The Wonder Weeks

According to the book, babies go through 10 predictable 'leaps' in their cognitive development during the first 20 months of a baby's life.[3] These leaps correspond to the emergence of levels in the perceptual hierarchy as the baby's brain grows and develops new capabilities.[15] A leap consist of two phases: A phase where the baby 'regresses' as though to a younger age, seeking secure closeness with the mother and being generally 'cranky', due to cognitive disruption of previously gained competencies, followed by a period where the baby is generally happy and outgoing, exploring new experiences with the newly gained cognitive skills. The leaps are predicted to occur at approximately 5, 8, 12, 19, 26, 37, 46, 55, 64 and 75 weeks old.[3][15] (Weeks are counted from the due date if the child is premature.)

This book derives from almost five decades of research, beginning at first with free-living chimpanzees in cooperation with Jane Goodall in 1971,[16][17] and then extending to humans. The existence of regular 'regression periods' and their correlation with stages of cognitive development are well documented in the literature of ethology[18] and child psychology,[19][20][12] and the findings underlying the book have been replicated in different countries and cultures by several independent research groups.[12][13][8][11] Further corroboration comes from independent lines of investigation into periodicity of early childhood illness[21] and peaks in the ages of infant mortality attributed to SIDS.[22] Beyond the scope of the book, additional regression periods have been documented, even into the teen years.[3] These findings conform to and help to confirm the comprehensive theoretical and research framework of Perceptual Control Theory (PCT).

Other conditions besides the challenges of controlling experiences within a new level of perception and cognition may cause insecurity and frustration in a child's life, "circumstances such as chaos, travel, moving house" and the like. The research underlying this book carefully controlled for such extraneous disturbances. Healthy, stable families were selected because the aim was to "understand normal development before venturing into understanding pathology".[10] When there are "special parental conditions, such as rigid schedule care or depression and phobias, ... [d]ips instead of peaks ... have been found at the regression periods. Peaks in crying could be found only in the first few months ... because the crying is practically nonexistent thereafter ... [and] under these special circumstances ... smiling appears to take over from crying as a means to try and get mother's attention."[10] Such difficulties and their effects on the developing child are serious matters for further research, but are not within the scope of this book.

Carolina de Weerth, a PhD student investigating Plooij's work under direction of Prof. P. van Geert,[4] tabulated behavior and tested cortisol levels in saliva and failed to find a clear pattern of greater fussiness or higher cortisol levels corresponding to the leaps.[7] Plooij tried to convince her that her conclusions were mistaken,[6] and objected to publication[4] of a paper co-authored with her supervisor, Prof. van Geert,[7] who told journalists that Plooij's behavior had been "very indecent" and that the claims made by Plooij's book contradicted the greater body of research on child development.[4] The controversy led to Plooij's departure from the University of Groningen.[4][5][6] "However, an increasing number of researchers are providing empirical support for [Plooij's] findings",[23] with multiple replications in diverse cultures and family circumstances.[19][20][11][12][13][8] Critical review by other researchers of van Geert's methodology and data analysis has suggested that "studies that fail to find regression periods must be carefully scrutinized to ensure they have not simply failed to see through the 'noise' in their data, such that they have mistaken their own failure to find the phenomenon for the phenomenon's nonexistence."[8] Re-analysis of the de Weert-van Geert data has shown that they actually confirm age-linked regression periods when extraneous challenges due to "special parental conditions" in three of the four mother-infant dyads in their study are taken into account (maternal depression and phobias, rigid schedules imposed for sleeping, contact, and feeding).[10][8] Subsequent research involving de Weerth concerns association of parental stress with childhood pathologies.[24][25][26]