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The Wonder Weeks

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The Wonder Weeks: How to Stimulate Your Baby's Mental Development and Help Him Turn His 10 Predictable, Great, Fussy Phases Into Magical Leaps Forward is a fortune-telling pseudoscientific best-selling book and mobile app masquerading as developmental psychology.[1] It was written by former professor Frans Plooij, and originally published in 1992 in Dutch.[2] It has been republished several times, in several different languages.[3] In the book Plooij claims that that the cognitive development of babies occurs in predictable jumps and that these "leaps" will indicate when the baby will be especially fussy.[4] However, a follow-up study by Plooij's PhD student, Carolina de Weerth, failed to find any evidence of predictable leaps, which is unsurprising because the idea that you can predict when babies will be fussy from week-to-week is completely ludicrous.[5]


According to the book, a baby should go through 10 predictable jumps or "leaps" in its cognitive development during the two years, with 8 in just the first year, counted from the due date if the child was premature.[6] These jumps consist of two phases; a week or so where the baby is generally unhappy, followed by a week or so where the baby is generally happy, due to discovering new things with the newly gained cognitive skills. The "leaps" are predicted to occur at 5, 8, 12, 19, 26, 37, 46, 55, 64 and 75 weeks old.[3] This is, a priori, suspicious that these cognitive leaps would be this predictable, because infant development is highly variable. For instance, the range of normal development for learning to walk ranges from 9 months to 18 months. The narrowest windows of cognitive development that are currently accepted in the literature typically span months.[7]. And these ranges, of course, refer to normal development, and developmentally delayed children may reach these milestones late, if at all.


The book is popular among parents who visit online forums such as baby center and mumsnet and in Facebook groups. Many parents believe that the book is "real" because of confirmation bias; because the windows of fussiness and good behaviour are close together, it's easy for parents to convince themselves (or others) that if their baby isn't fussy they're simply entering the "leap" early or late, and because infants are commonly fussy, nearly every baby will show some fussiness during these "leaps".

The appeal of the book is understandable; most people want to their baby to be happy, but babies aren't always consolable (see: colic). This book provides a pleasant explanation for this fussiness (they baby is simply becoming smarter! That's great!) and relieves the pressure on the parent to attempt to find some other explanation for the fussiness. It's human nature to want to explain phenomena, and explaining fussiness in an baby is no exception.

Reality sets in[edit]

A follow-up study by Plooij's PhD student, Carolina de Weerth, unsurprisingly completely failed to find any evidence of greater fussiness or higher cortisol levels corresponding to these supposed leaps.[5][4] This pissed off Plooij, who actually tried to prevent the British Journal of Developmental Psychology from publishing the study.[4] It didn't work; the study was published in 1998, and he was disgraced; his behaviour was so egregious that a coworker described his behaviour as "very indecent" to the press, and who also pointed out that his work contradicted the greater body of research on child development.[8] Plooij was fired in 1997 and thereafter left academia.[4][9][10] Carolina de Weerth's study was published in 1998.

At present, Carolina de Weerth is a full professor in her field, doing actual science and earning a modest government salary whilst running her own lab at Radboud University
 in the Netherlands,[11] whilst Plooij sits on his ass passively raking in the dough from book and app sales to literally millions of fleeced sleep-deprived parents despite the fact his work was debunked nearly 20 years ago.

External links[edit]


  1. "The Wonder Weeks App" (in en-US). The Wonder Weeks. 
  2. Oei, ik groei! De acht sprongen in de mentale ontwikkeling van je baby by Frans Plooij (1992). Kosmos Uitgevers. ISBN 9021522209.
  3. 3.0 3.1 Plooij, Frans X.; Rijt, Hetty van de (September 2017) (in en). The Wonder Weeks: How to Stimulate Your Baby's Mental Development and Help Him Turn His 10 Predictable, Great, Fussy Phases Into Magical Leaps Forward. Kiddy World Publishing. ISBN 9789491882166. 
  4. 4.0 4.1 4.2 4.3 Kok, Annemarie (1997-11-19). "'Oei, ik groei!' leidt tot bittere ruzie van wetenschappers". Trouw. Retrieved 14 June 2015. 
  5. 5.0 5.1 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. ISSN 2044-835X. 
  6. Plooij, Frans. "Groeisprongen". Retrieved 14 June 2015. 
  7. Speech and Language Developmental Milestones
  8. "'Oei, ik groei!' leidt tot bittere ruzie van wetenschappers" (in nl-NL). ""zeer onfatsoenlijk"" 
  9. Dirks, Bart (January 13, 1997). "Hoogleraar woedend over weerlegging theorie". Algemeen Dagblad (AD). Retrieved 14 June 2015. 
  10. Dirks, Bart (1998-01-14). "Positie Plooij onhoudbaar na openlijke kritiek op oud-promovenda; Auteur 'Oei, ik groei' ontslagen als hoogleraar". Volkskrant. Retrieved 14 June 2015. 
  11. Carolina De Weerth