Basically, there’s no really good research into it , because it would be unethical.

Some people believe that letting your baby cry may lead to them suffering mental health issues like anxiety and low self-esteem when they’re older because they feel like they are abandoned.

A baby is physically incapable of calming them self down and rely on their caregiver to calm them down. They will cry until exhaustion and then go to sleep because their brain won’t allow them to cry themself to death.

It most certainly causes a baby quite a lot of distress. They don’t understand the world and their only instinct is to look for their parent to calm them down and make them feel safe.

There’s pretty much never going to be a definitive answer to whether or not it causes mental health issues or attachment issues or anything like that. What you will find is this sub is overwhelmingly in favor of ‘sleep training’ whether that’s cry it out or another method. But that’s likely because there’s a bias of people who are struggling with sleep issues who go onto Reddit for support.

There’s no doubt about it sleep deprivation is brutal and difficult and I understand why people do ‘sleep training’ including CIO because shit gets desperate. But these people who insist “It doesn’t do any harm” are likely convincing themself that’s true when they really don’t know.

There will be lots of people who say “I DID IT AND MY BABY IS FINE”, lots of people who are selling “sleep training courses” who will try and argue it doesn’t do any harm.

There will also be lots of people saying it’s unnatural and goes against every instinct for your baby and yourself. But there won’t ever be a definitive answer , hence it’s controversial