What protects infants from Sudden Infant Death Syndrome?

SIDS research shows two things: first, Sudden Infant Death Syndrome almost always correlates with certain risk factors^{In individual cases, no one knows exactly how much these risks contribute, and it is important to mention that there are indeed SIDS cases without these risk influences, but these cases are very rare}. These factors are familiar to most parents and include things like prone sleeping position, an unfavorable sleep environment with heavy bedding, pillows, overly soft mattresses, cigarette smoke, etc.

However, SIDS research also shows that risks alone do not explain Sudden Infant Death Syndrome well (I explain this in detail here). Only a small fraction of infants die when exposed to the mentioned risks.

Therefore, the really exciting question in SIDS research is about resilience: What protects infants from SIDS?

This question is the focus of our recently published paper, in which we developed a new explanatory model for SIDS. We are a working group of experts in public health, SIDS epidemiology, developmental neurology, sleep research, anthropology, and pediatrics, and we are trying to understand SIDS from the perspective of evolutionary behavioral research and developmental neurology – hence our model is called: evolutionary-developmental model of SIDS.

And these are our assumptions

From an evolutionary perspective, the sudden death of an otherwise apparently healthy infant is simply a nonsense event: Why would an infant die after a long and costly pregnancy – without any identifiable reason? From an evolutionary standpoint, it would be more logical to assume that a human infant is equipped with enough protection to prevent such an event. Protection that allows the infant to keep his airways clear under all circumstances, for example. Or to maneuver his head out of a danger zone. This protection should either be factory-installed or develop in the course of normal development.

Confirmation from Infant Research

And indeed, infant research, which we intensively scrutinized for this question, confirms both: The infant starts his life with an innate protection program but must also expand and develop new protection strategies. The innate protective behavior is known to all parents – it consists of the newborn reflexes: The infant “knows” exactly that it must lift its head if it cannot get air through its nose. It also “knows” that it must turn its head if lifting the face is not enough. And then its arms automatically move forward protectively…

This innate, reflex-based protection program is extremely effective. But it also has a problem – a kind of predetermined expiration date: this program must largely dissolve at some point.

Needed: a different, new protective behavior

And this happens as the baby leaves the newborn period – that is, after the first month of life. The infant must now gradually develop finely controlled voluntary motor skills; the baby can no longer be just “pulled by strings”. Now new strategies for protection against dangers are needed, and they consist of the infant gradually switching its protective behavior to a complex, cortex-controlled, and ultimately “learned” program. And this switch from “innate” to “learned” protection, as shown by Myrtle McGraw’s experiments almost 100 years ago, happens between the 2nd and 5th months of life – precisely the period when Sudden Infant Death Syndrome is most prevalent (85% of SIDS deaths occur in this period).

Sometimes the transition is difficult

So it seems that some infants have problems with the transition from innate to acquired protection – and this fits in well with the apparent paradox that the first month of life actually represents a kind of “grace period” when it comes to SIDS: unlike the other causes of infant mortality, neonates are less likely to die from SIDS than the older infants!

However, this only raises further questions: Why do some infants manage this transition well and develop effective, “learned” protective behavior in good time, while others do not?

Learning handicaps

There is very detailed preliminary work that gives an answer to this question: Some infants probably have a “developmental handicap” – for them learning new protective behaviors does not work so well. Indeed, there are two possible reasons for such a learning handicap, both of which fit in well with the findings of SIDS epidemiology:

• On the one hand, it may be a biological handicap, i.e. a physical problem that makes it difficult for babies to develop new regulatory skills. For example, because they were exposed to adverse conditions in the womb that impaired their brain development (smoking during pregnancy should be mentioned here in particular, which actually plays at least a contributory role in a large proportion of SIDS cases)
• On the other hand, these “developmental handicaps” may also be due to a lack of practice opportunities.

Learning through experience

The latter consideration is incredibly exciting because SIDS research clearly shows that some SIDS cases are also associated with a lack of prior experience. In stark terms, with a lack of practice. The prone position, for example, is particularly dangerous if a baby sleeps in this position without prior experience^{Literature on this:
• Klonoff-Cohen, H. S., & Edelstein, S. L. (1995b). A case-control study of routine and death scene sleep position and sudden infant death syndrome in Southern California. Journal of the American Medical Association, 273(10), 790–794.
• L’Hoir, M. P., Engelberts, A. C., van Well, G. T., McClelland, S., Westers, P., Dandachli, T., Mellenbergh, G. J., Wolters, W. H., & Huber, J. (1998). Risk and preventive factors for cot death in the Netherlands, a low-incidence country. European Journal of Pediatrics, 157(8), 681–688. https://doi.org/10.1007/ s004310050911
• Li, D. K., Petitti, D. B., Willinger, M., McMahon, R., Odouli, R., Vu, H., & Hoffman, H. J. (2003). Infant sleeping position and the risk of sudden infant death syndrome in California, 1997–2000. American Journal of Epidemiology, 157(5), 446–455. https://doi.org/10.1093/aje/kwf226
• Mitchell, E. A., Thach, B. T., Thompson, J. M. D., & Williams, S. (1999b). Changing infants’ sleep position increases risk of sudden infant death syndrome. Archives of Pediatrics & Adolescent Medicine, 153(11), 1136–1141. https://doi.org/10.1001/archpedi.153.11.1136
• Moon, R., Oyen, N., Markestad, T., Skaerven, R., Irgens, L. M., Helweg-Larsen, K., Alm, B., Norvenius, G., & Wennergren, G. (1997). Combined effects of sleeping position and prenatal risk factors in sudden infant death syndrome: The Nordic Epidemiological SIDS Study. Pediatrics, 100(4), 613–621. https://doi.org/10.1542/peds.100.4.613} And it can also be shown for bed-sharing that babies who routinely sleep in the parental bed do not have an increased SIDS risk, but babies who unexpectedly (non-routinely) sleep in the parental bed do. And that brings us back to our evolutionary biological approach: Considering the development of the small Homo sapiens from this perspective, typical developmental and caregiving experiences should actually contain enough “practice opportunities” to successfully switch from innate (reflexive) to acquired (learned) protection!

The evolutionary “care package”

If we examine the typical experience framework of a human infant more closely, it always contains the same elements (we call it the “evolutionary care package”): being breastfed, being carried, sleeping with the mother, and generally a close, responsive interaction. Our hypothesis is now that these typical experiences provide an ideal developmental “practice environment” to “learn” adequate protective behavior.

How breastfeeding could protect

And that brings us back to exciting findings from infant research, this time from the 1960s. And to a British pediatrician, Mavis Gunther (here is a beautiful homage to her, who was called “Breast Lady” at that time because of her breastfeeding research). Mrs. Gunther showed that infants at the breast not only take in food or reduce stress – but also practice protective behavior. “Airway management,” that is. Because drinking at the breast – with a closed mouth – means a huge challenge to keep the small nose free while dealing with the milk-filled breast. This cannot always work, and what happens then, Mavis Gunther documented in her observations: when the little ones run into trouble and cannot breathe, they react more effectively and quickly next time! Obviously, they expand their protective repertoire through exposure. So it could well be that the species-typical experience of breastfeeding also helps them to cope better with other emergency situations.

And what about sleeping in a co-regulated context, together with a breastfeeding mother? Here, recent SIDS research (including the SWISS and CESDI cohort studies by my co-author Peter Blair) shows that co-sleeping in non-hazardous circumstances (no smoking, no alcohol, not on a sofa) is associated with a lower SIDS risk than sleeping in one’s own crib, at least for infants over 3 months of age (the risk is about the same for infants under 3 months of age).

But back to our model

We do not doubt at all that risks play a role in the development of SIDS, they do, we should avoid them as much as possible.

At the same time, however, the findings from experimental infant research, SIDS epidemiology, comparative behavioral research and developmental neurology evaluated for our model also show that the risks are most likely to have an impact if they are not countered by adequately developed protective behavior. And this protective behavior arises in the course of normal development under conditions that are evolutionarily expected for a small homo sapiens.

And that also makes sense pathobiologically. Because if you look at the long list of risk factors for SIDS, they basically represent one thing: increased challenges to physiological regulation. Of course, it is a huge challenge to keep your temperature in the green zone under a heavy comforter! Of course it’s a huge challenge to lie next to a drunk adult who doesn’t move to the side when baby needs it. Of course it’s a huge challenge to sleep on your stomach on a soft mattress – if baby wants to lift his head, his little arms sink in because there’s no support! And so on. And, of course, a baby’s little body is all the more likely to reach its limits if it has only been able to build up a few skills to meet these challenges during its development.

New recommendations?

Are we making new sleep recommendations with our work – for example with regard to the prone position? No, and that’s important to me.

In our work, we actually discuss up and down which sleeping position could be species-typical for a human baby; as an anthropologist and sleep researcher, Helen Ball has provided decisive impetus here. And yes, the prone position could be part of the evolutionary repertoire, but probably in an ‘accompanied’, co-regulated context, such as on the body, as can also be observed in other primates. Can this be transferred to the conditions here and now? We don’t know, and unlike in the bedsharing debate, there are no reliable findings from SIDS epidemiology that could show that the prone position is just as safe as the supine position under certain conditions. Such analyses have not yet been carried out. SIDS epidemiology can only show that the risk of prone positioning is higher if the mattress is too soft, or if the sleeping environment is otherwise dangerous, if there is no previous experience (see above), if the mother is a smoker, or drinks, if the baby is premature or born too small or is weakened by an infection. But whether a residual risk remains even without these circumstances: we simply do not know (yet). Neither do we, the authors of this paper.

So here is our central “balance” model, which I will also end with here – it contrasts our evolutionary-developmental model with the classic triple risk model for the development of SIDS (thanks Simon, for the great graphic!)

Original publication

Herbert Renz-Polster, Peter Blair, Helen Ball, Oskar Jenni, Freia De Bock: Death from Failed Protection? An Evolutionary-Developmental Theory of Sudden Infant Death Syndrome. Human Nature, Vol. 35(2), (2024); https://doi.org/10.1007/s12110-024-09474-6

For quick orientation…

… find a 2-page summary: here