I wouldn't say I have 50 (I was being more general) , but I still stand by my statement about correlation vs causation.
It's unfortunate that as a Teacher of the Deaf you've never seen a study that compared signers to non-signers. Shouldn't your teaching methods be based on methodology that has been proven by research to be effective? finding these studies on pubmed is *trivial*.
https://www.ncbi.nlm.nih.gov/pubmed/?term=sign+language+deaf+speech+children <-here is a good place to start.
http://www3.gallaudet.edu/Documents...al_Webcast_May_17_2016-for_upload-5-24-16.pdf <--more references supporting the use of ASL, not necessarily primary research, however.
https://www.ncbi.nlm.nih.gov/pubmed/19046778 <- study comparing signers to non-signers, found some differences in "The children in Group 1 (bilingual) had better verbal and manual expression whereas those in Group 2 (spoken) achieved better results in terms of speech intelligibility, auditory reception and grammatical closure" which is interesting because the new study said the same thing about intelligibility so maybe there is something there, but these few studies are still limited in scope.
http://pediatrics.aappublications.org/content/early/2015/12/18/peds.2015-1974 <--meta-analysis that came to no strong conclusions either way.
I think, given the research I have read, that I can come to these conclusions:
1. Signing does not have a negative effect on spoken language, and
1a. Therefore there is no reason to prohibit children/parents from learning it if they want.
2. Signing doesn't necessarily have a positive effect on spoken language acquisition either.
2b. This is probably due to most parents of children not being fluent in ASL, but that's just my guess.
3. For some children who are unable to fluently acquire spoken language despite intervention(s), they can still likely acquire a fluent signed and/or written language.
3b. Therefore prohibiting sign language will be a detriment to those children unable to acquire spoken language.
This newly published study adds to the body of scientific knowledge about Deaf education but is in no way definitive and should not be treated as such.
P.S. I am a published scientific author. Not in Deaf studied but in biomechanics, however the ability to analyze research is based on principals of science - which is irrespective of the field.