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. Xaviera Plas-Plooij is a third author of recent editions. It was first published in English in 2003^[1] as the translation of the 1992 Dutch book Oei, ik groei!^[2] The book claims that the cognitive development of babies occurs in predictably timed stages.^[3] This has long been a controversy in developmental psychology.^[4] Experts in child development have objected that sleep regressions are not so predictable.^[5] A chapter on sleep was added to the 6th edition in 2019.^[6][3] The publisher has produced a mobile app based on the book.^[7]

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.^[8] These developmental 'leaps' consist of two phases, A phase where the baby is generally unhappy, followed by a period where the baby is generally happy, due to discovering new things with the newly gained cognitive skills. They are predicted to occur at 5, 8, 12, 17, 26, 36, 44, 53, 61-62 and 72-73 weeks old.^[9]

Ethologists have documented predictable regression periods in the interactions of mothers and infants in many species, suggesting an early origin in evolution.^[10][11][12] In the course of a longitudinal (1971-1973) ethological study of chimpanzees in the wild, working with Jane Goodall,^[13] van de Rijt and Plooij published additional data demonstrating predictable regression periods in Chimpanzee mother-infant dyads.^[14][15][16] They hypothesized a new type of learning important in the evolution of human parenting, with reference to an explanation in control theory.^[17] They then applied the observational methods 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.^[18][19]

These behavioral observations have been correlated with stages of neurological development of the brain,^[20] and research into development of the central nervous system has shown that the periods of rapid change (PRC) begin at the same times as the observed regression periods.^[21] Consistent with increased stress, there is an uptick of illness during a regression period,^[22] and a correlation with SIDS.^[23] Parents' informal observations that babies master a cluster of new skills after each regression period has been verified.^[24]

With state funding for an independent replication study, 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 tested both behavior and cortisol levels in four infants, and failed to find any evidence of greater fussiness or higher cortisol levels corresponding to the leaps.^[25][26] She later suggested that this might be due to inadequate sample size.^[5] Plooij and other researchers objected that this was not a replication because the relevant data were obscured by extrinsic sources of stress which she failed to control.^[27][28] Plooij and van de Rijt-Plooij reanalyzed de Werth's data and demonstrated that the phenomenon can be seen when the effects of identified extrinsic sources of stress are factored out.^[29] According de Weerth, Frans Plooij tried to pressure her into not publishing the study.^[26][30][5] Plooij disputes this account.^[5] Plooij resigned his research affiliation with the university.^[26][31][32]