Tuesday, August 07, 2012

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.

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.


  1. "Why does the baby cry?"

    I'm only guessing; because life sucks?

  2. My experience is that babies don't cry unless they are unhappy. If there is no visible reason, my guess is that it is usually pain, perhaps associated with teething.

    The work you reference shows how far we have come. If you didn't see it, check out the episode of "Quarks & Co" which aired a few weeks ago. The moderator mentioned some things which people believed just a few decades ago, within my lifetime (if not yours). For example, nursing is bad, crying is good since it exercises the lungs and one can operate on babies without anesthesia because they don't feel anything anyway (apparently this was actually done!).

  3. Hi Bee

    Nice piece with explanations I’ve never considered before. As for my take I think babies cry because becoming self aware is a traumatic experience. Further one could argue this doesn’t get any better as we grow older, only the coping mechanisms become augmented and expanded.

    “Man alone is born crying, lives complaining, and dies disappointed.”

    -Samuel Johnson



  4. My experience is consistent with the "distress" hypothesis: That you as mother are not able to localize the root of the problem (maybe just strong bowel movement, or something similar, that the baby is not yet used to) does not mean there is no (percieved) problem. And things are still in such a simple state that it cannot judge if you can do anything about it. So it just cries when it feels pain (or similar) since that makes you help in case you can.

    Additional point: I heard that boys are more prone to show this type of early crying and that was attributed to the different geometry of male genitalia with respect to the digestive system.

  5. Hi Phillip,

    It is implausible that teething pain peaks daily in the late afternoon. Also, it should get worse as the baby gets older, not get better. Best,


  6. Hi Robert,

    To some point that's likely the case. But it doesn't make much sense, evolutionary, that infants invest energy in crying up to the level that it may trigger abuse by the supposed caregiver. If it's a sign of distress, it's one that evolution couldn't decrease to a more beneficial level. Why? Best,


  7. Hi Phil,

    Yes, it could be a side-effect of neural development. Unfortunately, this seems to me very difficult to test. I mean, I can imagine that one would be able to find a correlation (between development stages, neural activity, and crying pattern), but I don't see how one would test for a causation. Best,


  8. My guess is simply that the evolutionary choice was only "cry on distress" or "don't cry" and any differentiation to be too complicated.

    OTOH, it seems that evolution was quite effective to find the sound that is most distressing to parents. We found that my wife almost feels physical pain when our children cry whereas for me only to a lesser extend (which allows me reactions like "yes, I hear you but let me finish what I am currently doing, then I'll take care of you" which are almost impossible to her).

  9. Hi Bee,

    Yes it would be difficult to definitively test for, yet those silly little distractions that parents devise which often have their babies to stop crying has always had me to believe this to be true.



  10. "It is implausible that teething pain peaks daily in the late afternoon."

    I don't know when it would peak. Presumably it depends on when teeth grow (do they grow only at night like bones?). In general, at least in my experience, I suspect it is pain if there is no obvious reason, perhaps related to teething but of course there are other sources (mainly digestive).

    In any case, none of my children cried very much and one started sleeping all night through when he was two weeks old, so I'm lucky in that respect!

  11. Excessive crying not only pisses of parents end everyone else it would also draw predators in the wild making it a terrible "strategy" in itself, this is why I find it much more likely that it is a negative byproduct of something else which outweighs it's negatives.

    My hypothesis would be that human infants are messed up by "recent" (in evolutionary terms) accelerated increase in brain size.

    Kinda similar to how birth seems to be much harder for humans then for other mammals. The hypothesis I've come across is that evolutionary pressure to increase brain size is to blame - women reproductive tract evolution barely kept up. This link is more obvious but it's also possible that the fast growth of infant brain has other negative side effects, for example it may lead to some sort of psychological trauma/suffering experienced by infants.

  12. How does the caregiver know if the baby has a stuffy nose, an itch or a cramp?

  13. Crying could be a defensive play on adult sympathies in an early humanoid society. Warriors might pass over domiciles with crying babies in favor of attacking their warrior counterparts within the tribe.

  14. Yours is quite the list, Bee! One more - infants are fluid-buffered before entering the world. They have no callus layer atop their skin. Everything hurts when touched, hence lamb fleece in cribs rather than sheets. Easy test: Take a flatware spoon, gently touch the convex side to infant skin. That should trigger the siren, oh yes. Now, gently remove it. The walls will quake.

    Second degree burns regrow baby skin. It almost melts onto any smooth surface. If it is a cool smooth metal surface, pulling back is agony. Eventually new skin toughens toward the world and becomes useful, or it goes the coddled diversity route and cries forever.

  15. Hi Bee,
    I remember reading a book by anthropologist Sarah Hrdy dealing with the evolution of maternal instincts and infant behaviour and one of her points was that human infants need to exagerate their reactions because human mothers are the most likely of all primates to abandon their babies [given the investement in energy that a human child requires]

  16. If the infant cries unpredictably, it might make it not possible for the mother to go hunting/gathering (presumably the state in which humans spent a huge portion of their evolutionary history).

    Also, if she did try to carry such a baby and go foraging, the baby's cries might attract predators.

    So, if the mother is forced to be sedentary, and not expose herself to the dangers of going about, it improves hers and by extension her baby's chances of survival :)

    See how easy it is to come up with hypotheses. :)

  17. Actually hypothesis four, manipulation in order to increase food gains, seems quite feasible.

    However I would argue for a different mechanism as to why: Babies who cried more (ie. provided a form of negative reinforcement for the mother) might get fed more when competing with already existing siblings. Because a mother's resources are finite (there's only one of her), competition on this level could certainly offer survival advantages, as it provides negative reinforcement for the mother to attend first and foremost to the needs of the developing baby. This would conform to the fact that many of the mothers in the studies just thought that the crying babies needed food.

    Because over-crying is potentially deleterious, as shown, over-cryers would be selected against. This dynamic might lead to the sort of frequencies, "peak," and offset of the behavior that we see as typical, as these normal factors might ultimately have offered the greatest probabilities of survival.

  18. Also, the selfish gene notion could provide an explanation for the perceived strangeness at a baby competing with its siblings. It might not hold up in practice in this case, but it's certainly a thought.

  19. Hi Bee

    Have you considered that some other evolutionary step may have interfered with this communication channel between the baby and the outside world? There are lots of ways I can think that this might have happened. For example our ability to learn new things and concentrate on them may have distracted us from becoming familiar enough with the immediate needs of babies - we'd rather be doing something more interesting and we don't interact with babies enough (and in the right ways) to learn. We become irritated as a result of the perceived irrationality of the crying of the baby - something that may well be too recent in the evolutionary line to have affected this communication channel.
    I guess you could also come at this from the baby's point of view. Other factors that have given us an evolutionary advantage (large head, complicated brain, etc.) may cause the baby discomfort from what it may have evolved to expect, so it cries.

    I guess you could sum up this explanation as "there was a good reason once upon a time, but we've outgrown it and now it's a nuisance". C.f. the appendix as an organ.


    -- intonescience.wordpress.com

  20. Could be an existential thing.

    Marshall McLuhan's Three Stages of Man:

    1. Alarm

    2. Flight

    3. Exhaustion


  21. admiThank you for interesting theories on why babies cry.
    Recently, there has been a new approach speculating that babies should not cry at all, as witnessed in some tribes in Africa. Yes, babies in Africa don't make sounds to alarm the predators, nor are they testing the relationships of their parents (brilliant :) They are simply quiet.
    So there should be something that we are doing wrong. We respond to the cries with change-feed-sing, but there is definitely a missing soothing response, because the baby continues to wail.
    Evolutionary, we are born very immature because of the disproportion between the baby's head and mom's pelvis. That is why the first three months of our lives are frequently referred as the "fourth trimester". Babies suffer from over stimulation, they can't cope yet with zoning out light-noises-new environment. They long for the quiet surroundings of the womb. One of the most popular books on the matter is "Happiest baby on the block" by Dr. Karp. He elaborates on soothing reflex - once you swaddle rock and shush the baby, she instantly stops crying. He didn't develop anything new, these were practices widely spread centuries ago, forgotten by modern medicine.

  22. I'm well out of practice, but when I wasn't I could easily distinguish between a wide variety of causes for the babies I was around a lot. Including simple attention and/or interaction. Often when attention failed for this kind of cry a walk around different rooms or outside, or walk up within interaction distance of a variety of objects was effective. Males especially seemed to respond to the later more consistently, while girls seemed to get more attached certain individuals and less dependent on external interactions. The need for social interaction is universal though.

    I think you have a partial point with hypothesis D, but only in terms of timing and not cause per se. It is imperative that babies interact socially, even if it's one sided and seemingly pointless. Else they fail to develop properly. Even neurological development fails without these interactions, which are unmistakable in extreme case of neglect through a lack of interaction. Even certain medical treatments during developmental stages must take this into account or risk inducing a failure of neurological development in otherwise healthy organs.

    In earlier societies evening corresponded to the most optimal timing to receive these social interactions. As the baby ages, and can partake in more complex social interactions, the need to cry to induce these social interactions becomes less relevant and more dependent on circumstances than timing.

  23. I took a peek at the articles in the PDF. It looks like crying has multiple causes, but is mainly about the need for attention and delayed human development.

    Bard, in the section on colic, notes that hunter-gatherer babies don't have colic, but spend a lot of time being cradled in their mothers' arms or more likely in a sling within easy reach. (That way mom can gather with her quiet baby near at hand.) They still cry when they need something, but are not prone to unexplained crying jags.

    Bard also notes that human babies are sort of in their fourth trimester of development. Their skulls haven't sealed and their senses are limited. They can't even focus their eyes very far, so they need close interaction. You can't just yell, "I'm coming." It takes them a while to realize that crying can work, after which they learn to quiet down on their own when problems are minor.

  24. Hi Kaleberg,

    I didn't elaborate on this point, because the evidence is mixed (see also one of the later comments to the pdf). If I recall correctly, there are some studies saying that babies cry on the average less when they are carried more, but far from not crying at all. Other studies show that it doesn't matter much for the total amount of crying how quickly the mother responds (maybe because the baby adapts). The whole discussion around the "attachment parenting" is based on scientifically very shaky feet (there was a recent article in Time Magazine, which was pretty good actually). Best,


  25. I should have added: If you compare cultures, it is very difficult to single out a factor.

  26. I don't agree with you. I found (and still find) the theory of the attachment parenting pretty good, and it does work pretty well when is properly applied. My daughter is 2 1/2 y.o. now, and since she was born I started to "devorate" scientific papers and books on these topics.

    Well, I'm a theoretical physicist, not a neuroscientist or something like that LOL (meaning, I don't perfectly understand Neuroscience, Evolution, and so on), but I do think that Neuroscience explain quite well why babies cry, and how to soothe them. At least it worked to me and to my friends, and everybody that I heard about applying AP. We had great babies, almost no crying, ever, even today! 'Terrible two' is something I still don't know what it is. I'm not saying it's perfect! Nobody is, kids in general give a hard time to us. But definitely I know a bunch of "perfect babies" (I'm referring to the first three months, that you seem particularly worry about), and a bunch of places where these "perfect babies" are the norm. Definitely the answer is in our modern western society.

    I think (that is just my personal opinion) physicists in general are arrogant, thinking that all kind of Science should have the "same paths" that the Exact Science has. It just doesn't work like that.

    Regards from Brazil :-)


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