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The Wonder Weeks: A Stress-Free Guide to Your Baby's Behavior is a popular book "intended as a general information resource about how babies behave at various points during the normal development process, and why they behave that way"^[1] by physical anthropologist Hetty van de Rijt and ethologist and developmental psychologist Frans Plooij. It is based on ethological studies of mothers and infants in many species.^[2]^[3]^[4] After the death of the first author in 1993,^[5] their daughter Xaviera Plas-Plooij joined her father in revising and updating recent editions with new research, including a chapter on sleep.^[6] Supporting products from the publisher include a mobile app.^[7]

Basis

The book is based primarily on ethological studies of mothers and infants in many species as well as humans,^[8]^[3] correlated with evidence from developmental studies of the brain^[9] and from Perceptual Control Theory, which includes a Hebbian mechanism for development and learning, called reorganization (also proposed by Ashby),^[10] which predicts and accounts for this pattern of development.^[11]^[12]^[13]

A technical report of the Plooijs' first controlled replication of these ethological findings in humans, involving 15 Dutch mothers and their infants with extrinsic sources of stress carefully controlled, was published in the Journal of Reproductive and Infant Psychology in 1992,^[14] the same year as the first publication of this book for the general public.^[15]

Claims

On the basis of this research, and the observation of "predictable regression periods" in many species including humans, the book describes 10 predictable 'leaps' observed in a child's cognitive development during the first 20 months, with 8 in the first year, counting from the due date because development begins with conception.^[16] This is a period of tremendous growth in the brain. The brain reaches half its adult size within the first three months after birth.^[17] Brain volume is about 35% of adult volume 2–3 weeks after birth, doubles from term size in the first year of life and increases an additional 15% in the second to about 80% of adult size.^[18]

The 'regression period' is the first phase of a 'leap', during which the infant's previously confident control is disrupted by having to learn to perceive and control at a new and more abstract level of the growing perceptual hierarchy. This is followed by another period in which the baby is generally happy, due to discovering new things with the newly gained cognitive skills. The 'leaps' are predicted to occur at about 5, 8, 12, 19, 26, 37, 46, 55, 64 and 75 weeks of age (from the due date), although the timing for learning particular skills associated with them varies from one child to another.^[1]^[19]

Reception

Many child development experts are skeptical and say that it contradicts their training and the variability in the timing of skills acquisition. Others find it accurate and helpful.^[20] Brazelton found it to be mutually confirmatory with his own work.^[21]^[22]

Controversy

Since the beginning of the 20th century developmental psychologists have investigated whether the early childhood development process is gradual or punctuated by distinct biologically determined advances.^[23]^[24] The relation between developmental milestones or transitions, such as grasping, crawling or walking for the first time, has proven untenable.^[25]

Evidence for regression periods correlated with punctuated neurological developments followed by periods of equilibrium came from ethologists studying the interactions of mothers and infants in primates and many other species,^[3] suggesting an early origin in evolution of species.^[26]

Into this old controversy the Plooijs brought ethological evidence for intrinsically timed stages of human as well as primate infant development. Their first human study involving 15 Dutch mothers and their infants, with extrinsic sources of stress carefully controlled, was published in the Journal of Reproductive and Infant Psychology in 1992,^[14] the same year as the first publication of this book for the general public.^[27] These behavioral observations have been correlated with stages of neurological development of the brain.^[11] ^[9]

Plooij obtained state funding to oversee an independent replication. With this funding he obtained a one day a week position as a "Professor by special appointment" (Dutch: bijzonder hoogleraar) at his alma mater, the University of Groningen.^[28] He engaged Carolina de Weerth, a PhD student of Paul van Geert.^[29] At the conclusion of her research, she reported that she had found many periods of greater fussiness and higher cortisol levels other than at the predicted times.^[30]^[31] Plooij said that this was because three of the four mothers that she had recruited did not meet the stated criteria for a replication because their circumstances and behavior introduced extrinsic sources of stress for the infant which made it difficult to discern distress specifically at times of new developments.^[25]^[32]^[33]^[34]

Plooij objected to these results being published as a replication. He resigned his position, which was near its end, subsequently asked to rescind his resignation, and was refused.^[35] Dutch newspapers reported that de Weert was Plooij's PhD student and had refuted his research, and that Plooij had been dismissed.^[35]^[36] According de Weerth, Plooij tried to pressure her into not publishing the study.^[35]^[37] Plooij disputes this account,^[37] and de Weert's results were published jointly with van Geert as the lead author.^[38]

The controversy was made a focal topic in the journal Neuropsychiatrica with an article by Plooij, ^[25]

a rejoinder by van Geert and de Weert,^[39] and a refutation by Plooij.^[32]

The Plooijs' 1992 study has subsequently been independently replicated at universities in three countries, Oxford in England,^[34] Girona in Spain,^[40]^[41] and Gothenburg in Sweden.^[42] Plooij's research has continued.^[43]

References

^ ^a ^b Rijt, Hetty van de; Plooij, Frans X.; Plas-Plooij, Xaviera (2019). The Wonder Weeks: A Stress-Free Guide to Your Baby's Behavior. New York: W.W. Norton. ISBN 9789491882166.
^ "Before our research, such regression phases had been found by others in 12 other primate species and two lower mammalian species, indicating that this appears to be an old phenomenon, perhaps emerging during the very evolution of life on earth" ("Introduction" (by F.X. Plooij), The Wonder Weeks p. 15).
^ ^a ^b ^c Horwich, "Robert H." (1974). ""Regressive Periods in Primate Behavioral Development with Reference to Other Mammals"". Primates. 15 (2–3): 141–149. Retrieved 2024-02-25. Cite error: The named reference "Horwich74" was defined multiple times with different content (see the help page).
^ Xaveira Plas-Plooij: Shilton, Emma (2014-05-15). "An interview with Xaviera Plas Plooij". Hello Mrs. Shilts. Retrieved 2024-02-26. The research was carried out by my parents and to be honest they weren't looking for it. It was open research based on observations of Mums and babies in their natural habitats. Over time we noticed that there were peaks where babies would change. We noticed that there were a lot of similarities in their development resulting in different behaviours than what parents were used to before. The research showed we could predict when parents could expect their babies to behave differently and go through a phase.
^ Rojas-Rocha, Xochitl (2014-08-22). "Gombe chimpanzee calls available after 40-year wait". Science News. No. 22579. American Association for the Advancement of Science. Retrieved 2024-02-25.
^ Verhoeven, Eymeke (2018-03-07). "Je kind loopt nog niet? Maakt niet uit" [Your child isn't walking yet? It doesn't matter]. Nederlands Dagblad (in Dutch). Retrieved 2024-02-25.
^ "The Wonder Weeks App". The Wonder Weeks. Retrieved 2017-08-27.
^ "Before our research, such regression phases had been found by others in 12 other primate species and two lower mammalian species, indicating that this appears to be an old phenomenon, perhaps emerging during the very evolution of life on earth" ("Introduction" (by F.X. Plooij), ibid. p. 15).
^ ^a ^b Trevarthen, C.; Aitken, K. (2003). "Regulation of Brain Development and Age-Related Changes in Infants' Motives: The Developmental Function of Regressive Periods". In Heimann, Mikael (ed.). Regression periods in human infancy. Mahwah, NJ: Erlbaum. pp. 107–184. ISBN 0-8058-4098-2.
^ Ashby, W. Ross (1960). Design for a Brain: The origin of adaptive behavior. New York: Wiley.
^ ^a ^b Plooij, F. X.; Rijt-Plooij, H. H. C. van de (1990). "Developmental transitions as successive reorganizations of a control hierarchy". American Behavioral Scientist. 34 (1): 67–80. doi:10.1177/0002764290034001007. S2CID 144183592.
^ Mansell, Warren (2020). "Ten vital elements of perceptual control theory, tracing the pathway from implicit influence to scientific advance". In Mansell, Warren (ed.). The Interdisciplinary Handbook of Perceptual Control Theory: Living Control Systems IV. London: Academic Press. p. 589. ISBN 978-0-12-818948-1.
^ Plooij, Frans X. (2020). "The phylogeny, ontogeny, causation and function of regression periods explained by reorganizations of the hierarchy of perceptual control systems". In Mansell, Warren (ed.). The Interdisciplinary Handbook of Perceptual Control Theory: Living Control Systems IV. London: Academic Press. pp. 199–225. ISBN 978-0-12-818948-1.
^ ^a ^b Van De Rijt-Plooij, Hedwig H.C.; Plooij, Frans X. (July 1992). "Infantile regressions: Disorganization and the onset of transition periods". Journal of Reproductive and Infant Psychology. 10 (3): 129–149. doi:10.1080/02646839208403946. ISSN 0264-6838.
^ Oei, ik groei! (literal translation: Ai, I'm growing!) was published in 1992 by Zomer & Keuning Boeken BV, Ede and Antwerp.
^ The Wonder Weeks (2017 ed.) p. 23.
^ Holland, D.; Chang, L.; Ernst, T.M.; et al. (2024-10-01). "Structural Growth Trajectories and Rates of Change in the First 3 Months of Infant Brain Development". JAMA Neurol. 71 (10): 1266–1274. doi:10.1001/jamaneurol.2014.1638.
^ Gilmore, J.H.; Knickmeyer, R.C.; Gao, W. (2018-02-16). "Imaging structural and functional brain development in early childhood". Nat. Rev. Neurosci. 19 (3): 123–137. doi:10.1038/nrn.2018.1.
^ Xaveira Plas-Plooij: {{cite web |url=https://www.mrsshilts.co.uk/interview-xaviera-plas-plooij-wonder-weeks/ |title=An interview with Xaviera Plas Plooij |last=Shilton |first=Emma |date=2014-05-15 |website=Hello Mrs. Shilts |publisher= |access-date=2024-02-26 |quote=The research stopped at 20 months as this is when we realised that by then parents are able to decipher when their children are going through leaps. As children grow older into teenagers and young adults, there [are] 3 or 4 much bigger leaps such as puberty and further on the ‘mid-life crisis’. The research would be enormous if we studied all of those so it was best to concentrate on the first 20 months when parents need help making informed choices and information on how their children are developing.}
^ Wapner, Jessica (2020-04-16). "Are Sleep Regressions Real?". The New York Times. ISSN 0362-4331. Archived from the original on 2023-06-05. Retrieved 2023-12-17. In other words, timed leaps may be real, but the evidence hasn't proven if or in what capacity they exist. Neurologists are just beginning to understand brain patterns during the first two years of life. "We really are at the infancy of infant brain knowledge," said Dr. Hirsh-Pasek, the psychologist at Temple University. Some pediatricians, though, believe Dr. Plooij's theory is correct. Dr. Pamela Hops, M.D., a pediatrician in New York City, said that during her 20 years of practice, she has anecdotally seen and heard about changes in babies that perfectly align with the 10 leaps Dr. Plooij described. "I think he's spot on," said Dr. Hops, "shockingly so."
^ Sparrow, J. (2013). "Newborn Behavior, Parent–Infant Interaction, and Developmental Change Processes: Research Roots of Developmental, Relational, and Systems-Theory-Based Practice". Journal of Child and Adolescent Psychiatric Nursing. 26 (3): 180–185. doi:10.1111/jcap.12047. PMID 23909940.
^ Plooij, F. X. (2010), "The 4 WHY's of age-linked regression periods in infancy", in Lester, B. M.; Sparrow, J. D. (eds.), Nurturing Children and Families: Building on the Legacy of T. Berry Brazelton, Malden, MA: Wiley-Blackwell, pp. 107–119
^ Kalverboer, L. (1998). "Ontwikkelingssprongen in het duister: Over transities in de ontwikkeling" [Developmental leaps in the dark: On transitions in development]. Neuropraxis. 1. doi:10.1007/BF03070912. Verloopt het vroegkinderlijke ontwikkelingsproces geleidelijk of sprongsgewijs? Deze vraag houdt onderzoekers bezig sinds het begin van deze eeuw, toen de systematische studie van de ontwikkeling van het kind begon. [Is the early childhood development process gradual or in leaps? This question has preoccupied researchers since the beginning of this century, when the systematic study of child development began.]
^ Kagan, Jerome; Herschkowitz, Norbert (2014-01-01). "Preface". A Young Mind in a Growing Brain. New York: Psychology Press. p. xi. The argument that a select set of human psychological properties cannot appear until certain maturational events have occurred bothers a number of American and European social scientists. We understand their resistance to the suggestion that there are natural restraints on the time of emergence of some human competences. Western society values freedom of action and the continual contribution of experience to biological structures.
^ ^a ^b ^c Plooij, F. X. (1998). "Hersenveranderingen en 'sprongen' in de eerste 20 levensmaanden en de invloed van de context op gedragsmaten van regressieperioden" [Brain changes and 'leaps' in the first 20 months of life and the influence of context on behavioral measures of regression periods]. Acta Neuropsychiatrica. 10 (3): 63–66. doi:10.1017/S0924270800036590.;|quote= Het idee dat er een koppeling te leggen valt tussen rijping van het zenuwstelsel en ontwikkelings-mijlpalen of -transities, zoals voor het eerst grijpen, kruipen of lopen, is onhoudbaar gebleken. Wel valt er een koppeling in de tijd te leggen tussen hersenveranderingen en regressieperioden |trans-quote= The idea that there is a link between nervous system maturation and developmental milestones or transitions, such as grasping, crawling or walking for the first time, has proven untenable. However, a temporal link can be established between brain changes and regression periods …. Cite error: The named reference "Plooij1998a" was defined multiple times with different content (see the help page).
^ "Before our research, such regression phases had been found by others in 12 other primate species and two lower mammalian species, indicating that this appears to be an old phenomenon, perhaps emerging during the very evolution of life on earth" ("Introduction" (by F.X. Plooij), ibid. p. 15).
^ Oei, ik groei! (literal translation: Ai, I'm growing!) was published in 1992 by Zomer & Keuning Boeken BV, Ede and Antwerp.
^ Kreulen, Edwin (2024-08-18). "En-plooij, hij springt verder" [And Plooij, he jumps further]. Trouw. Retrieved 2024-02-25. Plooij, appointed as a special professor at the University of Groningen for one day a week, hired a PhD student who should substantiate the theory.
^ Kok, Annemarie (1997-11-19). "'Oei, ik groei!' leidt tot bittere ruzie van wetenschappers" ['Ouch, I'm growing!' leads to bitter arguments among scientists]. Trouw (in Dutch). Retrieved 2024-02-26. Volgens hoogleraar P. van Geert, die onderzoekster De Weerth de laatste jaren heeft begeleid, is het boek enerzijds gebaseerd op bestaande, betrouwbare ontwikkelingspsychologische kennis. [According to Professor P. van Geert, who has guided researcher De Weerth in recent years, the book is based on existing, reliable developmental psychological knowledge.]
^ de Weerth, C.; van Geert, P. (1998-03-01). "Emotional instability as an indicator of strictly timed infantile developmental transitions". British Journal of Developmental Psychology. 16 (1): 15–44. doi:10.1111/j.2044-835X.1998.tb00748.x. ISSN 2044-835X.
^ Cite error: The named reference trouw was invoked but never defined (see the help page).
^ ^a ^b Plooij, F. X. (1998). "Repliek op 'Empirische indicatoren voor regressies en sprongen bij baby's'" [Reply to 'Empirical indicators of regressions and leaps in infants']. Acta Neuropsychiatrica. 10 (3): 74–77. doi:10.1017/S0924270800036619.
^ Plooij, F. X.; Rijt-Plooij, H. H. C. van de (2003), "The effects of sources of "noise" on direct observation measures of regression periods: Case studies of four infants' adaptations to special parental conditions.", in Heimann, M. (ed.), Regression periods in human infancy, Mahwah, NJ: Erlbaum, pp. 57–80, ISBN 978-0805840988
^ ^a ^b Woolmore, A.; Richer, J. (2003). "Detecting infant regression periods: weak signals in a noisy environment". In Heimann, M. (ed.). Regression periods in human infancy. Mahwah, NJ: Erlbaum. pp. 23–39. ISBN 978-0805840988.
^ ^a ^b ^c Kok, Annemarie (1997-11-19). "'Oei, ik groei!' leidt tot bittere ruzie van wetenschappers" ['Ouch, I'm growing!' leads to bitter arguments among scientists]. Trouw (in Dutch). Retrieved 2024-02-26.
^ Dirks, Bart (1998-01-14). "Positie Plooij onhoudbaar na openlijke kritiek op oud-promovenda; Auteur 'Oei, ik groei' ontslagen als hoogleraar" [Plooij's position untenable after open criticism of former PhD candidate; Author 'Oops, I'm growing' dismissed as professor]. Volkskrant. Retrieved 14 June 2015.
^ ^a ^b {{Cite news |last=Wapner |first=Jessica |date=2020-04-16 |title=Are Sleep Regressions Real? |language=en-US |work=The New York Times |url=https://www.nytimes.com/2020/04/15/parenting/baby/sleep-regression.html |url-status=live |access-date=2023-12-17 |archive-url=https://web.archive.org/web/20230605155926/https://www.nytimes.com/2020/04/15/parenting/baby/sleep-regression.html |archive-date=2023-06-05 |issn=0362-4331 |quote=
^ Weerth, C. d., & Geert, P. v. (1998). Emotional instability as an indicator of strictly timed infantile developmental transitions. British Journal of Developmental Psychology, 16, 15-44
^ Geert, P. v.; Weerth, C. d. (1998). "Empirische indicatoren voor regressies en sprongen bij baby's" [Empirical indicators of regressions and leaps in infants]. Acta Neuropsychiatrica. 10 (3): 67–73. doi:10.1017/S0924270800036607.
^ Sadurní, M.; Rostan, C. (2002). "Regression periods in infancy: A case study from Catalonia". Spanish Journal of Psychology. 5 (1): 36–44. doi:10.1017/s1138741600005813. hdl:10256/1720. PMID 12025364.
^ Sadurní, M.; Rostan, C. (2003). "Reflections on regression periods in the development of Catalan infants". In Heimann, Mikael (ed.). Regression periods in human infancy. Mahwah, NJ: Erlbaum. pp. 7–22. ISBN 0-8058-4098-2.
^ Lindahl, L.; Heimann, M.; Ullstadius, E. (2003). "Occurrence of regressive periods in the normal development of Swedish infants". In Heimann, M. (ed.). Regression periods in human infancy. Mahwah, NJ: Erlbaum. pp. 41–55. ISBN 978-0805840988.
^ Plooij, F. X. (2010). "The 4 WHY's of Age-Linked Regression Periods in Infancy". In Lester, B. M.; Sparrow, J. D. (eds.). Nurturing Children and Families: Building on the Legacy of T. Berry Brazelton. Malden, MA: Wiley-Blackwell. pp. 107–119.

[TWW2019-1] Rijt, Hetty van de; Plooij, Frans X.; Plas-Plooij, Xaviera (2019). The Wonder Weeks: A Stress-Free Guide to Your Baby's Behavior. New York: W.W. Norton. ISBN 9789491882166.

[2] "Before our research, such regression phases had been found by others in 12 other primate species and two lower mammalian species, indicating that this appears to be an old phenomenon, perhaps emerging during the very evolution of life on earth" ("Introduction" (by F.X. Plooij), The Wonder Weeks p. 15).

[Horwich74-3] Horwich, "Robert H." (1974). ""Regressive Periods in Primate Behavioral Development with Reference to Other Mammals"". Primates. 15 (2–3): 141–149. Retrieved 2024-02-25. Cite error: The named reference "Horwich74" was defined multiple times with different content (see the help page).

[4] Xaveira Plas-Plooij: Shilton, Emma (2014-05-15). "An interview with Xaviera Plas Plooij". Hello Mrs. Shilts. Retrieved 2024-02-26. The research was carried out by my parents and to be honest they weren't looking for it. It was open research based on observations of Mums and babies in their natural habitats. Over time we noticed that there were peaks where babies would change. We noticed that there were a lot of similarities in their development resulting in different behaviours than what parents were used to before. The research showed we could predict when parents could expect their babies to behave differently and go through a phase.

[5] Rojas-Rocha, Xochitl (2014-08-22). "Gombe chimpanzee calls available after 40-year wait". Science News. No. 22579. American Association for the Advancement of Science. Retrieved 2024-02-25.

[Verhoeven-6] Verhoeven, Eymeke (2018-03-07). "Je kind loopt nog niet? Maakt niet uit" [Your child isn't walking yet? It doesn't matter]. Nederlands Dagblad (in Dutch). Retrieved 2024-02-25.

[7] "The Wonder Weeks App". The Wonder Weeks. Retrieved 2017-08-27.

[8] "Before our research, such regression phases had been found by others in 12 other primate species and two lower mammalian species, indicating that this appears to be an old phenomenon, perhaps emerging during the very evolution of life on earth" ("Introduction" (by F.X. Plooij), ibid. p. 15).

[Trevarthen-Aitken03-9] Trevarthen, C.; Aitken, K. (2003). "Regulation of Brain Development and Age-Related Changes in Infants' Motives: The Developmental Function of Regressive Periods". In Heimann, Mikael (ed.). Regression periods in human infancy. Mahwah, NJ: Erlbaum. pp. 107–184. ISBN 0-8058-4098-2.

[10] Ashby, W. Ross (1960). Design for a Brain: The origin of adaptive behavior. New York: Wiley.

[Plooij-ABS-11] Plooij, F. X.; Rijt-Plooij, H. H. C. van de (1990). "Developmental transitions as successive reorganizations of a control hierarchy". American Behavioral Scientist. 34 (1): 67–80. doi:10.1177/0002764290034001007. S2CID 144183592.

[Mansell-2020-12] Mansell, Warren (2020). "Ten vital elements of perceptual control theory, tracing the pathway from implicit influence to scientific advance". In Mansell, Warren (ed.). The Interdisciplinary Handbook of Perceptual Control Theory: Living Control Systems IV. London: Academic Press. p. 589. ISBN 978-0-12-818948-1.

[Plooij-2020-13] Plooij, Frans X. (2020). "The phylogeny, ontogeny, causation and function of regression periods explained by reorganizations of the hierarchy of perceptual control systems". In Mansell, Warren (ed.). The Interdisciplinary Handbook of Perceptual Control Theory: Living Control Systems IV. London: Academic Press. pp. 199–225. ISBN 978-0-12-818948-1.

[Plooijs1992-14] Van De Rijt-Plooij, Hedwig H.C.; Plooij, Frans X. (July 1992). "Infantile regressions: Disorganization and the onset of transition periods". Journal of Reproductive and Infant Psychology. 10 (3): 129–149. doi:10.1080/02646839208403946. ISSN 0264-6838.

[15] Oei, ik groei! (literal translation: Ai, I'm growing!) was published in 1992 by Zomer & Keuning Boeken BV, Ede and Antwerp.

[16] The Wonder Weeks (2017 ed.) p. 23.

[17] Holland, D.; Chang, L.; Ernst, T.M.; et al. (2024-10-01). "Structural Growth Trajectories and Rates of Change in the First 3 Months of Infant Brain Development". JAMA Neurol. 71 (10): 1266–1274. doi:10.1001/jamaneurol.2014.1638.

[18] Gilmore, J.H.; Knickmeyer, R.C.; Gao, W. (2018-02-16). "Imaging structural and functional brain development in early childhood". Nat. Rev. Neurosci. 19 (3): 123–137. doi:10.1038/nrn.2018.1.

[19] Xaveira Plas-Plooij: {{cite web |url=https://www.mrsshilts.co.uk/interview-xaviera-plas-plooij-wonder-weeks/ |title=An interview with Xaviera Plas Plooij |last=Shilton |first=Emma |date=2014-05-15 |website=Hello Mrs. Shilts |publisher= |access-date=2024-02-26 |quote=The research stopped at 20 months as this is when we realised that by then parents are able to decipher when their children are going through leaps. As children grow older into teenagers and young adults, there [are] 3 or 4 much bigger leaps such as puberty and further on the ‘mid-life crisis’. The research would be enormous if we studied all of those so it was best to concentrate on the first 20 months when parents need help making informed choices and information on how their children are developing.}

[20] Wapner, Jessica (2020-04-16). "Are Sleep Regressions Real?". The New York Times. ISSN 0362-4331. Archived from the original on 2023-06-05. Retrieved 2023-12-17. In other words, timed leaps may be real, but the evidence hasn't proven if or in what capacity they exist. Neurologists are just beginning to understand brain patterns during the first two years of life. "We really are at the infancy of infant brain knowledge," said Dr. Hirsh-Pasek, the psychologist at Temple University. Some pediatricians, though, believe Dr. Plooij's theory is correct. Dr. Pamela Hops, M.D., a pediatrician in New York City, said that during her 20 years of practice, she has anecdotally seen and heard about changes in babies that perfectly align with the 10 leaps Dr. Plooij described. "I think he's spot on," said Dr. Hops, "shockingly so."

[Sparrow2013-21] Sparrow, J. (2013). "Newborn Behavior, Parent–Infant Interaction, and Developmental Change Processes: Research Roots of Developmental, Relational, and Systems-Theory-Based Practice". Journal of Child and Adolescent Psychiatric Nursing. 26 (3): 180–185. doi:10.1111/jcap.12047. PMID 23909940.

[Plooij-2010-22] Plooij, F. X. (2010), "The 4 WHY's of age-linked regression periods in infancy", in Lester, B. M.; Sparrow, J. D. (eds.), Nurturing Children and Families: Building on the Legacy of T. Berry Brazelton, Malden, MA: Wiley-Blackwell, pp. 107–119

[23] Kalverboer, L. (1998). "Ontwikkelingssprongen in het duister: Over transities in de ontwikkeling" [Developmental leaps in the dark: On transitions in development]. Neuropraxis. 1. doi:10.1007/BF03070912. Verloopt het vroegkinderlijke ontwikkelingsproces geleidelijk of sprongsgewijs? Deze vraag houdt onderzoekers bezig sinds het begin van deze eeuw, toen de systematische studie van de ontwikkeling van het kind begon. [Is the early childhood development process gradual or in leaps? This question has preoccupied researchers since the beginning of this century, when the systematic study of child development began.]

[24] Kagan, Jerome; Herschkowitz, Norbert (2014-01-01). "Preface". A Young Mind in a Growing Brain. New York: Psychology Press. p. xi. The argument that a select set of human psychological properties cannot appear until certain maturational events have occurred bothers a number of American and European social scientists. We understand their resistance to the suggestion that there are natural restraints on the time of emergence of some human competences. Western society values freedom of action and the continual contribution of experience to biological structures.

[Plooij1998a-25] Plooij, F. X. (1998). "Hersenveranderingen en 'sprongen' in de eerste 20 levensmaanden en de invloed van de context op gedragsmaten van regressieperioden" [Brain changes and 'leaps' in the first 20 months of life and the influence of context on behavioral measures of regression periods]. Acta Neuropsychiatrica. 10 (3): 63–66. doi:10.1017/S0924270800036590.;|quote= Het idee dat er een koppeling te leggen valt tussen rijping van het zenuwstelsel en ontwikkelings-mijlpalen of -transities, zoals voor het eerst grijpen, kruipen of lopen, is onhoudbaar gebleken. Wel valt er een koppeling in de tijd te leggen tussen hersenveranderingen en regressieperioden |trans-quote= The idea that there is a link between nervous system maturation and developmental milestones or transitions, such as grasping, crawling or walking for the first time, has proven untenable. However, a temporal link can be established between brain changes and regression periods …. Cite error: The named reference "Plooij1998a" was defined multiple times with different content (see the help page).

[26] "Before our research, such regression phases had been found by others in 12 other primate species and two lower mammalian species, indicating that this appears to be an old phenomenon, perhaps emerging during the very evolution of life on earth" ("Introduction" (by F.X. Plooij), ibid. p. 15).

[27] Oei, ik groei! (literal translation: Ai, I'm growing!) was published in 1992 by Zomer & Keuning Boeken BV, Ede and Antwerp.

[28] Kreulen, Edwin (2024-08-18). "En-plooij, hij springt verder" [And Plooij, he jumps further]. Trouw. Retrieved 2024-02-25. Plooij, appointed as a special professor at the University of Groningen for one day a week, hired a PhD student who should substantiate the theory.

[29] Kok, Annemarie (1997-11-19). "'Oei, ik groei!' leidt tot bittere ruzie van wetenschappers" ['Ouch, I'm growing!' leads to bitter arguments among scientists]. Trouw (in Dutch). Retrieved 2024-02-26. Volgens hoogleraar P. van Geert, die onderzoekster De Weerth de laatste jaren heeft begeleid, is het boek enerzijds gebaseerd op bestaande, betrouwbare ontwikkelingspsychologische kennis. [According to Professor P. van Geert, who has guided researcher De Weerth in recent years, the book is based on existing, reliable developmental psychological knowledge.]

[:0-30] Weerth, C.; van Geert, P. (1998-03-01). "Emotional instability as an indicator of strictly timed infantile developmental transitions". British Journal of Developmental Psychology. 16 (1): 15–44. doi:10.1111/j.2044-835X.1998.tb00748.x. ISSN 2044-835X.

[trouw-31] Cite error: The named reference trouw was invoked but never defined (see the help page).

[Plooij1998b-32] Plooij, F. X. (1998). "Repliek op 'Empirische indicatoren voor regressies en sprongen bij baby's'" [Reply to 'Empirical indicators of regressions and leaps in infants']. Acta Neuropsychiatrica. 10 (3): 74–77. doi:10.1017/S0924270800036619.

[Plooij-plooij-2003-33] Plooij, F. X.; Rijt-Plooij, H. H. C. van de (2003), "The effects of sources of "noise" on direct observation measures of regression periods: Case studies of four infants' adaptations to special parental conditions.", in Heimann, M. (ed.), Regression periods in human infancy, Mahwah, NJ: Erlbaum, pp. 57–80, ISBN 978-0805840988

[Woolmore-Richer-2003-34] Woolmore, A.; Richer, J. (2003). "Detecting infant regression periods: weak signals in a noisy environment". In Heimann, M. (ed.). Regression periods in human infancy. Mahwah, NJ: Erlbaum. pp. 23–39. ISBN 978-0805840988.

[trouw-ruzie-35] Kok, Annemarie (1997-11-19). "'Oei, ik groei!' leidt tot bittere ruzie van wetenschappers" ['Ouch, I'm growing!' leads to bitter arguments among scientists]. Trouw (in Dutch). Retrieved 2024-02-26.

[volkskrant-36] Dirks, Bart (1998-01-14). "Positie Plooij onhoudbaar na openlijke kritiek op oud-promovenda; Auteur 'Oei, ik groei' ontslagen als hoogleraar" [Plooij's position untenable after open criticism of former PhD candidate; Author 'Oops, I'm growing' dismissed as professor]. Volkskrant. Retrieved 14 June 2015.

[Wapner-37] {{Cite news |last=Wapner |first=Jessica |date=2020-04-16 |title=Are Sleep Regressions Real? |language=en-US |work=The New York Times |url=https://www.nytimes.com/2020/04/15/parenting/baby/sleep-regression.html |url-status=live |access-date=2023-12-17 |archive-url=https://web.archive.org/web/20230605155926/https://www.nytimes.com/2020/04/15/parenting/baby/sleep-regression.html |archive-date=2023-06-05 |issn=0362-4331 |quote=

[38] Weerth, C. d., & Geert, P. v. (1998). Emotional instability as an indicator of strictly timed infantile developmental transitions. British Journal of Developmental Psychology, 16, 15-44

[39] Geert, P. v.; Weerth, C. d. (1998). "Empirische indicatoren voor regressies en sprongen bij baby's" [Empirical indicators of regressions and leaps in infants]. Acta Neuropsychiatrica. 10 (3): 67–73. doi:10.1017/S0924270800036607.

[Sadurní-Rostan02-40] Sadurní, M.; Rostan, C. (2002). "Regression periods in infancy: A case study from Catalonia". Spanish Journal of Psychology. 5 (1): 36–44. doi:10.1017/s1138741600005813. hdl:10256/1720. PMID 12025364.

[Sadurní-Rostan03-41] Sadurní, M.; Rostan, C. (2003). "Reflections on regression periods in the development of Catalan infants". In Heimann, Mikael (ed.). Regression periods in human infancy. Mahwah, NJ: Erlbaum. pp. 7–22. ISBN 0-8058-4098-2.

[Lindahl-2003-42] Lindahl, L.; Heimann, M.; Ullstadius, E. (2003). "Occurrence of regressive periods in the normal development of Swedish infants". In Heimann, M. (ed.). Regression periods in human infancy. Mahwah, NJ: Erlbaum. pp. 41–55. ISBN 978-0805840988.

[43] Plooij, F. X. (2010). "The 4 WHY's of Age-Linked Regression Periods in Infancy". In Lester, B. M.; Sparrow, J. D. (eds.). Nurturing Children and Families: Building on the Legacy of T. Berry Brazelton. Malden, MA: Wiley-Blackwell. pp. 107–119.

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 ==Controversy==
-Since the beginning of the 20th century developmental psychologists have investigated whether the early childhood development process is gradual or punctuated by distinct biologically determined advances.<ref>{{cite journal |last=Kalverboer |first=L. |date=1998 |title=Ontwikkelingssprongen in het duister: Over transities in de ontwikkeling |trans-title=Developmental leaps in the dark: On transitions in development |url=https://mijn.bsl.nl/ontwikkelingssprongen-in-het-duister/448212 |journal=Neuropraxis|volume=1 |issue= | pages= |quote=Verloopt het vroegkinderlijke ontwikkelingsproces geleidelijk of sprongsgewijs? Deze vraag houdt onderzoekers bezig sinds het begin van deze eeuw, toen de systematische studie van de ontwikkeling van het kind begon.|trans-quote=Is the early childhood development process gradual or in leaps? This question has preoccupied researchers since the beginning of this century, when the systematic study of child development began. |doi=10.1007/BF03070912}}</ref><ref>{{Cite book |last1=Kagan |first1=Jerome |last2=Herschkowitz |first2=Norbert |url=https://www.google.com/books/edition/A_Young_Mind_in_a_Growing_Brain/pIB5AgAAQBAJ?hl=en&gbpv=1&dq=Kagan+J+Herschkowitz+N+A+young+mind+in+a+growing+brain+2005+Mahwah,+NJ+Erlbaum+Associates+&pg=PP1&printsec=frontcover |title=A Young Mind in a Growing Brain |date=2014-01-01 |publisher=Psychology Press |isbn= |editor-last= |editor-first= |place=New York |page=xi |chapter=Preface |quote=The argument that a select set of human psychological properties cannot appear until certain maturational events have occurred bothers a number of American and European social scientists. We understand their resistance to the suggestion that there are natural restraints on the time of emergence of some human competences. Western society values freedom of action and the continual contribution of experience to biological structures. }}</ref> "The idea that there is a link between nervous system maturation and developmental milestones or transitions, such as grasping, crawling or walking for the first time, has proven untenable. However, a temporal link can be established between brain changes and regression periods.<ref name=Plooij1998a>{{cite journal |last=Plooij |first=F. X. |date=1998 |title=Hersenveranderingen en 'sprongen' in de eerste 20 levensmaanden en de invloed van de context op gedragsmaten van regressieperioden |trans-title=Brain changes and 'leaps' in the first 20 months of life and the influence of context on behavioral measures of regression periods |url=https://www.cambridge.org/core/journals/acta-neuropsychiatrica/issue/B6231D25939BE324F6EA457610FBEAAC |journal=Acta Neuropsychiatrica |volume=10 |issue=3 | pages=63-66 |doi=10.1017/S0924270800036590}};|quote= Het idee dat er een koppeling te leggen valt tussen rijping van het zenuwstelsel en ontwikkelings-mijlpalen of -transities, zoals voor het eerst grijpen, kruipen of lopen, is onhoudbaar gebleken. Wel valt er een koppeling in de tijd te leggen tussen hersenveranderingen en regressieperioden |trans-quote= </ref>
+Since the beginning of the 20th century developmental psychologists have investigated whether the early childhood development process is gradual or punctuated by distinct biologically determined advances.<ref>{{cite journal |last=Kalverboer |first=L. |date=1998 |title=Ontwikkelingssprongen in het duister: Over transities in de ontwikkeling |trans-title=Developmental leaps in the dark: On transitions in development |url=https://mijn.bsl.nl/ontwikkelingssprongen-in-het-duister/448212 |journal=Neuropraxis|volume=1 |issue= | pages= |quote=Verloopt het vroegkinderlijke ontwikkelingsproces geleidelijk of sprongsgewijs? Deze vraag houdt onderzoekers bezig sinds het begin van deze eeuw, toen de systematische studie van de ontwikkeling van het kind begon.|trans-quote=Is the early childhood development process gradual or in leaps? This question has preoccupied researchers since the beginning of this century, when the systematic study of child development began. |doi=10.1007/BF03070912}}</ref><ref>{{Cite book |last1=Kagan |first1=Jerome |last2=Herschkowitz |first2=Norbert |url=https://www.google.com/books/edition/A_Young_Mind_in_a_Growing_Brain/pIB5AgAAQBAJ?hl=en&gbpv=1&dq=Kagan+J+Herschkowitz+N+A+young+mind+in+a+growing+brain+2005+Mahwah,+NJ+Erlbaum+Associates+&pg=PP1&printsec=frontcover |title=A Young Mind in a Growing Brain |date=2014-01-01 |publisher=Psychology Press |isbn= |editor-last= |editor-first= |place=New York |page=xi |chapter=Preface |quote=The argument that a select set of human psychological properties cannot appear until certain maturational events have occurred bothers a number of American and European social scientists. We understand their resistance to the suggestion that there are natural restraints on the time of emergence of some human competences. Western society values freedom of action and the continual contribution of experience to biological structures. }}</ref> The relation between  developmental milestones or transitions, such as grasping, crawling or walking for the first time, has proven untenable.<ref name=Plooij1998a>{{cite journal |last=Plooij |first=F. X. |date=1998 |title=Hersenveranderingen en 'sprongen' in de eerste 20 levensmaanden en de invloed van de context op gedragsmaten van regressieperioden |trans-title=Brain changes and 'leaps' in the first 20 months of life and the influence of context on behavioral measures of regression periods |url=https://www.cambridge.org/core/journals/acta-neuropsychiatrica/issue/B6231D25939BE324F6EA457610FBEAAC |journal=Acta Neuropsychiatrica |volume=10 |issue=3 | pages=63-66 |doi=10.1017/S0924270800036590}};|quote= Het idee dat er een koppeling te leggen valt tussen rijping van het zenuwstelsel en ontwikkelings-mijlpalen of -transities, zoals voor het eerst grijpen, kruipen of lopen, is onhoudbaar gebleken. Wel valt er een koppeling in de tijd te leggen tussen hersenveranderingen en regressieperioden |trans-quote= The idea that there is a link between nervous system maturation and developmental milestones or transitions, such as grasping, crawling or walking for the first time, has proven untenable. However, a temporal link can be established between brain changes and regression periods &hellip;.</ref>
-Evidence for regression periods correlated with punctuated neurological developments followed by periods of equilibrium came from ethologists studying the interactions of mothers and infants in primates and many other species,<ref name="Horwich74">{{cite journal |last1=Horwich |first1=Robert H. |date=1974 |title="Regressive Periods in Primate Behavioral Development with Reference to Other Mammals" |url= https://www.researchgate.net/publication/226450864 |journal=Primates |volume=15 |issue=2–3 |pages=141–149 |doi= 10.1007/BF01742277|s2cid=6922407 |access-date=2024-02-26}}</ref> suggesting an early origin in evolution of species.<ref>"Before our research, such regression phases had been found by others in 12 other primate species and two lower mammalian species, indicating that this appears to be an old phenomenon, perhaps emerging during the very evolution of life on earth" ("Introduction" (by F.X. Plooij), ''ibid.'' p.&nbsp;15).</ref> The Plooijs brought new ethological evidence for intrinsically timed stages of human infant development into this old controversy. A technical report of the Plooijs' first controlled replication of these ethological findings in humans, involving 15 Dutch mothers and their infants with extrinsic sources of stress carefully controlled, was published in the ''Journal of Reproductive and Infant Psychology'' in 1992,<ref name="Plooijs1992">{{Cite journal |last1=Van De Rijt-Plooij |first1=Hedwig H.C. |last2=Plooij |first2=Frans X. |date=July 1992 |title=Infantile regressions: Disorganization and the onset of transition periods |journal=Journal of Reproductive and Infant Psychology |language=en |volume=10 |issue=3 |pages=129–149 |doi=10.1080/02646839208403946 |issn=0264-6838}}</ref>  the same year as the first publication of this book for the general public.<ref>''Oei, ik groei!'' (literal translation: Ai, I'm growing!) was published in 1992 by Zomer & Keuning Boeken BV, Ede and Antwerp.</ref> These behavioral observations have been correlated with stages of neurological development of the brain.<ref name="Plooij-ABS">{{cite journal | last1 =Plooij | first1 =F. X. | last2  = Rijt-Plooij | first2 = H. H. C. van de | title = Developmental transitions as successive reorganizations of a control hierarchy | journal = American Behavioral Scientist | volume = 34 | issue = 1 | pages =67–80 | date = 1990 | language = English | url = | jstor = | issn = | doi = 10.1177/0002764290034001007 | s2cid =144183592 }}</ref>
+Evidence for regression periods correlated with punctuated neurological developments followed by periods of equilibrium came from ethologists studying the interactions of mothers and infants in primates and many other species,<ref name="Horwich74">{{cite journal |last1=Horwich |first1=Robert H. |date=1974 |title="Regressive Periods in Primate Behavioral Development with Reference to Other Mammals" |url= https://www.researchgate.net/publication/226450864 |journal=Primates |volume=15 |issue=2–3 |pages=141–149 |doi= 10.1007/BF01742277|s2cid=6922407 |access-date=2024-02-26}}</ref> suggesting an early origin in evolution of species.<ref>"Before our research, such regression phases had been found by others in 12 other primate species and two lower mammalian species, indicating that this appears to be an old phenomenon, perhaps emerging during the very evolution of life on earth" ("Introduction" (by F.X. Plooij), ''ibid.'' p.&nbsp;15).</ref>
+Into this old controversy the Plooijs brought ethological evidence for intrinsically timed stages of human as well as primate infant development. Their first human study involving 15 Dutch mothers and their infants, with extrinsic sources of stress carefully controlled, was published in the ''Journal of Reproductive and Infant Psychology'' in 1992,<ref name="Plooijs1992">{{Cite journal |last1=Van De Rijt-Plooij |first1=Hedwig H.C. |last2=Plooij |first2=Frans X. |date=July 1992 |title=Infantile regressions: Disorganization and the onset of transition periods |journal=Journal of Reproductive and Infant Psychology |language=en |volume=10 |issue=3 |pages=129–149 |doi=10.1080/02646839208403946 |issn=0264-6838}}</ref>  the same year as the first publication of this book for the general public.<ref>''Oei, ik groei!'' (literal translation: Ai, I'm growing!) was published in 1992 by Zomer & Keuning Boeken BV, Ede and Antwerp.</ref> These behavioral observations have been correlated with stages of neurological development of the brain.<ref name="Plooij-ABS">{{cite journal | last1 =Plooij | first1 =F. X. | last2  = Rijt-Plooij | first2 = H. H. C. van de | title = Developmental transitions as successive reorganizations of a control hierarchy | journal = American Behavioral Scientist | volume = 34 | issue = 1 | pages =67–80 | date = 1990 | language = English | url = | jstor = | issn = | doi = 10.1177/0002764290034001007 | s2cid =144183592 }}</ref>
 <ref name="Trevarthen-Aitken03">{{Cite book |last1=Trevarthen|first1=C. |last2=Aitken|first2=K. |url=https://www.google.com/books/edition/Regression_Periods_in_Human_Infancy/mSZ5AgAAQBAJ?hl=en&gbpv=1&dq=regression+periods+in+human+infancy&printsec=frontcover |title=Regression periods in human infancy |date=2003 |publisher=Erlbaum |isbn=0-8058-4098-2 |editor-last=Heimann |editor-first=Mikael |place=Mahwah, NJ |pages=107-184 |chapter=Regulation of Brain Development and Age-Related Changes in Infants’ Motives: The Developmental Function of Regressive Periods}}</ref>
 Plooij obtained state funding to oversee an independent replication. With this funding he obtained a one day a week position as a "Professor by special appointment" (Dutch: bijzonder hoogleraar) at his alma mater, the [[University of Groningen]].<ref>{{cite news |last=Kreulen |first=Edwin |date=2024-08-18 |title=En-plooij, hij springt verder |trans-title=And Plooij, he jumps further |url=https://www.trouw.nl/home/en-plooij-hij-springt-verder~bef41ca1/ |url-status= |language= |issue= |publisher= |work=Trouw |location= |archive-url= |archive-date= |access-date=2024-02-25 |via= |quote=Plooij, appointed as a special professor at the University of Groningen for one day a week, hired a PhD student who should substantiate the theory.}}</ref> He engaged Carolina de Weerth, a PhD student of Paul van Geert.<ref>{{Cite news |last=Kok |first=Annemarie |title='Oei, ik groei!' leidt tot bittere ruzie van wetenschappers |trans-title='Ouch, I'm growing!' leads to bitter arguments among scientists |language=nl |website=Trouw |date=1997-11-19 |url=https://www.trouw.nl/home/-oei-ik-groei-leidt-tot-bittere-ruzie-van-wetenschappers~a361efcd/ |url-status= |access-date=2024-02-26 |archive-url= |archive-date= |quote=Volgens hoogleraar P. van Geert, die onderzoekster De Weerth de laatste jaren heeft begeleid, is het boek enerzijds gebaseerd op bestaande, betrouwbare ontwikkelingspsychologische kennis. |trans-quote=According to Professor P. van Geert, who has guided researcher De Weerth in recent years, the book is based on existing, reliable developmental psychological knowledge.}}</ref> At the conclusion of her research, she reported that she had found many periods of greater fussiness and higher [[cortisol]] levels other than at the predicted times.<ref name=":0">{{Cite journal |last1=de Weerth |first1=C. |last2=van Geert |first2=P. |date=1998-03-01 |title=Emotional instability as an indicator of strictly timed infantile developmental transitions |journal=British Journal of Developmental Psychology |volume=16 |issue=1 |pages=15–44 |doi=10.1111/j.2044-835X.1998.tb00748.x |issn=2044-835X}}</ref><ref name="trouw" /> Plooij said that this was because three of the four mothers that she had recruited did not meet the stated criteria for a replication because their circumstances and behavior introduced extrinsic sources of stress for the infant which made it difficult to discern distress specifically at times of new developments.<ref name=Plooij1998a/><ref name=Plooij1998b/><ref name=Plooij-plooij-2003>{{Citation | last1  = Plooij | first1 = F. X. | last2  = Rijt-Plooij | first2 = H. H. C. van de | author-link = | chapter = The effects of sources of "noise" on direct observation measures of regression periods: Case studies of four infants' adaptations to special parental conditions. | editor-last  = Heimann | editor-first = M. | title  = Regression periods in human infancy | url=https://www.google.com/books/edition/Regression_Periods_in_Human_Infancy/mSZ5AgAAQBAJ?hl=en&gbpv=1&dq=regression+periods+in+human+infancy&printsec=frontcover | year = 2003 | pages   = 57–80 | volume = | publisher = Erlbaum | place=Mahwah, NJ | publication-date = 2003 | isbn = 978-0805840988 }}</ref><ref name=Woolmore-Richer-2003>{{Cite book | last1=Woolmore | first1 = A. | last2  = Richer | first2 = J. | chapter = Detecting infant regression periods: weak signals in a noisy environment | editor-last  = Heimann | editor-first = M. | title  = Regression periods in human infancy | url=https://www.google.com/books/edition/Regression_Periods_in_Human_Infancy/mSZ5AgAAQBAJ?hl=en&gbpv=1&dq=regression+periods+in+human+infancy&printsec=frontcover | pages = 23–39 | publisher = Erlbaum | place=Mahwah, NJ | date = 2003 | isbn = 978-0805840988 }}</ref>