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The phylogeny, ontogeny, causation and function of regression periods explained by reorganizations of the hierarchy of perceptual control systems FX Plooij - The interdisciplinary handbook of perceptual control …, 2020

The Wonder Weeks: A Stress-Free Guide to Your Baby's Behavior is a book with advice to parents about child development by physical anthropologist Hetty van de Rijt and ethologist and developmental psychologist Frans Plooij. Their daughter Xaviera Plas-Plooij is a third author of recent editions. It was first published in English in 2003 as the translation of the 1992 Dutch book Oei, ik groei! The book claims that the cognitive development of babies occurs in predictably timed stages. Ever since the systematic study of child development began at the beginning of the 20th century researchers have disagreed whether this is gradual or in punctuated stages. Some figures in the child development field have objected that sleep regressions are not so predictable. A chapter on sleep was added to the 6th edition in 2019. The publisher has produced a mobile app based on the book.

Claims
The book describes 10 predictable 'leaps' observed in a child's cognitive development during the first 20 months, with 8 in the first year. Months are counted from the due date because development begins with conception. These developmental 'leaps' are said to begin with the baby becoming more insecure, clinging, and cranky, followed by a longer period in which the baby is more happy and learning new skills. They are predicted to occur at about 5, 8, 12, 17, 26, 36, 44, 53, 61-62 and 72-73 weeks old.

Scientific Basis
Ethologists have documented predictable 'regression periods' in the interactions of mothers and infants in many species, suggesting an early origin in evolution. In the course of a longitudinal (1971-1973) ethological study of chimpanzees in the wild, working with Jane Goodall, van de Rijt and Plooij published additional data demonstrating predictable regression periods in Chimpanzee mother-infant dyads, their correlation to illness, and the importance of the mother's interactions for the baby's growing independence and learning. They hypothesized a new type of learning important in the evolution of human parenting, with reference to an explanation in control theory. To test this hypothesis, they applied the observational methodology of ethology to humans. 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.

Research into development of the central nervous system has identified periods of rapid change (PRC) which coincide with the observed regression periods, and known stages of neurological development of the brain have been correlated with the behavioral observations. Consistent with the hypothesis of increased stress, a correlation between regression periods and upticks of illness has been reported, and a correlation with SIDS. One peer-reviewed study included verification of parents' reports that babies master a cluster of new skills after each regression period. Independent replication studies were carried out at universities in four countries, Groningen in the Netherlands, Oxford in England, Girona in Spain, and Gothenburg in Sweden. The failure of the first replication study was a subject of controversy.

Controversy
With state funding for an independent replication of the research with human mothers and infants, Frans Plooij obtained a temporary research position at the University of Groningen, where he had earned his degrees, and engaged a PhD student, Carolina de Weerth. She observed behavior of four infants and tested their cortisol levels as a measure of stress, and failed to find any evidence of greater fussiness or higher cortisol levels corresponding to the leaps. She later suggested that this might be due to inadequate sample size. Plooij objected that this was not a replication because the relevant data were obscured by extrinsic sources of stress which she failed to control. De Weerth and her dissertation supervisor, Paul van Geert, said that Frans Plooij tried to pressure her into not publishing the study; Plooij has disputed this account. Plooij resigned his research affiliation with his alma mater. De Werth's dissertation was published with van Geert as the lead author.

At a conference in 1997 (and published in 2003), Plooij and van de Rijt-Plooij reported that when extrinsic sources of stress which de Werth had identified are factored out from her data the regression periods are evident. The following year, 1998, the scientific controversy was made the focal topic of an issue of the journal Neuropsychiatrica with an article by Plooij, a rejoinder by van Geert and de Weert, and a reply by Plooij. Plooij has continued his research under other auspices.