This study aimed to show secular changes in the cephalic index (CI) in Japan when brachycephalization was reported to have ceased, and examined possible causes of the observed changes. Head length and breadth data measured on 4034 Japanese adults, born between 1910 and 1996, were used to examine the association between birth year and CI using linear and quadratic regression analyses. Possible causes of secular changes in CI were investigated by examining the secular changes in eight body-size variables and 16 environmental indicators, obtained from the government statistics records. Results showed that a trend reversal from brachycephalization to debrachycephalization occurred among the generation born in 1960–1964, followed by trend reversals in birth length (BL), energy intake, and birth weight (BW) in that order. Previous studies showed that the reverse trend in BW was related to a reduction in fetal growth rate, which resulted from worsening fetal nutritional status, which was also attributable to decreasing maternal energy intake. Since the head measurement at birth is approximately 60% of the adult size, the main cause of debrachycephalization may be worsening fetal nutritional status. From the maternal nutritional perspective, reverse trends in CI and BL began with rapid recovery from the post-war food shortage. However, the body mass index (BMI) of 20-year-old females decreased due to the very small increase in weight despite a rapid increase in height, which was partly achieved by weight loss after the age of 17. As speculated, debrachycephalization began when the negative effects of decreasing BMI among the young adult females outweighed the positive effects of the post-war recovery of the food situation. The results of this study may help to understand the plasticity of the human body.
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