Why are sign languages considered to be natural rather than constructed languages?

Question

Linguists consider sign languages to be natural languages. Surely they had been invented/constructed at some point in time by someone.

So why are they not categorized as constructed, but as natural languages? And in what circumstances could they be considered to be constructed languages?

Answer 1

The history of sign languages is more akin to that of creoles than that of a constructed language which later gained native speakers. Consider the following two examples:

The grammar of Esperanto and a large chunk of its vocabulary was created, codified and written down by L. L. Zamenhof. People who were interested learned his language according to his rules and later passed it on to their children, creating the first native speakers of it.

The development of Tok Pisin is described as follows: Contact between English speakers and locals of Papua New Guinea gave rise to a pidgin, an incomplete and bare-bones not-quite-language that was just good enough to communicate - everyone would have spoken it a bit differently at that point in time. It became used more commonly and was passed down as a native language to children, creating a full-fledged language. At no point however did its speakers sit down and decide on what grammar Tok Pisin ought to have.

Now compare these with the case of the Nicaraguan Sign Langauge, the currently only natural langauge which we have actively seen develop. Deaf children of Nicaragua previously communicated with the people closest to them with improvised home signs and gestures (in other words they had at best something akin to a pidgin). When they were brought together in a new school for the deaf, they had the need to communicate with each other. The school encouraged use of mouthing or fingerspelling Spanish, but the students, who were not native speakers of Spanish failed at learning these skills. Instead, they started communicating by using their own home signs, forming a pidgin that developed over the following years and was taught to younger students who later arrived. Over time, the language developed complexities similar to what you would find in other signed languages - evidence that it had developed beyond the initial pidgin stage.

It is clear that the example of Nicaraguan Sign Language is much more similar to that of Tok Pisin than that of Esperanto.

Answer 2

The constructed versus natural language distinction is less of a binary opposition, and more of a continuum. There are many spoken languages as well that can partially qualify as 'constructed', including Standard Italian and Nynorsk. Every language to some degree has been modified by someone's conscious choice. A conlang is set apart largely by the degree of conscious choice that's been applied to it.

Signed languages, though they are relatively new on a linguistic scale, are not typically made up by a majority of constructed grammar rules and so on. The signs themselves are often consciously created (though just as often not), but the grammar around them is mostly generated spontaneously as the language is used more and more - rules begin to coalesce as people need to use their language in more and more circumstances. No one sat down and decided that in ASL, for example, you could set up a location as a pronominal referent and direct future signs toward that location as a way of referring to that pronoun. It simply happened - someone started doing it, what they were doing was sufficiently clear to get across, and then it was grammar. In a conlang, that grammar would have been the result of a conscious choice.

Honestly, spoken languages could have a very similar history. There's nothing that sets signed languages apart from spoken ones in this regard except their newness - we've watched sign languages come into being out of nothing, while all spoken languages are derived from some ancestor that's been lost in the depths of time. Spoken languages could have had an absolutely identical genesis, for all we know.