which makes Danish sound to the others like the speaker is mumbling and constantly clearing their throat. It's something often joked about:
“Danish is not a language but a throat condition”,
“How do you speak Danish? –Just put a hot potato in your mouth and speak Swedish”,
or in this Norwegian comedy programme:


Of course, Danes have lots of training in understanding their spoken language despite all that, but this doesn't prevent them from also understanding Swedish with its more directly writing-mapped phonology. But Swedish speakers have much more difficulty reconstructing the omitted syllables in spoken Danish.

More detailed discussion in Gooskens et al: Explaining Danish-Swedish asymmetric word intelligibility - An error analysis (from Phonetics in Europe: Perception and Production).

Norwegian actually has its own difficulties, mostly because there isn't really such a thing as Norwegian language – almost everybody speaks some strong local dialect. It is urban Eastern Norwegian / Bokmål which is easy for Danes and Swedes, but some western and northern dialects differ from Bokmål more strongly than it does from Swedish. There is even a separate written form of Norwegian, Nynorsk, which is the official form in many western municipalities and represents these dialects better.
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    "some western and northern dialects differ from Bokmål" Be careful here. Bokmål is a standardized written Norwegian language. There is no such thing as spoken bokmål (unless oneone decides to recite written text exactly as written as a piece of art or something; the theater "Det Norske Teater" is known for doing that with Nynorsk). The officially recognized spoken languages in Norway are Norwegian, Sámi, Kven, Romani and Scandoromani (and Norwegian Sign Language if that counts), all presumably with more or less interintelligible dialect variations.
    – Arthur
    Commented Aug 16, 2022 at 8:48
  • @Arthur yes, but 1. Bokmål is a pretty close representation of the dialects spoken in and around Oslo, in the sense that an e.g. English speaker who has learned only written Bokmål has a decent chance of understanding somebody from Oslo if they speak slowly enough, but will understand very little of somebody from Tromsø or Hardanger 2. Bokmål and Nynorsk are written forms of the family Norwegian dialects whose speakers can talk to each other (each speaking their native dialect), whereas Sámi, Kven, Romani and Scandoromani are not intelligible at all to most Norwegians (or Danes). Commented Aug 16, 2022 at 8:57
  • I agree about Sámi, Kven, Romani and Scandoromani. They were just in the list for completion. My point was, when discussing spoken language, one should be very careful with using the terms "Bokmål" and "Nynorsk". Spoken, there is only one Norwegian language, it has many dialects, and none of the spoken dialects are "Bokmål" or "Nynorsk". Yes, some dialects are more faithfully represented by Bokmål, while some are better represented by Nynorsk. But those two are written languages, and written languages only.
    – Arthur
    Commented Aug 16, 2022 at 9:07
  • @Arthur I chose not to go into those details in a footnote that was mostly meant as a disclaimer (regarding that not all Norwegian is easy to understand for Danes). But yes, you're right. Though if we're going to be pedantic, spoken Nynorsk is also a thing, albeit not as a native dialect in any region (it's mostly used in theatre and by some TV presenters). Commented Aug 16, 2022 at 9:26
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    Don’t forget that several studies have shown that out of all the Scandinavians, the Norwegians are best at understanding their neighbours – and the Danes the worst. Norwegians in general find Danish easier to understand than Danes do Norwegian, even Bokmål-adjacent forms. So while Danish is definitely the phonetically challenged sibling, the result isn’t quite as expected here. (That’s also why I chose Spanish and Portuguese rather than Swedish/Norwegian and Danish in the example in my comment to JK’s answer.) Commented Aug 16, 2022 at 12:14

There are a lot of things factoring in here, the following list is by no means exhaustive

  • exposure to the other language: this is often asymmetric between the languages A and B, and more exposure means better intelligibility
  • Relative entropy: The distance between two languages measured by Kullback-Leibler divergence between suitable language models is asymmetric, so more additional information is needed in one direction than the other
  • Phonological differences: One of the languages may have a subtle phonemic contrast that the other language lacks, and native speakers of the other language miss some important distinction, reducing the mutual intelligibility
  • Generally, the existence of any "quirks" (like special grammatical constructions in morphology or syntax) reduces the mutual intelligibility.
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    For the third point, another possibility is that language A may have undergone recent sound changes such as vowel reductions, lenition, etc., which are not reflected in spelling and which did not occur in language B. That would give speakers of A an advantage, since they are used to equating the spelling (≈ original phonemes) with the changed sounds, whereas speakers of B are not. That is a big part of the reason why Portuguese-speakers find Spanish easier than vice versa. Commented Aug 15, 2022 at 11:14

Linguists would start with comparing social status of the languages. For example, there is a massive comprehensive asymmetry between Cantonese and Mandarin: more Cantonese speakers understand Mandarin better that Mandarin speakers understand Cantonese. That's because Mandarin has a privileged position in China as "the national language", which Cantonese speakers therefore probably know as well. You would then want to only consider monolingual / monodialectal speakers of A and B, but that is not a highly reliable demographic (you have to devise a good test to objectively assess knowledge of language that is less than fluent).

Cultural bias can also influence reported comprehension, that is, a speaker of A might actually understand B better than reported, but that could conflict with social requirements in A society to not "validate" B society: or, A and B may differ in the extent to which it is allowed to "guess" at what someone said.

A quasi-social factor is lexical familiarity owing to loanwords. Language A may have replaced a substantial portion of the original vocabulary with items from a language C (perhaps English or Arabic) which speakers of B also know, which gives B-speakers a comprehension advantage (A speakers no long know the old word for "hippo" which B-speakers still use).

  • Very surprised to see this (currently) as only the third most up-voted answer. This is surely the overwhelmingly greatest factor explaining asymmetries.
    – adam.baker
    Commented Aug 17, 2022 at 6:52
  • @adam.baker I got the feeling that this isn't what op had in mind. He doesn't say two neighboring populations vel sim., just two groups of speakers. Social factors must be looked at first, but that is a cultural evaluation, not necessarily one that's rooted in the languages themselves.
    – cmw
    Commented Aug 17, 2022 at 13:14
  • @cmw No, but I think the answer appropriately corrects the OP's assumption (which I imagine we all shared at one point) that linguistic factors are what would drive asymmetries in mutual intelligibility.
    – adam.baker
    Commented Aug 17, 2022 at 14:59

To me, this sounds like diglossia.

Suppose the speakers of language A understand language B a lot better than the speakers of language B understand language A.

Diglossia is when two related dialects or languages are used within a community. The textbook example for this would be Modern Standard Arabic (Language B) and, f.x., Egyptian Arabic (Language A).

Both of these forms of Arabic are media languages, Standard Arabic is the norm in literature and Egyptian Arabic is especially common in Egyptian cinematography. But as you can imagine, Standard Arabic is recognisable outside of Egypt from Morocco to Iraq. Whereas the Egyptian form of the language is limited to Egypt and people who watch Egyptian films.

Diglossia is not often brought up in my linguistic circles, because it is usually treated as a sociopolitical matter. It can occur outside of established language families. Another less obvious case of diglossia is Singlish (Language A) and English (Language B) in Singapore. Singlish is a creole amalgamating Hokkien, Malay and English; and it is definitely spoken. But it is by no means a media language, because of government policy and intellectual opinion to adhere to a more conservative form of English. So many people (linguists included) will reduce Singlish to "informal English" in their analyses.

In diglossia, the asymmetry in intelligibility has more to do with the functional usage of language among speakers A and B. Diglossia is not due to the fact that f.x. Persian (Language B) has a more extant vocabulary than Kurdish (Language A). Rather, an ethnic Kurd (Speaker A) might find themselves switching languages to fulfil separate functions.

Speaker A might find themselves using Persian in their studies at school but speaking Kurdish among their family. Speaker A has thus learned to do math in Language B, and has learned their family and kinship terms primarily in Language A. If now we bring in a monolingual, Iranian from Tehran (Speaker B) they will have grown up using only Persian in all domains of life.

When employing language for shared functions, Speaker A and B should not expect any problem. So Speaker A can check B's homework, and vice versa. But when language is employed for a function where the two would use different languages, that is where the asymmetry becomes apparent. If the Iranian speaker tells the Kurd that he will visit his "padar", the Kurd will immediately understand "father". But if the Kurd were to say that she will visit her "bav", now their Iranian friend will be confused. Thus we find that Speaker A and B seem to have an asymmetrical levels of intelligibility.

Many social scientists interpret this as being due to language B holding a prestige over language A, i.e. a kind of superiority. I prefer to think a bit differently, because on the other hand speaker A adapts by extending their own vocabulary and language rules to encapsulate both languages A and B (to a limited extent). So really, it's more a trade-off between understandability and having more ways to express oneself.

  • It’s only diglossia if the two variants are used within the same community and one is somehow ‘upper’ and the other ‘lower’. That’s not ruled out by the question, but it’s not suggested by it either. The examples given elsewhere (Spanish<>Portuguese, Danish<>Norwegian<>Swedish) are not cases of diglossia (as a whole), since they’re on equal footing and are not shared by the same community: each is the ‘upper’ variant in its own major community. Commented Aug 16, 2022 at 12:56
  • Yes, diglossia is not the only form of asymmetrical intelligibility. What my understanding of what @dobrze is describing in the body of the question was diglossia, hence my answer. I edited the quote just now to clarify. And then of course, your point of view raises the question of the boundaries of a community. What you demarcate as an improbable (only hypothetical) Spanish<>Portuguese community above might still exist as a border community in Iberia or South America, if not as a distinct entity on the Latin American web. Similar for the Nordics. Commented Aug 16, 2022 at 12:59
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    Yeah, I was running out of characters, but by “as a whole”, I meant by looking at the whole of the languages and their speech communities. There are of course pockets along the borderlines where both exist and are used in the same smaller community, but those are dwarfed by the vast majority of speakers in the non-shared areas that make up 99% of the communities in total. Commented Aug 16, 2022 at 22:57

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