The short answer is “no.” However, that doesn’t go without qualification. Studies have shown that signing drags communication levels forward. In fact, a baby taught to sign will be a full year ahead of another who hasn’t by age 2. That’s a significant improvement.
Up for debate is whether or not a baby really is smarter. By best measures, signing can add up to 10 IQ points. If you trust this measure, then babies who sign are smarter, but I think there’s more to it than this.
In my opinion, signing is an indication of your baby’s knowledge. Signing is an exhibit of the things they know, the things they are interested in and their personality. Signing shouldn’t be about making babies smarter, this is something I will argue vehemently. If you’re in it just for the IQ points, then you’re going to be in it for the wrong reasons. Yeah, these are nice side benefits, but the true benefit will come from connecting with your baby. To this aside, you’ll also be better able to manage your baby because you’ll know much more about their desires and needs.
So while studies show that babies measure higher in vocabulary and communication skills, and even IQ, I don’t think that babies are smart just because they sign. I think a lot has to do with the significant increase in the quality and quantity of time they spend interacting with adults. If signing is the excuse you need to interact with your baby, then go for it, but by all means interact with your darling for all the right reasons.