There have actually been precious few studies done on sleep training and for me, that's where a large part of the problem lies. However, the research that is there doesn't support extinction sleep training as a safe or effective method. It's been shown to reduce stress signaling and has been shown to be statistically ineffective by 6mo after training has taken place. There is a scientific consensus on the harm of high levels of stress on developing brains. Sleep training causes infants to stop signalling their stress, even though they still feel it, which disrupts the ability of the adult caregiver to sooth such stress. While I'm not going to argue that cio will cause a baby to become a serial killer or a moron, it's generally understood that any straying from the conditions in which our bodies evolved to perform best under will result in disimproved outcomes. And cio/cc/ferberizing/etc is a huge leap from what humans evolved to expect from caregivers.
You summed up the problem with infant sleep theories in your first sentence: precious few studies have been done. There are also just as many studies that say sleep training is safe as there are that say it's not. We can argue and throw studies out from both sides of the argument all day, but in the end the scientific community doesn't know for sure. Because of that, it does come down to the mother to make the right decision for her family based on her knowledge of her child.
High levels of stress are harmful on a developing brain. However, the anti-crying community uses this fact in misleading ways. The babies that the stress studies were done on were abused and neglected. You cannot turn around and say that
babies left to cry for a few nights are in the same category. Continual, sustained abuse is not even in the same ballpark as sleep training regardless of any feelings by those who find the process distasteful. Objectively, they are two completely different circumstances.
I am only pro-sleep training in certain situations. I do think it is overused. However, if used properly I think it can be beneficial. Saying that it will cause any unseen or long term damage is misleading because you cannot prove it. In fact, there are several large studies that have found otherwise. The best any mother can do is make a choice based on her individual child and hope that the next generation will have better answers.
Did you read the link? Do you even bother reading the studies at all? Do you critically evaluate them in the least? Because every single study saying "there is no harm" fails to use any physiological marker as an indicator.
Every single one of them use
parent reported attachment, stress, and sleep quality as a measurement of harm.
Every study that says "there could be harm long term, there
is harm short term" uses either physiological measurements of stress or blind-reported independent evaluations of attachment and dyad interaction.
Everyone strawmans the extreme stress studies saying that they're not comparable to cio, but completely ignore the plethora of published work on mild-to-moderate levels of stress that also point to disimproved attachment, cognitive function, and stress management.
ETA: to bolded: Pro-cio-ers always say that baby only cries for a few nights. This completely ignores that it's been shown that
signalling stops,
stress does not. Just because they stop telling you about their stress doesn't mean it's not there. The other study based on mother-reported sleep difficulties also shows that sometime between 4w and 6mo after training has taken place, the infants made to cio and the infants with mothers who wio report comparable sleep reports-- meaning that either sleep would have sorted itself in that time or the cio babies stopped sleeping as 'well'. A majority of the cio families had had to sleep train again within the 6mo. So it's
not a few nights for most families. It's who knows how many nights of unsignaled stress, to be repeated
again when the training wears off.
ETA again:
I have no interest in bashing the people, but I will bash the methods you may choose to use. Please don't confuse the two.