http://www.wsj.com/articles/what-your-babys-smile-can-tell-you-about-her-development-1448303794
Infants’ earliest grins are a primitive impulse but become a communication tool; know the types of baby smiles
http://www.wsj.com/articles/what-your-babys-smile-can-tell-you-about-her-development-1448303794