The Effects of Aging on Hair–More Than Just Amount 466 Knowledge gaps for diameter and age: From existing data on hair diameter and age, it would appear that the age for maximum diameter for females in the parietal region of the scalp is in the mid-forties. A more accurate determination of the true average peak age for female hair diameter awaits larger studies however, equally important are questions about age and scalp site variation and whether there are geo-ethnic differences, because there is no data for diameter versus age for those of African descent. Determination of the age for maximum diameter for males in all scalp sites also requires additional investigation, as there are no large studies of this type from infancy to advanced age. In addition, the rate of diameter decrease for women and men with advanced age requires additional large studies to more accurately elucidate. The biological causes of the peak age for scalp hair fiber diameter also remain an important knowledge gap in our understanding of the mechanisms underlying changes in hair fiber diameter with age. Hair fiber curvature and age: Nagase et al.18 studied hair curvature from the hair of 132 Japanese females, ages 10 to 70, and reported an increase in curvature with age. In this paper, the curl radius was measured and regressed against age, providing a statistically significant increase in hair fiber curvature with increasing age. The index of determination (r2) for the linear regression was 0.185, indicating that about 19% of the total variation in hair curvature among Japanese women’s hair within the studied age range can be explained by age. The curl radius values for the total subjects fell from approximately 10 cm to 1.25 cm, while the curl radius values on the regression line of curvature versus age fell from about 4.5 to 2.9. Nagase et al., in this same paper, also measured hair luster using both a contrast ratio for luster and a sensory method. Both measures showed a decrease in luster with increasing age. The regression line for the effect of hair luster and age among Japanese women shows that age accounts for about 18% of the total variation in luster among these 132 women (r2 = 0.18). These scientists also plotted hair luster versus the curl radius and found a significant linear regression