Yasuyuki Yamada1, Takeshi Ebara2, Chikae Yamaguchi3, Taishi Miyachi4, Naoto Shoji5, Taro Matsuki6, Hirohisa Kano6, Masataka Hirosawa7, Michihiro Kamijima6. 1. Juntendo University, Graduate School of Health and Sports Science, Inzai, Chiba 2701695, Japan; Nagoya City University, Department of Occupational and Environmental Health, Graduate School of Medical Sciences, Mizuho-ku, Nagoya, Aichi 4678601, Japan. 2. Nagoya City University, Department of Occupational and Environmental Health, Graduate School of Medical Sciences, Mizuho-ku, Nagoya, Aichi 4678601, Japan. Electronic address: ebara@med.nagoya-cu.ac.jp. 3. Nagoya City University, Graduate School of Nursing, Mizuho-ku, Nagoya, Aichi 4678601, Japan. 4. Nagoya City University, Department of Pediatrics and Neonatology, Graduate School of Medical Sciences, Mizuho-ku, Nagoya, Aichi 4678601, Japan. 5. Nagoya City University, Department of Occupational and Environmental Health, Graduate School of Medical Sciences, Mizuho-ku, Nagoya, Aichi 4678601, Japan; Asahi University, School of Health Sciences, Mizuho, Gifu 5010296, Japan. 6. Nagoya City University, Department of Occupational and Environmental Health, Graduate School of Medical Sciences, Mizuho-ku, Nagoya, Aichi 4678601, Japan. 7. Juntendo University, Graduate School of Health and Sports Science, Inzai, Chiba 2701695, Japan.
Abstract
BACKGROUND: A sex difference in the second-to-fourth digit ratio (2D:4D, males < females) has been described in Japanese fetuses and children, and its possible links to autism spectrum disorders (ASDs) have been discussed. Accordingly, this sexual difference in representative neonates merits examination. AIMS: This study aimed to examine 2D:4D measurements and sexual differences in Japanese toddlers aged 1.5 years. STUDY DESIGN AND SUBJECTS: The digit lengths used to calculate 2D:4D were measured using an easy-to-use photography method. A total of 1045 JECS-A (the Aichi regional sub-cohort of the Japan Environment and Children's Study) children (males, 523; females, 522) aged 1.5 years were analyzed. RESULTS: The mean ages for the males and females were 575.3 ± 13.1 and 575.9 ± 17.1 days, respectively. Histograms of left and right 2D:4D were normally distributed regardless of sex (left male, 0.909 ± 0.048; left female, 0.913 ± 0.049, d = 0.08; right male, 0.938 ± 0.055; right female, 0.937 ± 0.049, d = 0.02). Because of high dispersion in the data, t-tests did not support a significant sex difference in 2D:4D. Post-hoc statistical power was calculated as 0.124 and the effect size for the sex difference in 2D:4D was 0.036. CONCLUSIONS: This study failed to confirm sexual differences in 2D:4D in 1.5-year-old Japanese children. This may be because digit measurement is difficult in this group, resulting in reduced effect sizes, or because rapid growth attenuates the in utero sexual dimorphism. This evidence is useful for the light it casts on the extreme male brain theory of ASDs.
BACKGROUND: A sex difference in the second-to-fourth digit ratio (2D:4D, males < females) has been described in Japanese fetuses and children, and its possible links to autism spectrum disorders (ASDs) have been discussed. Accordingly, this sexual difference in representative neonates merits examination. AIMS: This study aimed to examine 2D:4D measurements and sexual differences in Japanese toddlers aged 1.5 years. STUDY DESIGN AND SUBJECTS: The digit lengths used to calculate 2D:4D were measured using an easy-to-use photography method. A total of 1045 JECS-A (the Aichi regional sub-cohort of the Japan Environment and Children's Study) children (males, 523; females, 522) aged 1.5 years were analyzed. RESULTS: The mean ages for the males and females were 575.3 ± 13.1 and 575.9 ± 17.1 days, respectively. Histograms of left and right 2D:4D were normally distributed regardless of sex (left male, 0.909 ± 0.048; left female, 0.913 ± 0.049, d = 0.08; right male, 0.938 ± 0.055; right female, 0.937 ± 0.049, d = 0.02). Because of high dispersion in the data, t-tests did not support a significant sex difference in 2D:4D. Post-hoc statistical power was calculated as 0.124 and the effect size for the sex difference in 2D:4D was 0.036. CONCLUSIONS: This study failed to confirm sexual differences in 2D:4D in 1.5-year-old Japanese children. This may be because digit measurement is difficult in this group, resulting in reduced effect sizes, or because rapid growth attenuates the in utero sexual dimorphism. This evidence is useful for the light it casts on the extreme male brain theory of ASDs.