The study summarized below was conducted on rats, but parent-child bonding at the physiological level is pretty much the same in all mammals - so this does translate well to humans.
The study found that the presence and nurturing behaviors of the mother (or father, or primary care-giver) toward the newborn directly shapes the wiring and function of the infant's brain. This is the first study to show this process (which is well-known) WHILE it is happening.
Pretty cool. The paper itself, of course, is behind a paywall, so below is the summary from Science Daily, followed by the abstract of the original article.
Date: July 17, 2014
Source: NYU Langone Medical Center
By carefully watching nearly a hundred hours of video showing mother rats protecting, warming, and feeding their young pups, and then matching up what they saw to real-time electrical readings from the pups’ brains, researchers have found that the mother’s presence and social interactions — her nurturing role — directly molds the early neural activity and growth of her offsprings’ brain.