"JPN individuals have more than 30% larger mean cross-sectional area (6,518 'units') than SEA (4,957 'units') and more than 50% larger than Africans (4,274 'units') and Caucasians (3,857 'units') (12)."Since population origin also explains variation in hair thickness, the EDAR SNP "1540T/C by itself cannot explain all the differentiation of hair fiber thickness between JPN and SEA" and other genetic and environmental factors must be responsible.
They do a control for population stratification, but it would have been interesting to see a STRUCTURE output of their sample. I assume there wasn't that much stratification in terms of the 23 markers that they chose to look at (those with high 'JPN+CHN vs. the rest of the HapMap pops' differentiation)
A replication study confirmed the EDAR gene to be a major contributor to population differentiation regarding head hair thickness in Asia
Akihiro Fujimoto, Jun Ohashi, Nao Nishida, Taku Miyagawa, Yasuyuki Morishita, Tatsuhiko Tsunoda, Ryosuke Kimura and Katsushi Tokunaga
Human Genetics V. 124, Number 2 / September, 2008
Abstract Hair morphology is a highly divergent phenotype among human populations. We recently reported that a nonsynonymous SNP in the ectodysplasin A receptor (EDAR 1540T/C) is associated with head hair fiber thickness in an ethnic group in Thailand (Thai-Mai) and an Indonesian population. However, these Southeast Asian populations are genetically and geographically close, and thus the genetic contribution of EDAR to hair morphological variation in the other Asian populations has remained unclear. In this study, we examined the association of 1540T/C with hair morphology in a Japanese population (Northeast Asian). As observed in our previous study, 1540T/C showed a significant association with hair cross-sectional area (P = 2.7 × 10−6) in Japanese. When all populations (Thai-Mai, Indonesian, and Japanese) were combined, the association of 1540T/C was stronger (P = 3.8 × 10−10) than those of age, sex, and population. These results indicate that EDAR is the genetic determinant of hair thickness as well as a strong contributor to hair fiber thickness variation among Asian populations.