This study shows why babies calm down when they are carried.  
​https://www.psychologytoday.com/blog/the-athletes-way/201304/the-neuroscience-calming-baby

​https://www.washingtonpost.com/amphtml/news/parenting/wp/2017/08/09/the-rise-of-the-postpartum-helper/

An article in the journal Pediatric Child Health reports how scientists at the University of British Columbia and BC Children’s Hospital Research Institute followed about 100 infants over four years. This new study shows that the amount a baby is touched can leave lasting, measurable effects — not just on behavior or growth, but all the way down to the molecular level of the DNA. Those changes, the scientists speculate, could have negative effects on the way the child grows and develops. When they compared the methylation between the children, they found that there were consistent differences between the low-physical-contact and high-physical-contact children. In effect, the cells of the low-contact children were less mature than they should have been given the child’s actual age.

Read more here: http://www.miamiherald.com/news/nation-world/national/article186889938.html#storylink=cpy