As I note in this evidence-based overview of colic, colicky babies seem to react differently to world.
Individual differences of temperament have been observed during the first few days postpartum (Tsuchiya 2011), and to some degree, you can predict which newborns will become colicky by seeing how they respond to being undressed, handled, or put down. The babies who are most upset by these maneuvers are most likely to develop colic (St James-Roberts et al 2003).
So maybe colicky babies are simply less tolerant of disruptions and transitions.
Consistent with this idea, several studies have found that colic symptoms improve when parents are instructed to stimulate their babies less (Lucassen et al 1998).
There’s another possibility, too: Colicky babies may also react differently to care-giving maneuvers that are meant to soothe them.
In normally-developing infants, some calming stimuli can trigger the release of endogenous opioids, self-produced painkillers that bathe the brain. Soothing touch can also trigger the brain to release oxytocin, a hormone and neurotransmitter that counteracts stress. Do babies with colic respond the same way?
As noted above, research suggests that the real difference between babies with and without colic is that colicky babies have more trouble calming down once they begin crying. In one controlled study, researchers found that normal and colicky babies had the same frequency of crying bouts. However, the babies with colic cried longer (Barr et al 1992).
This is consistent with the “Pleistocene” baby care experiments that I mention in my review of the research about colicky babies. Increased carrying and breastfeeding reduced crying in normal babies, but not in babies with colic (e.g., Barr et al 1991).
And an experiment with soothing flavors reported similar results. Ronald Barr and colleagues gave 6-week old babies a sugar solution to taste, and discovered that all babies—those with colic and those without—responded to the sugar by calming down.
But the calming effect lasted longer for normal infants. Babies with colic were more likely to resume crying two minutes later (Barr et al 1999).
Why these differences? Barr’s team speculates these babies may have a less responsive distress regulation system: Perhaps soothing stimuli are less effective triggers of endogenous opioids (Barr 1999). According to this idea, colic eventually improves because the opioid release system matures.
As of 2013, I haven’t found any studies that address the oxytocin and colic. It would be interesting to know if colicky babies release less oxytocin in response to soothing stimuli.
Meanwhile, it seems a good guess that some babies cry excessively and inconsolably because of differences in chemically-mediated regulation systems.
Barr RG, McMullan SJ, Spiess H, Leduc DG, Yaremko J, Barfield R, Francoeur TE, Hunziker UA. 1991b. Carrying as colic "therapy": a randomized controlled trial. Pediatrics. 87(5):623-30.
Barr RG, Young SN, Wright JH, Gravel R, and Alkawaf R. 1999. Differential calming responses to sucrose taste in crying infants with and without colic. Pediatrics. 103(5):e68.
Barr RG, Rotman A, Yaremko J, Leduc D and Francoear TE. 1992. The crying of infants with colic: A controlled empirical description. Pediatrics 90: 14-21.Barr 1999
Lucassen PL, Assendelft WJ, Gubbels JW, van Eijk JT, van Geldrop WJ, Neven AK. 1998. Effectiveness of treatments for infantile colic: systematic review. BMJ. 316(7144):1563-9.
St James-Roberts I, Goodwin J, Peter B, Adams D, and Hunt S. 2003. Individual differences in responsivity to a neurobehavioural examination predict crying patterns of 1-week-old infants at home Developmental Medicine & Child Neurology 45(6):400-407.
Tsuchiya H. 2011. Emergence of temperament in the neonate: neonates who cry longer during their first bath still cry longer at their next bathings. Infant Behav Dev. 34(4):627-31.
content last modified 5/2013
For references cited in my other articles about colic, click here.