Scientific History The Wonder Weeks

Scientific History The Wonder Weeks

Frans Plooij

Franciscus Xaverius (Frans) Plooij (Schiedam – Netherlands, 1946) is a Dutch behavioural biologist and former professor at the University of Groningen who became well known for his book The Wonder Weeks (translated from the original Dutch book Oei, ik groei!) written together with his wife Hetty van de Rijt.

Dr. Hetty van der Rijt tijdens het observeerden van chimpansees in Tanzania

Plooij studied biology and psychology at the universities of Nijmegen and Amsterdam [9]. His wife Hetty van de Rijt studied educational psychology and anthropology at the universities of Nijmegen and Cambridge. In Tanzania, they observed chimpanzees in the Gombe National Park with Jane Goodall (1971-1973). They processed the resulting data at Cambridge University (1973-1976) with Robert Hinde. Plooij then became a researcher at the department of developmental psychology of the University of Nijmegen (1976-1980) were he filmed and analysed mother-baby interactions in home settings [11]. Plooij obtained his PhD in the behavioural development of chimpanzee babies [14] at the University of Groningen in 1980 and his wife Hetty van de Rijt obtained her PhD in mother-baby interaction in chimpanzees [58] at the University of Cambridge.

Plooij went on to become head of the department for Research & Development of the Gemeentelijk Pedologisch Insitituut (Municipal Paedological Institute) of Amsterdam (1981-1993), where he ran various projects [6, 18, 24, 27, 33, 45, 46, 51, 52, 66, 67] and, among others, participated in the international SOCRATES-LINGUA project of the European Union, which dealt with the implementation and evaluation of an innovative teaching method for second and foreign languages for Kindergarten children [55].

Meanwhile, he and his wife published their findings on their chimpanzee work [1, 8, 10, 12, 13, 15, 16, 19, 23, 26, 34, 35, 37, 38, 56, 57, 59, 60, 61, 62]. An important discovery was the occurrence of ‘regression periods’, during which chimpanzee babies cling more to their mothers, went on the nipple more often and whimpered more. Similar, age-linked regression periods in early development had already been reported by Robert Horwich [4] on 12 different primate species and two other mammal species. This is clearly a phenomenon that must have originated at least 70 million years ago during the evolution of life on earth.

Strengthened by the findings of colleague scientist Robert Horwich from 1974, Plooij and Van de Rijt realized that these regression periods were likely to occur in humans as well. This prompted their research on human mother-infant interactions. They found 10 regression periods during the first 20 months of life and published their findings in 1992 [63]. During the second year of life they also found periods of conflict between the mothers and their babies, which followed shortly after the regression periods [64]. Meanwhile, Horwich independently reported regression periods in three orangutan infants, two baby gorillas, one chimpanzee infant and four human babies [5].

Plooij and Van de Rijt further discovered that shortly after a regression period babies became ill with surprising frequency. In 98 babies that were enrolled years later in the school for special education that was connected to the Gemeentelijk Pedologisch Instituut in Amsterdam they counted 140 illnesses and found that the start of these illnesses was not evenly distributed over age. They later replicated this retrospective data in a prospective, longitudinal study on babies, which showed peaks in illness shortly following regression periods [36].

In a review of the literature on the pre- and postnatal development of the central nervous system, Trevarthen and Aitken [53] showed that Periods of Rapid Change (PRC) occur in the central nervous system, whereby the earliest start of such PRCs coincides with the regression periods. Apparently, a baby is more clingy and fussy during a regression period because of drastic changes in the brain. Later, in cooperation with Spanish colleagues, Plooij showed that babies master a cluster of new skills after each regression period [49].

Perceptual Control Theory (PCT)

Plooij found that the Perceptual Control Theory (PCT) of Bill Powers [47, 48,] could explain the results of his research on the regression periods in babies. This theory is a functional model of the nervous system in which the control of the input or perception plays a crucial role and not the control of behaviour. Behaviour is the control of perception and not the other way around. Through a closed, negative feedback control loop the perception is kept at a certain target value. The PCT model contains a hierarchy of 10 different types of perception ranging from simple perceptions at the base of the hierarchy to complex perceptions at the top. Plooij discovered that this hierarchy is built up one level after the other during the sensorimotor period. At the start of each regression period a new level of perception is superimposed onto the already existing hierarchy of levels of perception [31, 37].

Based on these findings, Plooij and Van de Rijt developed the parental support and education program ‘Leaping Hurdles’ for the first 18 months of life. The program was implemented and evaluated [65]. In the evaluation study, an experimental group was compared with a control group. It turned out that ‘leaping hurdle’ babies scored much better mentally, socially and in terms of health (only the girls) than their peers from the control group. There were also a number of positive effects on the ‘leaping hurdle’ parents. It is one of the few evidence-based programs of its kind.

From 1993 until 1998, Plooij was professor by special appointment at the department of developmental and experimental clinical psychology at the University of Groningen, a chair that was set by the Foundation for Research into Psychosocial Stress (SOPS). Plooij was vice-president for information of the International Society for Human Ethology from 1989-1993, vice-president of the Institute Européen pout le Development de tous les Enfants (IEDPE), and a member of the editorial board of the scientific journal Ethology and Sociobiology. He was also a member of the advisory board of the Journal of Clinical Child Psychology and Psychiatry and a member of various international scientific societies in the field of child development and behavioural biology and of the New York Academy of Sciences.

During the nineties, Plooij and his wife founded a consultancy firm working for companies producing products for babies. They had a long working relationship with LEGO where they provided in-company training and tested toy prototypes. They were closely involved in the development of Lego Primo.

The Wonder Weeks (translated from the original Dutch book Oei, ik groei!)

Plooij is primarily known for the book The Wonder Weeks (Oei, ik groei!) that he wrote together with his wife, and in which he describes the regression periods as ‘leaps’. A leap includes a phase in which a baby goes through a regression period, characterized by clinginess, crying and crankiness. This phase is directly followed by a ‘leap’ forward in mental development.


The Dutch book Oei, ik groei! was published in 1992. It described age-linked leaps in the development of babies. The book quickly became popular with parents. In 1998, Plooij’s PhD student, C de Weerth, published a follow-up study on four babies in which Plooij’s findings were only confirmed for one of the four babies [28, 30]. Shortly thereafter, three replication studies followed in Sweden [7], Spain [50] and Great Britain [68], confirming Plooij’s findings. In 2003 these studies were published together in a book with a number of other studies on the topic [2, 3].

Plooij authored several very popular parenting books. The Wonder Weeks (Oei, ik groei!) has been translated in over 20 languages and is available worldwide. Plooij and his wife have a daughter, Xaviera, and three grandchildren.



Sources, notes and/or references

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  7. Lindahl, L., Heimann, M., & Ullstadius, E. (2003). Occurrence of regressive periods in the normal development of Swedish infants. In M. Heimann (Ed.), Regression Periods in Human Infancy (pp. 41-55). Mahwah, NJ: Erlbaum.
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  33. Plooij, F. X., Traudes, M. A., & Van Zijp, A. J. S. M. (1985). Directe observaties van opvoeder-kind interactie tijdens de afname van een pedodiagnosticum voor kleuters. In F. X. Plooij & M. G. M. Van den Dungen (Eds.), Hulpverleningspraktijk en dienstverlenend onderzoek. Handelingsplannen en directie observatie van opvoeder-kind interactie (pp. 81-103). Lisse: Swets & Zeitlinger.
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  35. Plooij, F. X., & Van de Rijt-Plooij, H. (1989a). Evolution of human parenting: Canalization, new types of learning, and mother-infant conflict. European Journal of Psychology of Education, 4, 177-192.
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  40. Plooij, F. X., & Van de Rijt-Plooij, H. (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 M. Heimann (Ed.), Regression Periods in Human Infancy (pp. 57-80). Mahwah, NJ: Erlbaum.
  41. Plooij, F. X., Van de Rijt-Plooij, H., Fischer, M., & Pusey, A. (2014). Longitudinal recordings of the vocalizations of immature Gombe chimpanzees for developmental studies. Scientific Data, 1, 1-10. doi:10.1038/sdata.2014.25
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