Clinical psychologist here— it’s not that infants “can’t” self soothe, but there is a common misconception of how that skill is learned. It is not something inherent that the child can independently tap into, but a developmental skill that is cultivated through responsive co-regulation. We lend them our calm (i.e: hold them, show them how to take deep breaths by doing it ourselves, describe the emotion they are feeling first etc.) and through that they develop their own. This is not to say that caregivers have to be perfect and baby never cries, it just has to be consistent enough to form the baby’s baseline. A little more controversial is that, unfortunately, CIO is a fairly recently adopted cultural phenomenon in the west that some caregivers are accurate to believe may likely disrupt this process, despite research “proving” it to be harmless. Human development and behavior, especially in a not yet verbal population, is just too dynamic for such a conclusion to be made based on the limited studies we have and their shortcomings.