Why Dr Harvey Karp's Five S's misunderstands evolutionary biology and isn't likely to help when you have an unsettled baby

This article is part of a collection inside The Possums Sleep Program called Deeper Dive, which explores the complex scientific, historical and social contexts in which families and their babies or toddlers live and sleep. You don't need to read Deeper Dive articles to be helped by The Possums Sleep Program.

About the Five S's (Happiest Baby) method, from the perspective of Neuroprotective Developmental Care (also known as NDC or the Possums programs)

Here I use existing research and also the theoretical frameworks of evolutionary biology and complexity science, to examine Dr Karp's Five S's. These are swaddling, soothing, shushing, swinging and sucking.

1. Swaddling or wrapping doesn't make babies sleep more or cry less as a pattern over time. To find out more about wrapping or swaddling and baby sleep (and why wrapping doesn't mimic being in the womb), please read here and here.

2. Holding your baby over your arm, on baby's side or tummy, with his tummy on your arm and his little back against your body as he looks out, has always been in both my personal and clinical experience an effective way to dial babies down. Because she has such a powerful biological need for rich and diverse sensory motor nourishment, your baby will love to see out instead of looking into your shoulder or body, at the same time as she feels the security of being held firmly against your body. You can find more here.

3. Shushing in the ear dials babies down because it is a very loud and interesting sensory experience! But I've been concerned about the impact on a baby's sensitive developing auditory system of shushing up close to baby's ear when I first saw Dr Karp demonstrate this to an audience of researchers at the 11th International Infant Cry Workshop, held in The Netherlands in 2011. You can find out about white noise, baby's hearing, and sleep here.

4. Rocking, swinging and jiggling are delightful forms of sensory motor stimulation for your baby, and a powerful way to dial baby down. But they are work intensive strategies, and keep parents inside the home during the day, or in the bedroom at night, rocking, swinging and jiggling, often for long periods. The Five S's reinforces the unhelpful belief that parents have to do work to get their baby to sleep. This misunderstands the sleep science (that is, how the two sleep regulators, body clock and sleep pressure function). Jiggling, swinging, and rocking are simply ways of dialling baby down while the sleep pressure rises. They serve the same function as bouncing on a fit ball, for example. But there are much easier and more enjoyable ways of meeting your baby's need for sensory motor stimulation while the sleep pressure rises. You can find out about this here.

5. Sucking is another form of sensory motor nourishment which dials babies down. But offering your baby the breast once he's swaddled or wrapped interferes with positional stability, risking nipple pain for you and dialled up behaviour at the breast for her. Since safe access to the breast isn't possible once swaddled, the Five S's seems to preference pacifier or dummy use. This could also explain why the Five S's results in babies weighing less at 12 months of age, a concerning outcome particularly for breastfed babies. You can find out about pacifier or dummy use here.

The research doesn't show that Dr Karp's Five S's (or The Happiest Baby) method makes babies sleep more and cry less as a pattern over time

I first became aware of Dr Harvey Karp and his work at the 2011 International Infant Cry Research Workshop. Dr Karp was a medical doctor and not a career researcher, much like myself, but he was keen to raise the profile of his Five S's (or The Happiest Baby) method. He demonstrated his method to a politely skeptical audience, since two United States paediatrician-researchers had just published the results of a randomised controlled trial of the Five S's method, finding no benefit to baby crying or sleep durations over the first six months of life, and also no decrease in parent stress.

Despite claims, there is still no (reliable, methodologically robust) research to demonstrate improved crying or sleep patterns over time with the Five S's (or Happiest Baby) method, nor with use of Dr Karp's mechanised bassinet, known as the Snoo.

Does the Five S's cause a calming response in babies?

Dr Karp's work aims to stimulate what he refers to as the calming response, using a prescribed, very specific or constrained kind of sensory motor stimulation.

In The Possums Sleep Program and Neuroprotective Developmental Care more generally, I don't use the term 'calming response'. Instead, I talk about the dial on your baby's sympathetic nervous system. This is an important difference because I believe parents need to know about the things that science tells us dial babies up, as well the things that might dial them down. The Possums Sleep Program aims to help you experiment with many different ways of keeping your baby as dialled down as you sensibly can, both as prevention, and then in response to dialling up during the very sensitive early months of your baby's life.

• The Five S's assumes that babies are fed before the Five S's are applied. Studies investigating the Five S's acknowledge the approach doesn't work (even in the short-term) if baby's crying relates to hunger. Satiation with milk has to come first, if the Five S's specific, prescribed form of sensory motor stimulation is to be effective. Yet feeding problems, which take multiple forms and are vital reasons why babies dial up, are not addressed in the Five S's method.

• Therefore there is no evidence that the Five S's is any more effective than simply breastfeeding to sleep, or bottle feeding to sleep if your baby needs to use the bottle.

• Are babies in the Five S's studies burped after the feed that is given before the Five S's are applied? The studies do not investigate this. Similarly, wrapping or swaddling is likely to wake babies after the feed (if burping hasn't). Is it possible that parents are engaging in practices which dial babies up, before applying the Five S's to dial their babies down? The Possums Sleep Program recognises that it's best to avoid dialling babies up in the first place, if at all possible.

• Research shows that the Five S's dials babies down after a vaccination. But why wouldn't parents simply offer baby the breast, or a cuddle as they step outside the clinic into a much richer sensory environment, to dial baby down after a vaccination?

• Although the Five S's may result in a short-term calming response, Dr Karp's method has not been shown to generalise to patterns of less crying and better sleep over time.

• There is no research which tries to tease out which elements of the Five S's might result in a short-term calming response.

The Five S's (or Happiest Baby) method misrepresents evolutionary biology

The Homo sapiens infant is an exterogestate foetus in the first nine months of life, born with certain biological expectations. You can find out what is required for the flourishing of the baby's brain here.

Dr Karp claims that the Five S's and the Snoo replicate the environment inside the womb. But from an evolutionary perspective, your baby doesn't expect or need to benefit from efforts to replicate the environment inside the womb in the first few months of life, because that developmental phase came to an end with the birth! You can find out about the evolutionary origins of an infant's biological hunger for rich and changing sensory motor stimulation from soon after the birth here.

Your baby has an evolutionary expectation of being in continuous contact with the milk, warmth, scent, sound and touch of the maternal or parental body from birth, which offers exponentially more complex environmental and sensory motor experience than occurs inside the womb. The human infant needs this environmental complexity neurologically, to optimise development.

Yet sensory motor experience and access to sucking are often severely restricted in Western contexts (partly due to dominant cultural tendencies to prescribe sleep routines and to space out feeds). A prescribed package of sensory motor stimulation like the Five S's or a limited form of sensory stimulation provided mechanically by the Snoo may address specific elements of this relative impoverishment of sensory motor experience, resulting in short-term dialling down of the baby.

But this very limited form of sensory nourishment doesn't address multiple underlying problems which result in crying and sleep problems, and doesn't translate into less crying and more sleep as a pattern over time.

It's my view, then, that Dr Karp's methods perpetuate a mismatch between a baby's environment of evolutionary adaptedness and dominant Western culture. My concern is that this continues to make postnatal life much harder for women or primary carers than it needs to be.

The Possums Sleep Program, and Neuroprotective Developmental Care or the Possums programs build on the latest understandings of evolutionary anthropology, with the aim of providing you with a holistic, biologically-aligned approach to caring for your baby which make the days (and nights) as easy and as manageable as possible.

Recommended resources

Will a Snoo Smart Sleeper bassinet help with baby's sleep and crying?

Wrapping, swaddling or restricting baby's arm movements: how to do it safely, and does it help with sleep, anyway?

Wrapping, swaddling or restricting baby's arm movements: possible downsides and busting more sleep myths

The holistic NDC or Possums 8-step approach to supporting baby's motor development

The NDC evolutionary perspective on positional plagiocephaly, motor development, and sleep

What your baby needs for best possible motor development

Selected research

Harrington JW, Logan S, Harwell C. Effective analgesia using physical interventions for infant immunizations. Pediatrics. 2012;129:815-822.

Karp HK. Evaluation of a womb-like sensory intervention to improve infant sleep. Sleep Medicine. 2023;100:S195.

McRury JM, Zolotor AJ. A randomized, controlled trial of a behavioral intervention to reduce crying among infants. The Journal of the American Board of Family Medicine. 2010;23 315-322.

Moller EL, De Vente W, Rodenburg R. Infant crying and the calming response: parental versus mechanical soothing using swaddling, sound, and movement. Plos One. 2019;14(4):e0214548.

Rosenberg KR. The evolution of human infancy: why it helps to be helpless. Annual Review of Anthropology. 2021;50:423-440.

Savage JS, Birth LL, M M. Effect of the INSIGHT Responsive Parenting Intervention on rapid infant weight gain and overweight status age age 1 year: a randomized clinical trial. JAMA Pediatrics. 2016;170:742-749.

Singh JK, Menahem S. The five "S's" and the "SNOO" Smart Sleeper - non-pharmacological interventions (NPI) to promote sleep and reduce crying of infants: a scoping review. Translational Pediatrics. 2023;12(8).