This overlap makes sense, because language and music have a lot in common. They are both governed by a grammar, in which basic elements are organized hierarchically into sequences according to established rules. In language, words combine to form phrases, which join to form larger phrases, which in turn combine to make sentences. Similarly, in music, notes combine to form phrases, which connect to form larger phrases, and so on. (pp. 38-39)
I'm a bit skeptical regarding the many examples of baby-mother interaction she provides, because, like so many others in her field, and in cognitive science generally, she assumes that all babies and mothers interact similarly, based on research typically limited to American and European subjects. Before attempting to universalize such evidence, it's important to compare it with evidence from non-Western societies, as well as various indigenous groups from a wide range of different world areas.
The above reservations do not apply to her most remarkable and exciting results, regarding a completely unexpected and indeed very surprising correlation between absolute (or "perfect") pitch and tone language. She made the astonishing discovery that among students who had received musical training by the age of five, fluent speakers of Mandarin, a tone language, were far more likely to have absolute pitch than a comparable group of students who grew up with English or some other nontone language. We're talking a huge difference, of 92% of "very fluent tone language speakers," as opposed to only 8% of English speakers. To determine whether the correlation were primarily genetic rather than linguistic, she tested East Asian students who grew up speaking a non-tone language and discovered that they too scored only about 8%. The correlation seems definitely associated with tone language rather than genetic inheritance.
Another important discovery concerns the pitch sensitivity of tone language speakers generally. It's always been assumed that the pitches of tone language are relative and not absolute, yet Deutsch learned that
not only were Vietnamese and Mandarin speakers very sensitive to the pitches that they hear, but they can produce words at a consistent absolute pitch. . . We found that their pitches were remarkably consistent: when compared across days, half of the participants showed pitch differences of less than half a semitone (p. 42).
In the next post, I'll explain why I attach such importance to these results.