JOURNAL OF COSMETIC SCIENCE 104 differences are shown in Figure 5a where signifi cance is based on LSD and Duncan meth- ods of analyses where the output gives the same divisions. Ellipticity values show no signifi cant differences across all ethnicities for LSD, Scheffe and Duncan analyses. P- values are shown in the respective graphs. Figure 4. Mechanical properties as a function of curl class n = 12,050 fi bers. (a) Mean Young’s modulus ± standard error. (b) Mean break stress ± standard error. Figure 5. Geometrical properties according to ethnicity n = 12,050 fi bers. (a) Mean x-sectional area ± stan- dard error. (b) Mean ellipticity ± standard error.

2008 TRI/PRINCETON CONFERENCE 105 Figure 6. Mechanical properties according to ethnicity n = 12,050 fi bers. (a) Mean Young’s modulus ± standard error. (b) Mean break stress ± standard error. Figure 6 shows the mechanical data for the different ethnic groups studied. Young’s mod- ulus decreases in the order of AA JA KE LI GA and break stress decreases from AA JA LI GA KE. For the statistical groupings, LSD and Duncan methods give an output of the same divisions with differences shown in the graphs. If both Young’s mod- ulus and break stress data are considered, the mechanical integrity of African-American hair is shown to be greater compared to the other ethnicities whereas hair from Kenya is more fragile. To the authors’ knowledge, this is the fi rst time that such ethnic compari- sons have been made and statistical differences have been observed. To gain insight on possible differences in the structure of hair from the populations studied, DSC was performed to evaluate the thermal characteristics. Denaturation temperature (Td) values give information about the cross-link density of the matrix in which the intermediate fi laments are imbedded while the change in enthalpy during denaturation (ΔHd) gives the energy required to denature α-helical content within the intermediate fi laments. Wortmann et al. and Marsh et al. reported differences in Td and ΔHd as a function of treatment (30,31) however, values did not signifi cantly differ when Wortmann and Deutz compared the ther- mal properties of hair from people of different races (20). Figure 7 shows the thermal behav- ior of hair from the different countries in the present study. Td and ΔHd values range from 151.7 to 153.1°C and 17.2 to 19.1 J/g, respectively, and are consistent with reported values (20,21). Td values decreased from AA LI GA KE JA and ΔHd decreased from AA GA KE LI JA. Td values showed more statistical differences between ethnic groups compared to ΔHd (Figure 7). Thus, variations in the cross-link density between ethnic groups were more readily detected compared to differences in the α-helical content. Over- all, African-American hair had the highest thermal integrity and Jamaican hair had the lowest, which is not consistent with data from the mechanical studies. One might expect that hair with higher mechanical integrity would also be more structurally robust in regards