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Theories around the causes of Sudden Infant Death Syndrom
Here is the presentation of an article published in Human Nature in late 2024, focused on understanding the causes of Sudden Infant Death (SID) and developing a theory on the pathogenesis of this condition.
While it is established that there are risk factors: environmental (parents who smoke), behavioral (placing the infant on their stomach), and social (precarity), the fact that the vast majority of children exposed to these risks survive has been attributed to a biological vulnerability in the victims.
The authors raise the limitations of this "Triple Risk" theory in the pathogenesis of SID, since among children exposed to the risks, only a small minority will die. The authors highlight the many questions that remain around SID by reviewing the literature.
Thus, for some children a given risk will be strong while for others the same risk will be very low: the authors explain this through the child's developmental characteristics. This "evolutionary and developmental" theory conceptualizes SID as the result of the intersection between the environmental challenges the child faces (the risk landscape) and the developmental resources the child has accumulated (the protective landscape).
The authors explore these protective factors in detail, describing their origin according to evolutionary theory, and explaining why, in their view, this approach goes beyond the current risk-based theory. They raise, in particular, the following questions:
The grace period between 0 and 4 weeks: why is a newborn less at risk than a 3-month-old baby?
Boys are at higher risk of SID (60–40 ratio) and are more often placed on their stomachs. Yet, when looking at the relative risks of the prone position in studies, girls have 3 times the risk that boys do: why would girls be at higher risk in the prone position?
Similarly, 2/3 of infants who die in their own crib are boys, whereas in cosleeping the sex ratio is even: why would boys be protected during cosleeping?
Why is the prone position less risky during cosleeping than when the baby sleeps alone?
Regarding the prone position: it appears to be riskier when it occurs occasionally than when it is the infant's habitual position and the child has experienced it repeatedly.
The same holds for parental bed-sharing, which seems less risky when habitual than when occasional: habitual cosleeping does not appear to be associated with an increased risk of SID compared to occasional cosleeping.
The authors' explanations are based on evolutionary theory and the analysis of epidemiological studies.
The grace period
Indeed, the risk of SID is low from 0–4 weeks, and multiplies by 5 between 2 and 4 months. The same pattern is observed for the risk of suffocation. This occurs even though the human infant is highly immature at birth, which runs counter to the general distribution of infant mortality, where the more immature the child, the greater the risk.
Several hypotheses exist regarding the increased risk between 2 and 5 months. One behaviorist theory (Burn and Lipsitt, 1991) holds that newborns are born with certain innate reflexes (subcortical reflex behavior), which progressively, during a transitional period, give way to behaviors acquired through learning (cortical responses), dependent on the environment and on biological readiness. For example, if you cover a newborn's nostrils, this triggers a series of behaviors: turning the head to the side, tilting the head back, bringing the hands to the face, then starting to cry and pushing the object away from the face, followed by a sigh of relief. → These behaviors are more effective in newborns than in older infants.
The authors emphasize two factors:
Biological readiness:
some children have more developed innate reflexes than others, entering the transition and learning period with a lower starting level of learned skill.
Several studies show the learning capacities of newborns, who can modify their behavior in response to a stimulus even while asleep. These behaviors are especially important for learning to cope with the prone position, where a set of regulatory and motor skills must be established to prevent the risk of airway obstruction and suffocation, including waking up and signaling adequately.
When infants are placed prone and their breathing is experimentally compromised (by covering the face, or exposing them to higher CO2 levels), prone infants engage a series of behaviors: rooting, lifting the head, turning it to the side, accompanied by sighs, sounds, arm and leg movements, full arousal, and crying.
But these behaviors vary between children—for example, turning the head to both sides was more effective in babies experienced with the prone position. The effectiveness of these protective behaviors did not depend on age.
→ There are behaviors learned during prone sleep. Asleep prone with the face against the mattress, infants experience brief periods of asphyxia from rebreathing exhaled air, followed by brief arousals with active movements, including turning the head to both sides.
LESSONS FROM BREASTFEEDING
The authors view breastfeeding as training for the development of protective capacities in the event of asphyxia. Breastfed infants frequently experience obstruction of their nostrils and learn to free themselves by turning their heads, and these behaviors become increasingly effective with repeated exposure.
It is therefore possible that this lack of experience with nasal obstruction in bottle-fed infants plays a role in the higher rate of SID among bottle-fed babies. It is also possible that, instead of learning to free themselves, some infants learn to become habituated to asphyxia, making them more vulnerable once exposed to a longer or more severe airway obstruction.
Epidemiological studies supporting the developmental perspective
Some studies highlight early vulnerability as a risk factor for SID—for example: low Apgar score at birth, low birth weight, postnatal respiratory distress, and prolonged hospitalizations. → Developmental delay due to prematurity or low birth weight increases the risks of the prone position (fewer motor capacities to free the head), but postnatally acquired behaviors also play a role.
In the Nordic epidemiological study on sudden infant death syndrome, 92% of victims found face-down on the mattress belonged to the group of infants inexperienced with the prone position (L'Hoir et al., 1998). Similarly, Mitchell, in his analysis of the New Zealand sudden infant death cohort, found that only 10% of victims who had ended up in the prone position during their last sleep were experienced prone sleepers (Mitchell et al., 1999b).
The authors hypothesize that systematically placing infants on their backs contributes to the peak incidence of SID at 2–5 months: when infants learn to roll from side to side, they may end up on their stomachs, unable to perform protective movements they have not been trained to make.
How to explain the variations in risk of Sudden Infant Death Syndrom for the prone sleeping position
The risk of SID in the prone position is greater if the infant is too warm, is ill, has heavy blankets, or a soft mattress: this represents an increase in adaptive challenges. Other risks are developmental and reflect the child's maturity:
Age, and the development of protective strategies during the transition and loss of innate reflexes.
In-utero developmental risk factors: tobacco exposure, prematurity, low birth weight.
Experience: the prone position is riskier for inexperienced infants, or those who have had fewer opportunities to learn.
Sex: girls are less often placed in the prone position, and are therefore less experienced. Boys may have more developed motor capacities due to genetics and a greater ability to turn.
Influence of the sleep environment: there appears to be greater risk in the prone position for a baby sleeping alone in their crib than for a baby cosleeping. → Cosleeping babies may have enhanced developmental capacity because, from birth, they are subject to stimulation from proximity, and to intense interaction and co-regulation, with the mother's help for any challenges arising from position. Breastfeeding tends to produce a back- or side-lying position rather than prone.
Influence of day versus night: infants placed prone during the day appear to be at greater risk of SID than at night. It is assumed that nap conditions carry more risk: lack of supervision, being placed on a sofa or prone without prior experience, by a caregiver less familiar with the child, without cosleeping or co-regulation.
The dangers of the prone position thus appear to depend on two intertwined factors, and when there is an imbalance between them, this results in SID:
being developmentally ready
environmental challenges: soft mattress, heavy blankets, heat
EVOLUTIONARY PERSPECTIVE
The authors hypothesize that the protective factors against SID, which allow for the development of protective capacities, depend on the type of interaction with the environment. Human infants are the most developmentally immature of all altricial mammals and develop in a context of co-regulation with the parent: breastfeeding, cosleeping, intense proximity, babywearing: the evolutionary package.
This proximal caregiving is what the human infant expects from an evolutionary standpoint, and it affects development, including the development of protective capacities against SID.
Breastfeeding allows the infant to develop strategies in case of suffocation: breastfeeding can be seen as anti-asphyxia training, guided by the mother. Breastfed infants are less often placed prone.
Babywearing is a biological adaptation of our species. In constant balance against gravity, in a context of social and environmental stimulation, it promotes muscular development of the neck, legs, and trunk, and likely also constitutes training against asphyxia.
Cosleeping: in an evolutionary context, human infants shared their mother's sleep surface in close proximity, to protect them from predators, insects, or drops in temperature. Solitary sleep is a very recent development in our history. In cosleeping, sleep cycles synchronize between mother and infant, and babies spend more time in active sleep, allowing for easier arousal and greater motor activity.
Cosleeping provides enormous stimulation for the infant's developmental capacities and for co-regulation, and when begun from birth, it does not increase the risk of SID—indeed, it likely subjects the infant to training that protects their airway. All the sensorimotor and developmental stimulation of proximal caregiving would therefore contribute to increasing the child's protective capacities.
Which sleep position is safest?
The choice of position should take into account this evolutionary package, which should support the infant's survival. Here the prone position appears paradoxical: it is associated with deeper sleep, longer sleep duration, fewer arousals and less crying, and less motor activity or reaction to noise than the supine position. This position is, moreover, preferred by healthy infants who sleep alone once they are able to roll onto their stomachs. This also explains why parents, despite official recommendations to place the infant on their back, continue to use the prone position. The prone position also appears to support motor development.
Similarly, the side-lying position is associated with a higher risk of SID (risk of rolling onto the stomach), but it is the habitual position for breastfed infants sleeping close to their mothers. Mothers frequently reposition their baby in a half-asleep state, onto the side or back, and breastfed infants seem to spend more than half of their sleep time in this position. The protective factor here appears to be the position taken by the mother: the "cuddle curl," a C-shaped position that prevents the baby from rolling onto its stomach.
The authors evoke the hunter-gatherer context, where the prone position on the mother's body, whether moving or asleep, would have been the biological norm—a context of conscious co-regulation and proximity—whereas being placed prone on a hard surface would have posed a predation risk. Side- or back-lying positions next to a sleeping mother would also have been possible. So from an evolutionary standpoint, neither the prone nor the supine position can be considered the biological norm for our species.
The authors hypothesize that the prone position on a parent's body, in a context of co-regulation, is a safe learning context for the infant, providing stimulation and co-regulation, with the parent regularly checking on the baby. There may even be deeper co-regulation—of temperature, of breathing—with stimulation that supports brain maturation. This would allow the baby to safely learn, on their parent, to cope with the challenges of the prone position. However, this remains an unproven hypothesis, and the supine position remains the only one recommended for safety.
If one accepts the hypothesis of the proximal-caregiving package as the norm for our species, the authors point out that our modern environment could contribute to the risk of SID: an understimulating, solitary environment, with little physical proximity or interaction, is not conducive to protective learning. A child exposed to tobacco smoke or left alone in a room is not a situation anticipated by evolution and carries a risk of SID. Prematurity or in-utero exposure to toxins may prevent the development of protective capacities due to developmental delays.
The authors go on to note the lack of studies validating these hypotheses. They refer to other countries where caregiving practices differ from Western ones (cosleeping, babywearing, frequent interaction with family), and where SID rates are lower (though data collection may also be less reliable there): in Asian countries, cosleeping is the norm and SID rates are low. In the United Kingdom, SID rates are lower among ethnic groups where cosleeping practices are common: 5 times lower SID rates among people of Indian, Pakistani, and Bangladeshi origin, non-English white people, and Black African people, compared to English white people.
Regarding breastfeeding, the authors highlight:
the rich stimulation environment for developmental capacities, often linked to cosleeping, with greater maternal supervision (regularly checking on the baby)
the practice of airway protection during feeds, which may allow the baby to learn to wake more easily
In conclusion,
Government recommendations in several countries advise against cosleeping, setting parents up for failure and exhaustion, since most babies will only accept sleeping in close contact with their mother. This dissonance between recommendations and parents' instinctive practices certainly contributes to affecting the mental health of mothers, who are vulnerable and often in a state of postpartum hypervigilance. This article raises hypotheses about:
the protective mechanisms babies develop through experience
the protective mechanisms against SID associated with breastfeeding
the prone position on the sleeping mother's body, likely the most evolutionarily probable, which is moreover used during skin-to-skin contact and helps stabilize premature infants.
Bibliography
Renz-Polster, H., Blair, P. S., Ball, H. L., Jenni, O. G., & De Bock, F. (2024). Death from Failed Protection? An Evolutionary-Developmental Theory of Sudden Infant Death Syndrome. Human Nature, 35(2), 153–196. https://doi.org/10.1007/s12110-024-09474-6
Terms used: "Cosleeping" refers to sharing a sleep surface between the baby and an adult. I preferred this term over "cododo," which in France is more commonly used to refer to a crib placed as a "side-car" attached to the parental bed.
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