The cry-it-out method is usually attributed to Ferber. His sleep method, however, is actually a variation of the CIO method. So is Weissbluth's. The CIO method I'm talking about is the one maybe your mother or grandmother used. It's the one my own pediatrician told my mother to use with me so she could stop spending her nights driving around only to have me finally fall asleep and then awaken as soon as she turned off the car. It's the method Mary's father recommended when he saw she was having difficulty getting her first child to sleep.
What does this method entail? It's pretty simple: put the baby down asleep at bedtime and don't return to the room until the end of the sleep period. So, at nap-time, you put the baby down cooing and gurgling, walk out of the room, close the door, and don't return for two hours -- even if your baby is crying the entire time. Same thing at nighttime.
According to those who use/d this method, after 3-5 days the baby should stop screaming and start sleeping. Why does this method work in such a short period (though it may feel like forever as you're listening to your little one wail)? Crying, some claim, actually acts as an amnesic (Fagen, Ohr, & Fleckenstein, 1985). When little Johnny is crying in bed and you go to him, take him out of bed, and comfort him, he's learning that crying gets you to come to him. His crying is rewarded. If, however, you don't respond, then he'll forget you used to come when he cried in his bed, and instead of crying, he'll sleep.
Other researchers, however, claim that crying raises cortisol levels, adversely altering the baby's brain and making it more likely that little Johnny will have PTSD as an adult. (Admittedly, I couldn't find an actual study verifying this claim in any way. All I could find was this opinion paper. If anyone knows of an actual study, please let us know!) This claim has some merit. It's true that stress raises cortisol levels, and if a baby isn't getting his reward, then that can be stressful.
So who's right?
Fagen, J. W., Ohr, P. S., Fleckenstein, L. K. (1985). The effect of crying on long-term memory in infancy. Child Development, Vol 56(6), 1584-1592.