...of Egyptian Arabic.
Julie immigrated from Great Britain to Cairo at 21 when she married an Egyptian. She got no formal instruction and never learned to read or write in Arabic.
Laura studied Egyptian Arabic in the last year of her undergrad. She studied it at the graduate level, too, and then she settled in Cairo, married an Egyptian, and became a Standard Arabic teacher at a university there.
13 teachers of Arabic as a foreign language evaluated their speech. Julie and Laura were rated as native speakers by 6 of them. Furthermore, the two women were pros at discriminating between Arabic dialects and judging whether sentences were grammatically correct.
BOOM! You have to understand that in my textbook on second language acquisition, this study stands out like a sore thumb. Almost all researchers subscribe to the Critical Period Hypothesis, which says that after puberty, you'll never properly learn a second language, because of some biological change in the brain. Any yet if you travel, you meet people like Julie and Laura rather frequently. Researchers must not travel.
Even for this study, you see the researchers scrambling to say that Julie and Laura are biologically unique ("Julie herself is left-handed and has skin allergies"). No. They created environments for themselves and went for it.