Chapter 14 465 20s to the late 40s. Courtois et al. studied 10 Caucasian adult male subjects (ages 25–49) by making observations periodically over a 14-year period. These scientists demonstrated that the diameter of hair shafts decreased with increasing age beginning at age 25. Correspondingly, a reduction in the duration of the growth period also occurred. In addition, the time interval separating the loss of a hair in telogen and when a replacement hair appeared in anagen also increased. This study shows that hairs on the same male Caucasian after age 25 become finer during the next 14 years in agreement with the conclusions of Otsuka and Nemoto15 on age and fiber diameter on different Japanese males. This effort by Courtois et al. on adult Caucasian males also supports our conclusions on the effects of age on the diameter of scalp hair of male Caucasians for the French Canadian data of Trotter and Dawson. Table 5 summarizes calculated rates of diameter decrease per year over 10-year periods from the data of Otsuka and Nemoto on Japanese men’s and women’s hair. This data clearly shows the effects of aging in a practical way. However, it is unfortunate that we have not been able to find larger studies on Caucasian and African men analogous to Otsuka and Nemoto’s on Japanese hair, so that we could be more quantitative in our conclusions on the effects of age on the scalp hair of men. Table 5. Instantaneous rates for estimation of diameter changes per year at 10-year intervals Age Males15 Females15 Females16 35 -0.53 -0.02 +0.20 45 -0.80 -0.35 -0.043 55 -1.07 -0.80 -0.29 The instantaneous rates for Robbins et al. data16 were calculated from the first derivative of the quadratic model equation by substituting the mean ages 35, 45, and 55. The quadratic model was also used for the male data for the Japanese study15 while a cubic model was used for the females of the Japanese study, once again using the first derivatives of the model equations to calculate the instantaneous rates.