I have a 9 month old and her ability to sleep through the night is probably the most important thing in my life right now. Sleep training is common practice now and widely accepted, but was that always the case?
I'm curious about Roman times. What were the expectations? Was it different for wealthy citizens vs commoners? Was it not a thing at all?
I'm not optimistic that a proper answer to this question is possible. I've been looking through the literary sources (searching for various combinations of infan-, puer-, dormi-, cun-. fle-, vagi-) and at best I can find an expectation that crying infants require attention and should be fed. So, possibly, not in favor of crying it out?
Seneca writes:
Sic consolamur crustulo pueros, sic infantium fletum infuso lacte conpescimus
This way we comfort children with a cookie, this way we still the crying of infants by giving them milk. (Sen. Ep. 99)
Ovid has a crying infant get immediate attention from the wet nurse:
at puer infelix vagit opemque petit. / territa voce sui nutrix accurrit alumni,
... but the unlucky child is wailing and requires help. The wet nurse, startled by the voice of her ward, comes running, ... (Ov. Fast. 6,146-147)
Maybe stretching things a bit, Livy and Florus also have the she-wolf (as an image of ideal motherhood?) react to the crying of Romulus and Remus by approaching and breastfeeding them.
There's a book titled Infant Weeping in Akkadian, Hebrew, and Greek Literature by David A. Bosworth (Critical Studies in the Hebrew Bible 8, Winona Lake 2016) but I haven't been able to find anything for the Romans. The topic does not seem to come up in research on Roman sleeping habits (cf., for example, the RAC article on sleep).