My mother just had a baby girl yesterday and I have always been the youngest so I have never held a baby before until yesterday. When I held her for the first time yesterday my immediate instinct was to start bouncing her softly/ bobbing her up and down. I started talking with my mom and wanted to know why it is the human beings instinct when holding a baby?
It isn't instinctive. Many new parents have difficulty learning how to rock and hold babies.
Generally, though, most people learn it through behaviours through their childhood.
An odd quirk, though, is that babies often like to be rocked faster than the person holding them would expect - we first encountered this when we became parents: our kids liked being rocked nearly twice as fast as I would have thought comfortable, but after asking people and searching online this seems the norm.