The experiences of Genie and Victor demonstrate that people have a critical period of time when they best learn language, which goes from birth to the beginning of puberty.
It is known that children of just four weeks of age can differentiate about to forty consonants. This have been proven by measuring the changes in the rhythm of sucking and heartbeat when listening to the consonants, which reflects that the brain differentiates them involuntarily. This number is higher than the number of consonants a person is able to hear in his life, since English only contains 24 consonants. The rest of the consonants would be used by other languages. At six years of age, the ability to differentiate consonants to those that have not been exposed has been greatly reduced. These data contrast perfectly with the indisputable fact that after puberty it is not possible to learn a second language naturally. The mother language is learned without effort and is perfectly mastered, while a second laguage requires a lot of effort and is not pronounced, even with decades of use, in an entirely natural way.