I am surprised this required research. How is this not obvious ? What's next ? Algebra skills and essay writing? You could pretty much do this research for every thing we ever learn from school to college and publish endless amount of papers.
https://en.wikipedia.org/wiki/Universal_grammar
https://en.wikipedia.org/wiki/Poverty_of_the_stimulus
But it's hard to present the context of the argument when describing an individual research paper, so maybe the research summary ends up sounding at first like "when they're little, children learn how to speak their languages!", which definitely sounds like "wait, why does someone even bother publishing a paper about that?".
Ability to speak however comes very naturally without special training. Human societies independently developed isomorphic grammar. For example Navajo Indians helped American soldiers translate their messages into Navajo and back so Germans wont understand it.
This sort of evidence lead scholars like Chomsky to believe that human brain might have some in-built ability aided by evolution to parse some basic grammatical rules.
It literally takes years of practice for children to speak well. Maybe it's natural but we tend to dismiss the difficulty because it happens early.
I would think if we had dedicated hardware for grammar subsequent languages wouldn't be so difficult to learn either. It's easier to learn algebra than a second language.
We are unfortunately very aware that when isolated, kids don't learn to speak. So it isn't innate.
No, what's happening is she's mimicking the adults in the house. She's observing and repeating. She's done the same thing with dolls: put them on her shoulder, pat them, make shushing noises ... because we do that with her; because she see's others do that with other babies.
"Instruction"? No. But the only "innate" behaviors this child has involve eating and curiosity. Everything else is spurred by that innate curiosity.
(David Foster Wallace wrote a long piece about this in 2001 called "Tense Present" http://harpers.org/wp-content/uploads/HarpersMagazine-2001-0... and there's also a long story about nationalist movements trying to design, and teach, a standard prestige version of their national languages, which does require grammar instruction to create and maintain because it's somewhat different from all or most students' home language varieties)
Written vs. spoken language - may have different grammatical norms
Non-native speakers - want explicit instruction because they lack native-speaker intuition about some aspects of grammar
Appreciating language intellectually - curiosity and appreciation about how your language works, independent of your ability to produce sentences other speakers accept as correct
That is, to count you need to understand number, but to make decisions about scale and quantity, you don't.