The Ischium/Pubic Index for Sex Determination from Fragmentary Pelvic Material in Prehistoric New Zealand Populations
New Zealand Journal of Archaeology, N.14, p.159-166, 1992
Posted: 17 May 2018
Date Written: March 1, 1992
The use of quantitative osteometric techniques for sex determination has to be restricted to populations genetically similar to those for which parameters have already been established. Such techniques are highly population-specific (Iscan 1983). It is possible to achieve a consistency in sex determination of fragmentary pelvic material using an ischium/pubic index as long as the index has already been established from a larger group of individuals of a genetically similar population.
The following description of a method of sex determination for prehistoric New Zealand skeletal material uses an ischium/pubic index, as it is a technique which can be used when the remains of other parts of the pelvis are marginal or inadequate for other methods of sex determination. The term ischium/pubic index has caused some confusion. It was erroneously taken as ischial length x 100/pubic length by Wood (1976) and Baker (1975), although the error was corrected by Mobb and Wood (1977). The term pubo-ischial index had been proposed to avoid such an error (Williams and Warwick 1980: 390), but has not been widely accepted. A further problem arises with comparison of the index between different studies, because Washburn originally measured from the fusion point of the pubis, ilium and ischium within the acetabulum, and Mobb and Wood measured from the rim of the acetabulum. However, it is consistent with current metrical studies (Berge et al. 1984; Steudal 1984) to take the pubic and ischial lengths from the centre of the acetabulum, irrespective of the actual fusion point of the bones within the socket.
The ischium/pubic index can be used on isolated skeletal material when the range of 'male' and 'female' indices have already been established for a similar population, as long as it is remembered that the sex determination is only consistent with that undertaken on a larger prehistoric population, and is only as accurate as the sex determination in the larger group. In this NZ population in the 13 'females', the ischium/pubic index ranged from 105.8 to 127.5. The mean from the total of both sides was 115.6. In the 16 'males', the ischium/pubic index ranged from 95.75 to 106.2. The mean was 100.5. An assessment of sex of particular prehistoric New Zealand material within this population could be made from fragmentary pelvic material if the index was below 105 ('male ') or above 107 ('female'). If the index was within the overlap area, other means of sex determination may be more appropriate.
When Richman et al. (1979) discussed the ischium/pubic index, they noted there was always an overlap when it was used on individuals of known sex. In the Howard University skeletal collection, 28 of the 95 were in such an overlap area. In the Thieme and Schull (1957) sample from the Terry collection, 40 of the 200 were in the overlap area. If, for this New Zealand collection, ischium/pubic indices between 105.0 and 107.0 are taken as lying in the overlap zone, only 3 of the 49 measurements fall within this area. The more discrete nature of the index in this population should not be assumed to reflect a more pronounced degree of sexual dimorphism; it may be a reflection of the methods of making the initial sexual identification. It is likely that some estimation of the sub-pubic angle, which is secondary to pubic length, was used in the initial sex determination.
For a comparison of results with other racial groups, it is useful to compare Washburn's data from Negro and White populations with these data. Both males and females in racial groups with longer trunk length relative to total stature show a relative elongation of the pubic bone, reflected in a higher ischium/pubic index. This may be taken as confirmation of Rosenberg's (1988) study of the relationships between stature and pelvic proportions in different racial groups, but she considered body weight to be more significant than stature in determining the relative pubic bone length. She apparently did not consider whether the different body proportions in shorter populations with a greater trunk length relative to total stature might account for the longer pubic bones, rather than total body weight.
The findings in this study would be consistent with the functional biomechanical explanation of the Polynesian physique attributed to cooler climate adaptions, as suggested by Houghton (1990). Both males and females show relative elongation of the pubic bone, compared with other groups. It is suggested that this occurs secondarily to great trunk size relative to stature and overall muscular development in both sexes.
Keywords: Ischium/Pubic Index, Sex Determination, New Zealand Prehistoric Skeletal Material
JEL Classification: Z10
Suggested Citation: Suggested Citation