Why does the baby cry? Fact sheet.

Gloria at 2 months, crying.

Two weeks after delivery, when the husband went back to work and my hemoglobin level had recovered enough to let me think about anything besides breathing, I seemed to be spending a lot of time on The One Question: Why does the baby cry? We had been drowned in baby books that all had something helpful to say. Or so I believe, not having read them. But what really is the evolutional origin of all that crying to begin with? That’s what I was wondering. Is there a reason to begin with?

You don’t need a degree to know that baby cries if she’s unhappy. After a few weeks I had developed a trouble-shooting procedure roughly like this: Does she have a visible reason to be unhappy? Does she stop crying if I pick her up? New diaper? Clothes comfortable? Too warm? Too cold? Is she bored? Is it possible to distract her? Hungry? When I had reached the end of my list I’d start singing. The singing almost always helped. After that, there’s the stroller and white noise and earplugs.

Yes, the baby cries when she’s unhappy, no doubt about that. But both Lara and Gloria would sometimes cry for no apparent reason, or at least no reason that Stefan and I were able to figure out. The crying is distressing for the parents and costs the baby energy. So why, if it’s such an inefficient communication channel, does the baby cry so much? If the baby is trying to tell us something, why haven't hundred thousands of years of evolution been sufficient to teach caregivers what it is that she wants? I came up with the following hypotheses:

A) She doesn’t cry for any reason, it’s just what babies do. I wasn’t very convinced of this because it doesn’t actually explain anything.

B) She cries so I don’t misplace or forget about her. I wasn’t very convinced of this either because after two months or so, my brain had classified the crying as normal background noise. Also, babies seem to cry so much it overshoots the target: It doesn’t only remind the caregivers, it frustrates them.

C) It’s a stress-test. If the family can’t cope well, it’s of advantage for future reproductive success of the child if the family breaks up sooner rather than later.

D) It’s an adaption delay. The baby is evolutionary trained to expect something else than what it gets in modern western societies. If I’d just treat the baby like my ancestors did, she wouldn’t cry so much.

So I went and looked what the scientific literature has to say. I found a good review by Joseph Soltis from the year 2004 which you can download here. The below is my summary of these 48 pages.

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First, let us clarify what we’re talking about. The crying of human infants changes after about 3 months because the baby learns to make more complex sounds and also becomes more interactive. In the following we’ll only consider the first three months that are most likely to be nature rather than nurture.

Here are some facts about the first three months of baby’s crying that seem to be established pretty well. All references can be found in Soltis’ paper.

• Crying increases until about 6 weeks after birth, followed by a gradual decrease in crying until 3 or 4 months, after which it remains relatively stable. Crying is more frequent in the later afternoon and early evening hours. These crying patterns have been found in studies of very different cultures, from the Netherlands, from South African hunter-gatherers, from the UK, Manilia, Denmark, and North America.
• Chimpanzees too have a peak in crying frequency at approximately 6 weeks of life, and a substantial decline in crying frequency by 12 weeks.
• The cries of healthy, non-stressed infants last on the average 0.5-1.5 seconds with a fundamental pitch in the range of 200-600 Hz. The melody is either falling or rising/falling (as opposed to rising, falling/rising or flat).
• Serious illness, both genetic and acquired, is often accompanied by abnormal crying. The most common cry characteristic indicating serious pathology is an unusually high pitched cry, in one case study above 2000 Hz, and in many other studies exceeding 1500 Hz. (That’s higher than most sopranos can sing.) Examples are: bacterial meningitis 750-1000 Hz, Krabbe’s disease up to 1120 Hz, hypoglycemia up to 1600 Hz. Other abnormal cry patters that have been found in illness is biphonation (the simultaneous production of two fundamental frequencies), too low pitch, and deviations from the normal cry melodies.
• Various studies have been conducted to find out how well adults are able to tell the reason for a baby’s cry by playing them previously recorded cries. These studies show mothers are a little bit better than random chance when given a predefined selection of choices (eg pain, anger, other, in one study), but by and large mothers as well as other adults are pretty bad at figuring out the reason for a baby’s cry. Without being given categories, participants tend to attribute all cries to hunger.
• It has been reported in several papers that parents described a baby’s crying as the most proximate cause triggering abuse and infanticide. It has also been shown that especially the high pitched baby cries produce a response of the autonomic nervous system, measureable for example by the heart rate or skin conductance (the response is higher than for smiling babies). It has also been shown that abusers exhibit higher autonomic responses to high-pitched cries than non-abusers.
• Excessive infant crying is the most common clinical complaint of mothers with infants under three months of age.
• Excessive infant crying that begins and ends without warning is called “colic.” It is often attributed to organic disorders, but if the baby has no other symptoms it is estimated that only 5-10% of “colic” go back to an organic disorder, the most common one being lactose intolerance. If the baby has other symptoms (flexed legs, spasm, bloating, diarrhea), the ratio of organic disorder goes up to 45%. The rest cries for unknown reasons. Colic usually improves by 4 months, or so they tell you. (Lara’s didn’t improve until she was 6 months. Gloria never had any.)
• Colic is correlated with postpartum depression which is in turn robustly associated with reduced maternal care.
• Records and media reports kept by the National Center on Shaken Baby Syndrome implicate crying as the most common trigger.
• In a survey among US mothers, more infant crying was associated with lower levels of perceived infant health, more worry about baby’s health, and less positive emotion towards the infant.
• Some crying bouts are demonstrably unsoothable to typical caregiving responses in the first three months. Well, somebody has to do these studies.
• In studies of nurses judging infant pain, the audible cry was mostly redundant to facial activity in the judgment of pain.

Now let us look at the hypotheses researchers have put forward and how well they are supported by the facts. Again, let me mention that everybody agrees the baby cries when in distress, the question is if that’s the entire reason.

1. Honest signal of need. The baby cries if and only if she needs or wants something, and she cries to alert the caregivers of that need. This hypothesis is not well supported by the facts. Baby’s cries are demonstrably inefficient of bringing the baby the care it allegedly needs because caregivers don’t know what she wants and in many cases there doesn’t seem to be anything they can do about it. This is the scientific equivalent of my hypothesis D which I found not so convincing.
2. Signal of vigor. This hypothesis says that the baby cries to show she’s healthy. The more the baby cries (in the “healthy” pitch and melody range), the stronger she is and the more the mother should care because it’s a good investment of her attention to raise offspring that’s likely to reproduce successfully. Unfortunately, there’s no evidence linking a high amount of crying to good health of the child. In contrast, as mentioned above, parents perceive children as more sickly if they cry more, which is exactly the opposite of what the baby allegedly “wants” to signal. Also, lots of crying is apparently maladaptive according to the evidence listed above, because it can cause violence against the child. It’s also unclear why, if the baby isn’t seriously sick and too weak to cry, a not-so-vigorous child should alert the caregivers to his lack of vigor and trigger neglect. It doesn’t seem to make much sense. This is the scientific equivalent of my hypothesis B which I didn’t find very convincing either.
3. Graded signal of distress. The baby cries if she’s in distress, and the more distress the more she cries. This hypothesis is, at least for what pain is concerned, supported by evidence. Pretty much everybody seems to agree on that. As mentioned above however, while distress leads to crying, this leaves open the question why the baby is in distress to begin with and why it cries if caregivers can’t do anything about it. Thus, while this hypothesis is the least controversial one, it’s also the one with the smallest explanatory value.
4. Manipulation: The baby cries so mommy feeds her as often as possible. Breastfeeding stimulates the production of the hormone prolactin; prolactin inhibits estrogen production, which often (though not always) keeps the estrogen level below the threshold necessary for the menstrual cycle to set it. This is called lactational amenorrhea. In other words, the more the baby gets mommy to feed her, the smaller the probability that a younger sibling will compete for resources, thus improving the baby’s own well-being. The problem with this hypothesis is that it would predict the crying to increase when the mother’s body has recovered, some months after birth, and is in shape to carry another child. Instead however, at this time the babies cry less rather than more. (It also seems to say that having siblings is a disadvantage to one’s own reproductive success, which is quite a bold statement in my opinion.)
5. Thermoregulatory assistance. An infant’s thermoregulation is not very well developed, which is why you have to be so careful to wrap them warm when it’s cold and to keep them in the shade when it’s hot. According to this hypothesis the baby cries to make herself warm and also to alert the mother that it needs assistance with thermoregulation. It’s an interesting hypothesis that I hadn’t heard of before and it doesn’t seem to have been much studied. I would expect however that in this case the amount of crying depends on the external temperature, and I haven’t come across any evidence for that.
6. Inadequacy of central arousal. The infant’s brain needs a certain level of arousal for proper development. Baby starts crying if not enough is going on, to upset herself and her parents. If there’s any factual evidence speaking for this I don’t know of it. It seems to be a very young hypothesis. I’m not sure how this is compatible with my finding that the Lara after excessive crying would usually fall asleep, frequently in the middle of a cry, and that excitement (people, travel, noise) were a cause for crying too.
7. Underdeveloped circadian rhythm. The infant’s sleep-wake cycle is very different from an adult’s. Young babies basically don’t differentiate night from day. It’s only at around two to three months that they start sleeping through the night and develop a daily rhythm. According to this hypothesis it’s the underdeveloped circadian rhythm that causes the baby distress, probably because certain brain areas are not well synched with other daily variations. This makes a certain sense because it offers a possible explanation for the daily return of crying bouts in the late afternoon, and also for why they fade when the babies sleep through the night. This too is a very young hypothesis that is waiting for good evidence.
8. Behavioral state. The baby’s mind knows three states: Sleep, awake, and crying. It’s a very minimalistic hypothesis, but I’m not sure it explains anything. This is the scientific equivalent of my hypothesis A, the baby just cries.

Apparently nobody ever considered my hypothesis D, that baby cries to move herself into an optimally stable social environment which would have developmental payoffs. It’s probably very difficult a case to make. The theoretical physicist in me is admittedly most attracted to one of the neat and tidy explanations in which the crying is a side-effect of a physical development.

So if your baby is crying and you don’t know why, don’t worry. Even scientists who have spent their whole career on this question don’t actually know why the baby cries.