The levels of stress hormones washing over the brain are increased when a baby is left alone to cry for long periods of time. A baby's brain is too immature to regulate emotions on it's own.

Too often it is thought that a baby will be "spoiled" through repeatedly meeting expressed needs. This is actually the opposite of what results.
Consistently soothing and comforting an infant leads to a calmer and emotionally healthy child.

Below is an excerpt from the writings of Heidi Holmen. I encourage you to go to this valuable website by Deborah Hage, MSW, and read the very impressive story Heidi shares. I often share this story when I do brain presentations to clearly demonstrate how quickly the developing brain adapts to experiences early in life.

Having a primary emotional bond with an attuned caregiver who predictably meets a baby's emotional needs is foundational for healthy psychic development and the ability to form healthy relationships with others. The ability to process and regulate one's own emotional state effectively, to delay gratification, to problem solve, to have empathy for others, are all critical for a healthy and secure sense of self. These are skills that are developed through the formation of healthy attachment.
In early developmental stages, an agitated or hyperaroused baby must be soothed by her primary caregiver in order to feel calm, safe and secure. The baby actually experiences her own emotional state as an extension of her caregiver. If her emotional needs are consistently met, over time the baby is able to take on some of this emotional regulation herself. Throughout early childhood, skills of self awareness and self regulation are reinforced. From there the child is able to begin to process the emotions of others, to develop a sense of empathy and to form healthy relationships beyond the primary one. These are lessons with clear and lifelong repercussions.
On the other hand, lack of healthy attachment can have devastating emotional repercussions. During the heightened state of arousal that occurs when a baby’s cries are not answered, she grows increasingly frustrated, distressed, angry, or hopeless. When her needs are not met, or met unpredictably, the baby does not develop trust. She might shut down emotionally and remain stuck at this stage of emotional development. Stunted emotional development can also negatively affect cognitive development.

Here is a link to the Love Your Baby packet to help parents get ideas on how to establish attachment during everyday life. http://www.braininsightsonline.com/brainDevelopmentProducts.asp

By Deborah McNelis, BrainInsights http://networkedblogs.com/bnEcj