I believe that widespread writing is responsible for a lot of the grammatical complexity of European languages. Most languages that were not widely written, or that did not have a central source for correct writing, tend to have simple Grammer, but the cultures that wrote a lot tend to have very complex grammar. This is just my personal observation.
The creole and pidgin languages are what happen when you strip English or French of the artificial complexity imposed by layers of centralized rules.
Writing has no effect on the "complexity" of the grammar. Language is almost always acquired by children in the absence of writing; a 4-5 year old who is just starting to read already fully understands almost 100% of the language's grammar and going forward will mostly only acquire vocabulary.
Now, whether writing slows down the course of language change is an open (and interesting) topic.
The stereotype that Western languages like Latin and Russian are "complex" because they have things like case systems is a curious ethnocentrism of English speakers. If you dig deep enough all languages are phenomenally complex.
Why do you say that writing has no effect on complexity? The longer a writing tradition a language has, the more complex its grammar is. And complexity can be measured objectively.
By complexity, I mean having additional words for particular things, instead of describing with more words. For example, changing words to indicate future or past tense.
If children learn this or not is completely irrelevant - a child is capable of learning extremely complex communication forms - this has nothing at all to do with the history of a language.
If complexity can be "objectively measured", then surely you can provide a citation to a peer-reviewed article that supports your hypothesis?
Or has your objective measurement only been performed in secret?
The conventional wisdom, as I was taught it, is that the role of writing in language development and evolution is so small that it's only hypothetically detectable. That's partly because it's only in modern times that literacy has been widespread, but also because the vast majority of humanity learns to speak before learning to read - generally years sooner.
Anecdotes aren't data, but in the absence of data I'd note that Navajo has a legendarily tricky grammar and no tradition of writing, while I believe that Mandarin Chinese grammar is said to be fairly straightforward once you get past the phonology (which I have not, so I cannot say.)
I wrote in my parent comment that this was based on personal observation. There is no such thing as conventional wisdom on such topics, there are just widespread opinions:
Note, there are two points we are discussing here:
1. Is there a way of saying that some grammar is more complex that others - objectively?
2. Does writing influence the complexity of the grammar?
Now, for both cases, I base my opinion of observation, and not off any scientific papers. Since you are doing the same, the only way we can settle this duel is by trying to show as many examples one way or the other to resolve the issues.
Regarding the first question, I'll just assume that we both accept that there is difference in complexity in grammar between languages. Or do you not think so?
Regarding the second question, I'll say this:
* Chinese Grammar is simple, but it's very clear that Chinese style writing cannot have the same influence. If you do not know a chinese word, you cannot write it, and you cannot read it. As such, you are not likely to use it, and the word will tend to die. In western style writing, you can attempt to write a word from having heard it once. It's also much easier for words to live on, because words are easily copied.
* Unwritten Creole and Pidgin Languages always do very similar things to western languages - they strip it of certain grammatical constructs, and it ends up being like most unwritten languages - past tense and future tense are created by adding words, and not by changing words.
Ignoring everything else, I wanted to answer this one:
> 1. Is there a way of saying that some grammar is more complex that others - objectively?
I think so. English verbs, for instance, are significantly easier to conjugate than most other European languages. There's just less stuff to remember.
Another example with verbs is the subjunctive, which has all but disappeared from English (I wish I were, rather than I wish I was), but is still very much required in a language like Italian, even in the present tense: (Credo che sia importante instead of credo che e` importante, which is translated as "I believe it's important").
When you take complexity out of one part of a natural language, it ends up sneaking back in to another part.
English verbs by themselves are easy to conjugate, but English also uses a large number of modal verbs (I did write, he is hacking, we used to travel, she had better shut up) that these other languages lack. So while a native English speaker learning Spanish has to learn a lot of verb conjugations, a native Spanish speaker learning English has to learn a lot of modal verbs, and the special rules for conjugating verb phrases that contain them.
I'm curious, do you consider English grammar simpler than German? (The few people I've talked to about linguistics consider German to be a harder language for English speakers to learn than Spanish, which seems like it must be due to grammatical differences...)
English is a MUCH easier language grammatically than German. Just the fact that German has gender assignment on words increases the complexity of the German Grammar by a large amount.
Proto-Germanic (the common ancestor of English, German, Scandinavian etc.) was hardly ever written, but its reconstructed grammar is quite complex. Similarly complex as today's Icelandic.
Now I don't know much about this reconstruction process and how error-prone it is. But it's the best we can do for non-written languages of the past.
I don't know anything about the complexity of contemporary non-written languages.
Writing was more widespread in classical Latin than in modern Spanish. Classical Latin, the type that we read today, was a high brow language, used by educated people. The normal people used Vulgar Latin.
Creole and pidgin languages are special cases though. They are simple because they are relatively new constructions, rather than because they are unwritten.
The creole and pidgin languages are what happen when you strip English or French of the artificial complexity imposed by layers of centralized rules.