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.
[3] 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.
[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 1971-1973 longitudinal 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, the correlation of illnesses with these, and the importance of the mother's interactions for the baby's growing independence and learning.
[25] One peer-reviewed study included verification of parents' reports that babies master a cluster of new skills after each regression period.
[26] Independent replication studies were carried out at universities in four countries, Groningen in the Netherlands,[27] Oxford in England,[28] Girona in Spain,[29][30] and Gothenburg in Sweden.