The first social smile may appear as early as 6 weeks, but it's unlikely to be a regular feature for quite a while. (babycenter.com)
Once your babies begin social smiling and laughing, their sleep starts to develop, too. (twinloveconcierge.com)
The first time this happened to me, it was when my son began to do social smiles while my daughter did not - even after two weeks. (twinloveconcierge.com)