As I was thinking about language development in infants recently, I concluded that babies should be spoken to as adults so that they get an accurate sense of their parents’ language early on. By using “baby talk” when speaking to an infant, you can actually delay his or her cognitive development by making the language more confusing. Your baby will wonder if he is supposed to talk in a high pitched expressive voice all the time or use an even paced fluid tone, which will muddle his general understanding. However, my conclusion led to another set of curiosities. When should you start censoring what you say so that your baby doesn’t begin to repeat it?
 
According to recent studies, your baby will be able to hear you as early as his ears develop in the womb, but will only begin listening at three months of age. Though he is listening this early, he obviously won’t be able to comprehend sentences or even words. He is actually listening for tones so that he recognizes his family and also so that he knows how the language should sound in a basic way. If you’ve ever listened in on a language you don’t understand, you know that they sound very different from your own even without a basic understanding of each word. Eventually, listening will turn to talking, and your baby will start making sounds and words of his own. When he turns 18 months old, you should expect repetition to start happening. Your baby will repeat almost everything he or she hears without any filter, which is why it’s important you start censoring yourself and guests so that swears and insults don’t resurface at daycare. Of course, you should actually make sure you stop swearing before that in case your little one stores any words in his or her memory for later use.

Using bad words or speaking harshly in front of your three month old will have no consequences, assuming you and your partner aren’t yelling or creating a stressful environment. (If that’s the case, other consequences will arise.) However, be prepared to stop suddenly when your child enters the phase of word repetition. No matter what you say, it will get repeated as your baby makes his or her way through the indexes of language. Even when you think no one is listening, your baby is taking mental notes.

Source: Edy Veneziano et al: From One Word to Words: Repetition Patterns on the Way to Structured Speech. Journal of Child Language Volume 17 Issue 3 pp. 633-650 October 2010