Many of the differences between signed and spoken languages can be attributed to their different modalities: one makes use of our visual-gestural system, and the other of our aural-oral system. Differences in dexterity, processing time and other factors combine to create different modes of expression. I looked at these differences in my previous post on sign languages.
However, sign languages differ from languages like English in various other ways too. Most of these differences are sociolinguistic: social factors have a large impact on how we use language, and sign languages are no exception.
Minority status
Sign languages are minority languages: they exist in a context in which the majority of people are speaking another, usually more socially prestigious language. There are plenty of spoken minority languages too, like Welsh.
Daily contact with another language results in large amounts of borrowed vocabulary, of one kind or another. In BSL, this can be through mouthing whilst signing, or through the use of finger-spelling. Various words will enter the lexicon through finger-spelling, and become gradually simplified until they conform to the phonology of the sign language.
There are also various social effects, such as deliberately suppressing or encouraging the use of the minority language, which then gives rise to linguistic changes.
Age
It is also almost universally the case that sign languages are younger than spoken languages. There are very few spoken languages around today which do not have their origins in spoken languages stretching back hundreds of years or longer. Creoles are young spoken languages.
Sign languages, on the other hand, are only passed on when Deaf children can learn them from Deaf adults, which may not happen for a variety of societal reasons. As a result, many sign languages used today are less than a century or two old.
BSL can be traced back to the 1600s, and ASL to the 1800s, though without a written record, it has hard to say how much has been retained.
BSL can be traced back to the 1600s, and ASL to the 1800s, though without a written record, it has hard to say how much has been retained.
Unwritten
Out of the several thousand spoken languages, only 73 have enough recorded writing to have a "literature". No known signed language has a written literature.
There have been various attempts at devising a writing system for a signed language, but so far none of them has caught on except among academics.
There have been various attempts at devising a writing system for a signed language, but so far none of them has caught on except among academics.
Vocabulary size
An English dictionary might contain 1.5 million words, though the Center of Reading Research at Ghent University, Belgium, puts the non-inflected lexicon of English at around 60,500 words. A proficient native speaker will have a vocabulary of around 40,000 words. (A mid-to-high proficiency second language speaker will have 6,000 to 20,000 words.)
By comparison, a current sign language dictionary might contain a couple of thousand words. The Auslan sign bank contains 4000 signs, and that appears to be about equal to a native speaker's vocabulary. There are certainly not thousands of signs which a native speaker does not know!
This is probably due to a combination of the factors above: any technical vocabulary is in the majority language instead; specialised vocabulary used mostly in writing is also in the majority language, and so new technical vocabulary takes longer to become standardised; and without a written record, and with only a short history, the language does not accumulate synonyms and archaic terms.
I do not have vocabulary figures for an unwritten language or a creole to compare.
Lectures which I have had in BSL, which obviously require a similar standard of specialist vocabularly to those in English, involve a reasonable amount of finger spelling, with such words being shortened as they were repeated. It may be that, in daily use, the researchers have individual signs for those concepts, and they were just unknown to the interpreter. But the interpreter works with them often enough to know the English technical terms, so I must assume not yet!
By comparison, a current sign language dictionary might contain a couple of thousand words. The Auslan sign bank contains 4000 signs, and that appears to be about equal to a native speaker's vocabulary. There are certainly not thousands of signs which a native speaker does not know!
This is probably due to a combination of the factors above: any technical vocabulary is in the majority language instead; specialised vocabulary used mostly in writing is also in the majority language, and so new technical vocabulary takes longer to become standardised; and without a written record, and with only a short history, the language does not accumulate synonyms and archaic terms.
I do not have vocabulary figures for an unwritten language or a creole to compare.
Lectures which I have had in BSL, which obviously require a similar standard of specialist vocabularly to those in English, involve a reasonable amount of finger spelling, with such words being shortened as they were repeated. It may be that, in daily use, the researchers have individual signs for those concepts, and they were just unknown to the interpreter. But the interpreter works with them often enough to know the English technical terms, so I must assume not yet!
Conclusion
Sign languages differ from languages like English, and Spanish, and Mandarin, in ways that have nothing to do with modality. But these are not differences between signed and spoken languages: these are the differences between English and Welsh, French and Haitian Creole, Portuguese and Ngandyera. These are the differences of history and social status.
Reference
Vocabulary estimations (check your own, and contribute to linguistics research): http://vocabulary.ugent.be
Statistics on status and writing systems of languages: http://www.ethnologue.com