The Anatomy of Crying
But why do babies who are not in pain cry so much? Clearly some crying is an attempt to communicate something -- hunger, a wet diaper, boredom. That still doesn't explain why some babies shriek and others merely fuss, or why the shrieking doesn't stop once the problem is remedied. Interestingly such crying is unique to Homo sapiens. "All primates exhibit Brazelton's crying curve," says Joseph Soltis, PhD, a research scientist at the National Institutes of Health, "but only human babies have the extreme manifestation we call colic."
Blame biology. Most mammals are relatively self-sufficient at birth. A horse gives birth to a foal that will walk and even run on its first day. "Because of our large brains, the head of a fully developed baby would be too large to pass through the human pelvis," explains Dr. Soltis. Pediatrician Harvey Karp, MD, author of The Happiest Baby on the Block (Bantam), calls the first months of life the fourth trimester. "A newborn human is more like a fetus than a baby," he says. "The best way to calm her is to replicate as closely as possible the conditions of the womb."
Humans' early eviction from the womb means we are born with lots of still-developing neural connections. So erratic circadian rhythms -- in common parlance, having days and nights mixed up -- may be behind the pattern of babies' crying. "A grown-up's rhythms keep us alert until early evening, at which time the calming drive takes over," says Dr. Soltis. "A baby's 'alertness drive' kicks in at three weeks, but its opposite, the calming reflex, doesn't develop until three or four months. That's why an infant may start crying in late afternoon (after a full day of stimulation) and be unable to stop -- because she has no way to calm herself. (Significantly, in Dr. Karp's research, premature babies don't develop colic until two weeks after their due date.)
While it's impossible to know what an infant is thinking and feeling during a wailing session, the body is a fairly reliable indicator of stress. And in research conducted by Barbara Prudhomme White, PhD, assistant professor at the University of New Hampshire in Durham, colicky infants' physiological indicators of stress (such as heart rate and levels of the stress hormone cortisol) were no different from those of other infants. As for parents' stress levels during these crying bouts, it's a safe bet they'd be sky high.