Babies can tell if you are anxious or upset. Just as an example...you are taking your baby to get vaccinated so you are nervous knowing that they will be hurt and upset. Your baby picks up on this and, in turn, becomes anxious and upset making the visit to the doctor worse. That's why they say in a situation where your baby may be upset, you should remain calm and upbeat so they are less likely to think...mommy is nervous. I should be nervous as well. Also, I saw a documentary which quoted studies about the effects of parental behavior on devoloping brains. Just like a baby does not remember learning how to walk, but they remember how to walk. A baby may not remember that time you got really upset and started screaming and yelling, but they will remember it another way and this will have a negative affect on them. The documentary is called bomb in the brain. You can watch it on youtube.