[halter73]

While I agree that correcting grammar isn't usually that productive, I think that mixing up "your" and "you're" is probably indicative of a native English speaker.

I doubt that "your" and "you're" are homophones in most other languages. As a native English speaker myself, I often slip up and write "your" when I mean "you're" because they both sound the same in my head and it's quicker to type "your".

I think that a non-native speaker would be less like to make this mistake, because it requires the writer to decide to contract the pronoun and the verb and then forget to use an apostrophe.

I think this is far more likely to happen if you're simply typing out a stream of consciousness than if you're translating your thoughts from a different language.

[ludwigvan]

Yes, as a non-native, I agree that it is not very common (almost never) for me to mix them up because I prononunce them differently.

Another reason non-natives don't do this mistake might be that usually they are thought the written language first, so schools concentrate on grammatical issues more.

[imarg]

Adding another datapoint, as a non-native speaker myself, I believe I've rarely (if ever) mixed them up.

As the parent commenter said, I've also noticed in the past that this mix up occurs usually among native English speakers. I've noticed it with other languages too, that native speakers often mix up homophones, probably because as a native speaker you write as you would speak and either you don't care or don't even think about it.

[nzp]

As a non-native speaker I agree with this. But I've noticed something interesting: before I was aware that mixing up the two was common with native speakers it would never happen in my own writing, but once I realized people do make this mistake I became susceptible to making it too. I still don't, but now I sometimes have to make a conscious effort to differentiate the two. It's amazing where social learning can show it's effects. (And BTW, I think I would just kill myself if I ever mixed up affect and effect, I see that a lot, and for some reason it annoys the hell out of me.)