Regarding leaving a baby alone to "cry it out..." PLEASE DON'T DO IT!
From Peaceful Parenting Blog:
"What are the emotional consequences of crying for the infant when she is left unattended? Bowlby and colleagues initiated a series of studies where children between the ages of one and two who had good relationships with their mothers were separated from them and left to cry it out. Results showed a predictable sequence of behaviours: The first phase, labeled “protest”, consists of loud crying and extreme restlessness. The second phase, labeled “despair”, consists of monotonous crying, inactivity, and steady withdrawal. The third phase, labeled “detachment”, consists of a renewed interest in surroundings, albeit a remote, distant kind of interest. Thus, it appears that while leaving babies to cry it out can lead to the eventual dissipation of those cries, it also appears that this occurs due to the gradual development of apathy in the child. The child stops crying because she learns that she can no longer hope for the caregiver to provide comfort, not because her distress has been alleviated...
According to attachment theory, many babies are born without the ability to self-regulate emotions. That is, they find the world to be confusing and disorganized, but do not have the coping abilities required to soothe themselves. Thus, during times of distress, they seek out their caregivers because the physical closeness of the caregiver helps to soothe the infant and to re-establish equilibrium. When the caregiver is consistently responsive and sensitive, the child gradually learns and believes that she is worthy of love, and that other people can be trusted to provide it. She learns that the caregiver is a secure base from which she can explore the world, and if she encounters adversity she can return to her base for support and comfort. This trust in the caregiver results in what is known as a secure individual.
Children who do not have consistently responsive and sensitive caregivers often develop into insecure individuals, characterized by anxious, avoidant, and/or ambivalent interactions. Long-term studies have shown that secure individuals, compared to insecure individuals, are more likely to be outgoing, popular, well-adjusted, compassionate, and altruistic. As adults, secure individuals tend to be comfortable depending on others, readily develop close attachments, and trust their partners. Insecure individuals, on the other hand, tend to be unsettled in their relationships, displaying anxiety (manifesting as possessiveness, jealousy, and clinginess) or avoidance (manifesting as mistrust and a reluctance to depend on others)."
Dangers of Leaving Baby to Cry It Out (CIO)