Reddit mentions of The Language Instinct: How the Mind Creates Language

> Yes, those are factors that could plausibly affect the learning curve.

Sure, plausibly they could affect the learning, but that's a bit different than 'it doesn't make sense that every language is as easy to learn', don't you think?

> If we take any language, and then artificially make it twice as difficult to learn by making new, difficult rules to the grammar, it would (almost by definition) mean that such a language would be more difficult to learn to a toddler.

But that's the thing - even very complex grammar rules such as grammatical genders or Finnish-style agglutinated suffixes (and their order!) are pieces of cake to toddlers. They just pick them up very rapidly while second language learners struggle with these for decades.

Here's a paper on the magnificient Stephen Pinker on the topic of language acquisition and also touching a little bit on the different languages topic. If you're interested, his book The Language Instinct is a more in-depth look at the issue at hand and a fun read. Pinker, in general, is a wonderful writer and I'd especially recommend his book The Blank Slate for an interesting look into humanity.

The study you linked to is interesting, but using different parts of the brain - in this case, to decode the tonality of the language - doesn't really say anything about difficulty. Note that the researches uses the term 'different' while the journalist invokes 'more difficult'. This would explain, however, why speakers of tonal languages more commonly have perfect pitch, though..