Sudden infant death syndrome (SIDS), while not a disease, is a serious concern in infants under 12 months of age. The cause of sudden infant death syndrome is unknown, but a variety of environmental and social factors have been associated with an increased risk of SIDS. A significant amount of research has been devoted to the understanding of why SIDS occurs. It is a mystery that has perplexed society for thousands of years and continues to do so today. This paper will examine this devastating syndrome from an evolutionary perspective.

"And this woman’s child died in the night; because she has overlaid it" 1 Kings 3: 19-20. This passage taken from the Bible is the earliest reference to an unexpected death in a child (Dulbecco 1997). It wasn’t until the late 1800’s that S.W. Fearn entitled a letter "Sudden and Unexplained Death in Children," which resembles modern descriptions of Sudden Infant Death Syndrome (SIDS). Finally, in 1970, SIDS was recognized as a prominent medical concern that took the lives of over 290,000 infants in the western world (McKenna 1992). This resulted in a tremendous increase in the study of SIDS, which gained support from the United States government.

One definition of SIDS states that it is "the death of an infant under one year of age, which remains unexplained after the performance of a complete postmortem investigation…" (Dulbecco 1997). From this definition, combined with many others, SIDS can be described as a "diagnosis of exclusion", meaning that all other possibilities have been ruled out (McKenna 1992). Because of this broad diagnosis, it is possible that SIDS does not result from one cause, but from multiple causes.

There are many proposed causes and risk factors of SIDS. These proposed causes are determined after an autopsy and an investigation of the death scene. The most widely believed risk factor is when the baby is put to sleep in the prone position, or lying on its stomach. Data taken from a research in the state of Washington in 1996 shows that babies placed in the prone position are three times more likely to die from SIDS (Dulbecco 1997). In 1994, a campaign was formed entitled "Back-To-Sleep" to educate the public of the dangers of the prone position (Olendorf 1999).

The temperature of the environment during sleep is also a major risk factor. The overheating of the infant causes the child to sleep more deeply, potentially making it more difficult to awaken when short of breath (Olendorf 1999). Anne-Louise Ponsonby at the University of Tasmania in Australia suggests that a warm environment could lead to a sudden death by hyperthermia, or overheating, which then leads to a sudden fall in blood pressure and a diminished oxygen supply to the brain (Magills 1998).

Another risk factor is the use of tobacco and/or narcotics during pregnancy. Tobacco use increases the infant’s chance of SIDS by three times the average risk. Infants that were exposed to tobacco during pregnancy have been shown to have impaired arousal due to the low oxygen in the blood, also known as hypoxia. Narcotic use during pregnancy has much more severe effects, by increasing the child’s risk for SIDS 10 times. As the intensity of the withdrawal from the exposed narcotic in the infant increases, so does the risk for SIDS (Dulbecco 1997).

Male gender has also proven to be a risk factor. Studies have shown that the male to female ratio of SIDS death is 3:2. There is no evidence to show why males are more likely to die from SIDS than females (Olendorf 1999).

Premature or low birth-weight infants are two to four times at risk for SIDS. As the severity of the prematurity or low birth increases, so does the likelihood of death by SIDS (Olendorf 1999). This is believed to be true because the prematurity of the infant determines how developed the respiratory system is.

Asphyxiation, associated with soft bedding, is another proposed cause of SIDS. Soft bedding such as pillows, waterbeds, sheepskins, and beanbags, may contribute to trapping stale air around the infants’ mouth and nose. This results in the baby inhaling too much carbon dioxide, and not enough oxygen, resulting in asphyxiation by carbon dioxide (Olendorf 1999).

The most widely accepted cause for SIDS is the failure to breathe, or the apnea hypothesis. This hypothesis is defined as the cessation, or pause, of airflow to the nose and mouth. The pause is called the central apnea, and usually lasts about 15 seconds. The Sleep Apnea Hypothesis, associated with SIDS, states that there is an abnormality in the regulation of the infants breathing activity during the night. This abnormality is believed to be related to elevated levels of hypoxanthine in the blood. Hypoxanthine is an indicator of prolonged hypoxia, which is low oxygen content in the blood. Following this shortage of oxygen, the infant would go into a coma, then eventually die (Dulbecco 1997).

There is seasonal variation among SIDS victims, where higher incidence occurs during fall and winter (Dulbecco 1997). An infant born during the spring months is less likely to die from SIDS. The reason is because the infant’s first six months of life, which is when 90% of SIDS occurs, takes place during the spring and summer months. Age is a critical factor among infants who are under six months of age during the winter months. This seasonal risk occurs because of the association with cold weather, viral illness, and overdressing. A recent study in New Zealand examined the significance of the temperature, and found that infants who died of SIDS during the winter months were more likely to be overdressed. They also found that parents tended to overdress their infant because they believed that warming the infants reduced the risk for viral infections (Magills 1998).

Race is another factor considered when a post-mortem investigation occurs. In the United States, rates of SIDS ranges from 1.6 to 2.3 for every thousand births, with substantial ethnic divergence. The following are rates out of 1000 live births:
 

Asian- .50
Whites- 1.3
Latinos- 1.7
Blacks- 2.9
Low Economic- 5.0
Indians- 5.9

A study conducted by Davis and Gantley consisted of mothers and infants co-sleeping in a laboratory. This experimental work has shown that patterns of breathing may interact during sleep (Magills 1998). Other sensory stimuli that may result from parent-infant co-sleeping is the exchange of stimuli such as sound, movement, touch, vision, gas, carbon dioxide, and/or temperature. Because infants are so neurologically immature at birth, they greatly rely on this stimulus. In fact, human infants are only born with 25% of their developed brain; the other 75% develops after birth. As a consequence of this immaturity, the human infant is forced to rely on external support, especially during the first 6-12 months of life (McKenna 1992). An experiment was done to test the infant’s dependency using a simulated breathing teddy bear. Its purpose was to reflect the babies breathing pattern and give the infant a "surrogate companion". The results of the study showed that the infants were very responsive. The ability for the infant to be able to reach out and touch the bear during the night gave the baby a sense of security. The results also showed that infants who had the breathing teddy bear present for at least 2 weeks had quieter sleeping patterns. The final conclusion was that the teddy bear apparently stimulated neurological development within the extremely immature infant (1992).

Support for the underdeveloped brain proposal lies within the fossil record. Approximately four million years ago, natural selection began to favor upright locomotion (bipedalism). Two million years later, with the rise of humans (Homo erectus) came a conflicting evolutionary trend; increased brain size for learning complexity or a smaller birth canal needed for bipedalism. Natural selection apparently favored the smaller birth canal. The resulting dilemma is the birth of neurologically immature infants. For the infants to be able to fit through the decreased birth opening, the human skull has to become smaller, causing the brain to also decrease in size. This in turn affects the brain’s development. It is believed that parent-infant co-sleeping evolved so the infants could finish their biological and social gestation after birth (McKenna 1992).

Other significant findings in co-sleeping revealed that on average both mother and infant experience more arousal’s throughout the night, therefore spending less time in the deeper stages of sleep (stages 3-4), and more time in the lighter stages of sleep (stages 1-2) (McKenna 1992). On average, during co-sleeping experiments, mothers were highly responsive to the status of their infants (McKenna et al. 1999). Also, affection from the mother to the infant throughout the night such as kissing, hugging, patting, rubbing etc. kept the infant in the lighter stages of sleep (1-2). Another obvious advantage of having the mother near the child during the night is the ability to respond to an infant in physical distress or from preventing the child from rolling into the prone position (1999). This also helps prevent potentially dangerous situations such as the baby being short of breath or choking (1999).

On the other hand, bedsharing or co-sleeping can also be a dangerous risk factor if it is practiced under the wrong conditions. Low socioeconomic groups almost always have a higher rate of SIDS, and co-sleeping may be occurring under other adverse circumstances. Also, co-sleeping is dangerous when there is passive smoke present while the infant is in the bed, or if the bed has a soft mattress and/or many pillows, covers, etc. (McKenna 1992). Although there are risks associated with co-sleeping, the benefits often outweigh the risks.

Bedsharing or co-sleeping can also be associated with breast-feeding. Studies have shown that routine bedsharing infants breast-feed twice as frequently. Breast milk contains fewer calories than formula and therefore the infant needs to feed more often. Most experts believe that while breast feeding itself does not reduce the risk for SIDS. The frequent contact with the mother is what is beneficial (Magills 1998).

A study was done with monkeys to test the response of infants when separated from their caregivers. Results showed that when the monkey infants were separated for as short as three hours, they began experiencing significant effects, such as decrease in body temperature, a release of stress hormones, cardiac arrhythmia’s, and sleep disturbances. Compared to monkeys, human infants are far less neurologically developed at birth; therefore, the effects on human infants would not be as significant (McKenna 1992). It is speculated that natural selection probably favored infants and children who protested (by crying) when removed from their caregivers. The reason being that infants who cried during the night to receive parental contact perhaps had a better chance of survival when and if something was wrong (McKenna et al.1999). It could also be argued that during evolution, co-sleeping eliminated the infants who were "less fit" and were unable to protest when a breathing deficiency occurred (McKenna 1992).

All of the above proposed causes are highly speculative. SIDS is not a disease, but a mysterious syndrome that has confounded of so many researchers in society today. The researchers have become frustrated with the misunderstanding of such a devastating event, and no treatment has yet been discovered. Their only suggestion to parents is to be knowledgeable of the risk factors. As of now, education is the only "treatment".

Magills Medical Guide. 1998. Volume 3. Pasadena, California: Salem Press Inc.

McKenna J., S. Mosko, and C. Richard. 1999. Evolutionary Medicine. Pages 53-74. W. Trevathan, E.O. Smith, J. McKenna, eds. New York: Oxford Press, 1999.

McKenna J., E. Thoman, T. Anders. 1993. Infant-Parent Co-Sleeping in an Evolutionary Perspective: Implications for Understanding Infant Sleep Development and the Sudden Infant Death Syndrome. Pediatric Review. Volume 16. Pages 263-282. Claremont, California, 1993.

Olendorf, D., et al. eds. 1999. The Gale Encyclopedia of Medicine. Volume 4. Michigan: Gale Research Farmington Hills.