Many parents these days are using sign language as a way to communicate with their babies, and finding that the babies will use signs back. I did some poking around the web to see what I could see on this subject.
The most frequently-referenced book in the field appears to be Linda Acredolo and Susan Goodwyn's Baby Signs. The book has its own web site. There are a few photos from Acredolo's research at her university page.
There's a page of discussion from the UC Berkeley Parents mailing list online. The people who contributed to this discussion are positive about the idea of teaching your baby to sign. One parent suggests using standard American Sign Language (ASL) instead of the made-up baby signs promoted by some books, as a way of making it possible for your child to grow up bilingual.
Jane Nichols discusses her own experience with teaching ASL to a young child. Once again she confirms what others have said: "At first I was worried that teaching Sarah to sign would slow her speech process. I found it to be the opposite. At 17-months Sarah could say more than 50 words and enjoyed her accomplishment of communicating."
After a session of web searching, a few things are obvious:
I'm not going to bother linking to the dozens of news stories based on info from Acredolo & Goodwyn's publicist. The Fresno Bee's coverage is better than most, since they made an effort to seek out other researchers for comment. The verdict on early signing is still universally positive.
There's a roundup of the printed research on signing and babies. The conclusion: "The findings of almost all of the studies is that using both spoken English and sign language with an infant can greatly increase the rate and efficiency of language development."
The bottom line appears to be that teaching your baby to sign is not especially controversial. No one's out there screaming about the dangers of early signing, and all the studies I can find agree that signing babies do better on verbal communication as they grow older. The controversy is over whether to use the sort of made-up signs promoted by Acredolo, Goodwyn, and their followers, or standard ASL. Certainly it's easier to use your own idiosyncratic signs, or the simple ones you'll find in baby signing books. But over the long run, if your child learns any ASL they'll have one more recognized language to share with the world.