Aside from the emotional issues, there are the physiological, which in my mind cannot be separated anyway, as nature intended the mother and baby to be emotional about this.
Here's a really interesting piece of research:
"Research by Christopher Coe in the United States suggests that although behaviours may be altered by such methods as leaving babies to cry it out, the physiological effects remain. Coe took infant squirrel monkeys and separated them from their mothers, and then monitored both their levels of 'distress calling' - that is, crying - and the cortisol levels in their blood. After a time, the monkeys ceased calling for their mothers and seemingly had overcome their distress. However, their cortisol levels remained high, and their immune systems and the development of their brains and other systems were negatively affected."
Coe, C.L., Glass, J.C., Wiener, S.G., Levine, S. (1983). Behavioral, but not physiological, adaptation to repeated separation in mother and infant primates. Psychoneuroendocrinology, 8(4): 401-409
Research has also been done that shows that babies being held by their mothers regulate their breathing better than those left to sleep alone. Interestingly, cultures that don't leave their babies to sleep alone or to cry it out, have no term for SIDs, or what the British call 'cot death.' This is because nature intended for a mother to hold a new baby while his nervous system is undeveloped. Other studies show the dangers of babies who might be thirsty or hungry being left to cry and then suffering dehydration. Even the American Academy of Pediatrics, who are known to be conservative to a point of being non-commital, warns against scheduling the feedings of young babies, due to the number readmitted to hospitals every year due to dehydration.
Interestingly, nature got it right here too, because breastmilk alters its composition to provide just water to fend off dehydration when the baby sucks lightly. The mother is programmed to want to offer to nurse when the baby is restless, and to read its cues that it is thirsty, but we have moved so far from our instinctual selves that we often miss these cues. The formula companies have persuaded mothers that they can equal God's design, or even better it.
Of course, it can be argued that the baby 'learns' by sleep-training, but I would question exactly what he learns. Why is everything, especially 'sleeping through the night', a stress in our society? I think a huge factor is that we fail to consider the basic physiology of woman and baby, and don't trust that nature knows best. There is a problem developing in the Western world because man is losing his trust in God's design, and is starting to believe that he can improve on it. God created in babies the instinct to want to suckle. He created in mothers the instinct to want to hold their babies. When a mother nurses a baby, sleep hormones for both mother and baby are released. The baby falls asleep in the mother's arms, and she rests as her baby rests. The baby needs to be held and mother wants to hold the baby.
We will never know if it harms a child long-term if he is left to cry, but the few pieces of research done, show that it does do short-term harm. I don't think that you can pin point an aspect of a child's personality and put it down to cio, or any one experience of their childhood - each individual is a result of a complex mixture of nurture and nature. But cio cannot be an experience that adds a plus to a child's personality or self-esteem. It is logical to me that it adds a negative. But you wouldn't know how the child would have turned out without it.
I see the "cio does no harm" as being in line with "my parents spanked me, but I turned out OK." But I might have made a few disastrous relationships, I might have had problems with friendships, I might have felt the need to meddle with addictive substances, I might have bullied or been bullied as a teenager, I might have underachieved academically ...........the possibilities are endless. How many of those things could be directly attributed to being spanked it is impossible to say, but they are a result of the sum total of the experiences of childhood.