A brief background of Genie’s story can be found here:
A statement was mentioned at the end of this reddit user’s post, source to be confirmed:
The Genie case remains to this day the most studied individual case in the history of linguistics and developmental psychology. It confirmed the critical period theory: words can be learned at age 13, but the grammatical structure of language is only built before puberty.
In 1957, a girl was born in California, the fourth child of Clark and Dorothy Wiley. Her birth name would be protected for her entire life by the Californian authorities. The press, the scientific community, and the public would only know her by the pseudonym Genie.
by u/elnovorealista2000 in USHistory
However, we do not know Genie’s baseline to say that “the critical period theory of grammatical structure of language can only be acquired before puberty, as elsewhere (see description section of the youtube video above) quoted “Clark Wiley (her abusive father) was told by the doctor that his daughter had a learning disability” as the reason of him locking her away from the world.
Accounts report that she had been considered developmentally delayed very early in life. But crucially, researchers could never establish with certainty whether she had a pre-existing cognitive/developmental impairment or whether her later difficulties were primarily consequences of extreme deprivation and abuse. Modern discussions of the case explicitly identify this as a problem: possible pre-existing disability, severe neglect, malnutrition, trauma, and social/cognitive deprivation are all potential confounding variables.
Further information derived from the original scientific report shared on my developmental psychology module:
Genie’s case history
Genie was discovered at 13 years 7 months old.
From about 20 months old, she had been kept in extreme isolation and physical restraint.
Human interaction was extremely limited, and her father punished her for making sounds.
There was no TV or radio, and her father and brother did not normally speak to her.
When discovered, she:
did not speak
did not cry or make vocal sounds
had difficulty standing, chewing and swallowing
was severely physically and socially underdeveloped.
After entering hospital, however, her physical, cognitive and emotional development improved considerably
Had Genie already acquired language?
Essentially, no — or extremely little.
Initially, the researchers couldn't be completely sure whether she understood language but simply couldn't produce it. This distinction is between:
competence = knowledge/understanding of language
vs.
performance = ability to actually produce/use it.
Eventually, their testing suggested that her comprehension was only slightly better than her speech. She seemed to understand a few individual words, but there was little evidence that she possessed an underlying linguistic system.
So the researchers regarded what happened afterward as something close to first-language acquisition at nearly 14 years old, rather than simply teaching an existing speaker to talk.
Link to paper: https://www.jstor.org/stable/412222