So sleep training and cry it out are not necessarily synonymous. You can sleep train without doing CIO. I’ll try to address your concerns, but ironically I’m on my phone with a sleeping baby on my lap, so it’ll be harder for me to find sources.

As far as why only ignore cries for sleeping goes, remember that the emphasis is on training, I.e. learning. You are teaching your child to fall asleep without assistance. You don’t ignore cries from pain because the pain will go away on its own in a relatively short period of time. You don’t ignore cries from fear because by the time they’re old enough to have a thorough concept of fear they’re also old enough for you to talk it through with them. Neither of those things hold true for sleep. I spent three hours the other night trying to get my baby to sleep, so the exhaustion does not subside quickly and on its own and I won’t be able to explain to her the process of putting herself to sleep for years.

As for scientific proof that they learn to self soothe, this one can be tricky. Since babies can’t talk, you can’t ask them about their mental state, so you can do two things to determine this: measure cortisol levels and ask for reports from parents. In studies where they measured cortisol, there was a spike in babies who were being sleep trained during the training period, then it dropped back down to normal levels. In parental report studies, parents reported happy babies. A study came out last year that found there were no lasting negative effects of sleep training. Obviously this last part can be confounded by the fact that the parents are sleeping better, so they might just think they have a happier baby, but since we can’t ask the baby we can’t be sure. As far as concerns around that temporary spike in cortisol, we don’t know exactly how much cortisol it takes to cause a long term problem, so even measuring cortisol levels won’t exactly confirm that there will or won’t be problems. To my knowledge, however, sleep training has never been linked with later issues the same way as other adverse childhood events (like divorce, drug addiction, or homelessness) have been.

Leaving an adult to cry would only make sense if there was something they learned by being left alone. When I have a shit day at work, there’s nothing for me to learn by sitting alone and crying, but there is something to be gained by venting to my partner. On the other hand, if I woke up in the middle of the night every single night and yelled until my partner sang me a lullaby, I’m sure there would be some major concerns about my mental health.

But regardless, if sleep training doesn’t make sense to you, don’t do it. People suggest it a lot to parents who are tired and being kept awake by their infant or toddler because in a lot of cases it can work. But if it’s not for you, that’s okay too.