What little I know of it does suggest a couple of things (but my knowledge might be slightly outdated):
- It's true that no language is fundamentally unlearnable, not even as an adult (although you can of course always debate what "fluency" means exactly - to this date, I'm fairly certain that there are certain kinds of mistakes I make in English which native speakers don't make).
- A lot of the complexity does come from things like vocab and usage patterns, so even for a language where the grammar is "easy" (whatever that means - suppose we have a definition for that) it's not like it would be "easy" to speak properly. English in particular seems to have this "easy to learn, hard to master" problem.
- when we talk about grammar, there are definite differences in complexity, at least when we focus on certain areas (such as complexity of words). English is really just very simple in that regard, so is e.g. Chinese.
- contrary to GP's assumption, there is some evidence to believe that the causality goes in the opposite direction: English is not widely spoken because it's easy, it has become easier as part of being widely spoken as a second language.[1]
- relatedly, it appears that languages with very few non-native speakers (such as indigenous languages spoken only by a couple hundred of people) tend to have rather complicated grammatical features
[1] https://journals.plos.org/plosone/article?id=10.1371/journal...
I know little about this as well, but there’s been some research done by…… John McWhorter, I think? A quick search through my Big List of Things to Read (3000 articles and counting!) gives https://elementy.ru/downloads/elt/mcwhorter_creole_grammars.... and https://benjamins.com/catalog/slcs.71, which you may want to have a look at if you’re interested. David Gil is also well-known for his insistence that Riau Indonesian is simpler than most other non-creole languages.
> https://journals.plos.org/plosone/article?id=10.1371/journal...
Ah yes, this one. I remember carefully reading it a while ago, and I ended up very sceptical about its conclusions. For one thing, I don’t agree with rankings such as ‘Isolating > Concatenating’ and ‘Agent > Agent & Patient’ (though these do admittedly seem to be common assumptions). The ranking as a whole is a bit weird also — I’d love to know how they concluded that Tiwi is less complex than Dyirbal, for example. But I’m more suspicious of the linear fits, which appear extremely bad — I’m not convinced you can conclude anything from a fit of R²=8.9% (for Afro-Asiatic) or R²=14.1% (for Sino–Tibetan). In fact, only one of the graphs has R²>90% at all. And I note that they very carefully avoid giving p-values — except for the overall ‘complexity against population’ graph, a very significant correlation which however disappears when languages are split into groups by family (Simpson’s Paradox again?).
(Though keep in mind that I’m not great at statistics and any of the above could be incorrect.)
Also, they rely entirely on WALS, which isn’t a great source of linguistic data. It’s good for other things, but I very regularly find that a good proportion of its data is incorrect. It’s fine if you want to, say, see roughly what proportion of languages use active–stative alignment, but not for much else. (Though I suppose you could argue that the CLT applies and the errors will cancel out. But as I said, I’m bad at statistics and wouldn’t know how correct this is.)
(Personally, I’m sceptical that there’s a link between complexity and population. Witness Swahili, the lingua franca of a good chunk of East Africa. Or Quechua, the language of the Inca Empire. The main reason most widely-spoken languages are ‘simpler’ is that they disproportionally come from IE, ST and Austronesian, which aren’t the most morphologically complex families anyway.)
> Also, they rely entirely on WALS, which isn’t a great source of linguistic data.
the state of linguistic data is appalling in general. We have over 7000 languages in the world and yet, most of them are barely described, with grammars often being terribly inconsistent. I don't really know how to solve that problem except, in fact, hoping that the errors somehow do cancel out as you suggested.
I've heard this said often but I don't believe it. For one thing, ancient Greek was once the lingua franca around the Mediterranean (e.g. the New Testament was written in Greek koine) and it's hard enough to this native Greek speaker, so at the very least a language being easy or hard doesn't have much to do with whether it becomes a common tongue. I think English seems easy because its grammar works a bit like a word-stack, you can put words in order and make a phrase, whereas for other languages you have to modify words with pre- and suf-fixes. So in English you can sort of ignore grammar and only keep syntax in mind, whereas in other languages you have to manage both at once.
But, and this is just a theory, I suspect that there is no human language that is really "hard" to learn. Given that most languages must be spoken by at least a few thousand people over a few generations, I guess that every language is eventually optimised to be learned and used without trouble given the average linguistic ability of human beings.