As far as I can make out, none of the papers which show a raised cortisol response from certain types of parenting have actually been able to tell us what levels of cortisol are bad for a child.
As far as I understand it, cortisol levels rise at any type of stress or stimulation; so a child who is having a great time at a party will have higher levels of cortisol than a child of the same age who spends the entire day wrapped up in a blanket listening to lullabies. A toddler who falls out with a friend and bashes him on the head will probably have higher levels of cortisol than a toddler who is protected from that kind of situation by only being around gentle adults who arrange everything around him.
Now we can all agree, I think, that the latter way - avoiding all stress- is not the ideal way to raise a child. But where do we draw the lines? What is a good, safe level of cortisol and what is the sign of harm being done? Do we know? Does anyone know?
Can we do anything better or more reasonable than using common sense to decide what kind of stress situation might be worth a rise in cortisol levels because of other benefits.
I am sure ds, when he was a baby, sometimes had higher cortisol levels because he had to wait while I was attending to his older sister. But that is offset against the enormous benefit of having an older sister, a benefit that becomes more apparent the older he gets.
When I had to stop breastfeeding after a few months to change medications, that really felt like I was letting ds down. But that had to be offset against the fact that I was a far safer person to be in charge of a baby when I was no longer suffering from the side effects of a medicine that was turning me into a zombie; I could easily have dropped him on his head, I was that out of it. I imagine that's the sort of situation that might make somebody go down the road of sleep training: realising they are so sleep deprived that they are no longer safe to look after the baby.
I am glad that I never had to try any kind of sleep training because other things worked for us. But then I'm sure the women who were able to breast-feed for a year because they don't have to worry about medication are glad about that too. We all do the best with the situation we are given.