The Critical Period Hypothesis suggests that there is a certain time frame within which a human needs to be exposed to human language in order to acquire a first language. If not exposed to human language during this period, an individual will learn little to no language. However, since an instinct for learning language is innate, what is the nature of that instinct? If some part of language is innate, what part is acquired from the environment? The answers to these questions set up a distinction between the principles of universal grammar and the parameters of an individual language. If we try, we can weave these various ideas into a comprehensible whole.

The distinction between principles and parameters comes from Chomsky, and we can turn to his protege, Stephen Pinker for a clear definition in his 1994 book, The Language Instinct,: “The piece of information that makes one language different from another is called a parameter.” (p. 111) “Chomsky suggests that the unordered super-rules (principles) are universal and innate . . . All they [children] have to do is learn whether their particular language has the parameter value . . .”(p.112)

This suggests that we should consider two different periods in the acquisition of a first language. First, these ideas claim that every normal human is born with some capacity for language. What is inborn is a set of principles called "universal grammar" by Chomsky. However, if we add Lenneberg's ideas, the inborn capacity will only be realized if a child is exposed to human language within a certain time frame. The cases of feral children suggest that the critical period for triggering the innate instinct ends quite early, perhaps around age 6, although we don't know if other forces are also at play in these cases as well. Thus we may need to describe the critical period as the period of time within which the inborn language principles of universal grammar are activated in order to begin the parameter setting activity. The parameter setting period, then, may overlap with but be a different time frame than the critical period. It has been suggested that different parameters within a particular language are set at different times. From Munro and Mann's work, we may speculate that pronunciation is the first parameter set, Dr. John Madden (personal communication), has suggested that the pragmatic parameters are the last set, and that the process continues even through early adulthood.

If we wish to add a distinction between the critical period and the parameter setting period, we will need to overlay one on the other.

We also considered Vygotsky's ideas of how language and thought interrelate, and we can add this to our visualization. If we look closely, we can see that Vygotsky considers thought and language to begin to intertwine at about age two, when a child is in the overlap between the first words stage and the two word stage. Undoubtedly, the connection of meaning to utterances functions as one of Vygotsky's indications that thought and language are interrelating.

We might want to consider that the first decline in the language capacity seems to occur at approximately age three; just after the time when thought and language should begin to intertwine. More exactly, Vygotsky says the two lines of thought and language begin to curve towards each other at age two, and if the first drop in language learning occurs at about age three, this can be taken to mean that the attachment of thought to one particular language is what causes the reduction in language learning capacity. Why this might occur is open to speculation. Questions about what is really happening with feral children arise, too. Is their language ability lost because the critical period for its activation over? Have their brains simply developed differently so that thought and langauge remain separate forever? Can we have language without thought attached? Can we have thought without language? Or does the lack of development in either one prevent development in the other? What about cases like Christopher, who has reduced cognitive abilities in many areas, but has high capabilities for language? There is still much we do not know for sure, and many of these issues will return to the forefront when we consider second langauge acquisition.