Age‐at‐death assessment using the os pubis and the auricular surface of the ilium: a test on an identified Asian sample

This paper reviews the results of blind tests of two morphological methods of age‐at‐death estimation. These tests were performed on a known age‐at‐death and sex sample taken from a collection of a Thai population. The first technique is based on the age related changes of the pubic symphysis according to the Suchey‐Brooks system, and the other concerns the metamorphosis of the auricular surface of the ilium elaborated by Lovejoy and colleagues. This is the first time that these methods have been tested on skeletal material from Asia. The results indicate that, for both methods, bias and inaccuracy increase with age and true age tends to be underestimated. As a consequence, age‐at‐death assessment based on these two techniques should be avoided on Asian archaeological series or forensic cases. Copyright © 2004 John Wiley & Sons, Ltd.     

Age estimation of children from prehistoric Southeast Asia: are the dental formation methods used appropriate?

Inter-population differences in skeletal and dental growth and maturation are acknowledged frequently in the biological anthropological literature. These growth differences have implications for the reliability of standards for the estimation of age at death of archaeological subadults. The number of archaeological projects that are recovering human burials from non-European contexts, including Southeast Asia, and the increasing interest in subadult bioarchaeological studies provides the impetus for investigating this issue of ageing subadult individuals from these populations. This paper aims to address some of the problems of the representativeness of ageing standards for non-European children in bioarchaeology. This is achieved through a literature review of the issue of growth variability and age estimation, and a basic comparison of the commonly applied age estimation method based on North American children with a dental formation study of modern Thai children. Although these studies do not employ similar methods the Thai study is the only comparable data available and therefore serves as a starting point to address these issues. The results raise an important question for bioarchaeologists of the appropriateness of available ageing methods. In addition this paper emphasises the need for the use of appropriate methodologies in the collection and presentation of dental formation data.     

Estimation of adult skeletal age‐at‐death: statistical assumptions and applications

We examine the statistical assumptions underlying different techniques of estimating the age‐at‐death of a skeleton from one or more age indicators. The preferred method depends on which property of the distribution of the data in the reference sample is preserved in the skeleton to be aged. In cases where the conditional distribution of age given indicator is preserved, we provide ‘look‐up’ tables giving essentially unbiased age estimates and prediction intervals, using a large reference sample and the auricular surface and pubic symphysis age indicators. Where this assumption is violated, but the conditional distribution of indicator given age is preserved, we find that an alternative model which attempts to capture the biological process of development of an individual has some attractive features, which may make it suitable for further study. Copyright © 2006 John Wiley & Sons, Ltd.     

Don't throw out the baby with the bathwater: Estimating fertility from subadult skeletons

Recent research interest has focused on the bioarchaeology of children. Although paleodemography is essential for accurate reconstructions of lifestyle and health in past populations, currently there is no published technique for estimating fertility and life expectancy at birth for skeletal populations in which adults are under‐enumerated. This paper provides a formula to predict Gross Reproductive Rate (GRR) from the proportion of young infants to subadults in a skeletal population. The formula was developed from 98 of Coale and Demeny's Female Model West Life Tables, which represented diverse fertility and mortality rates. The formula's accuracy was examined using independent samples from historical and archaeological cemeteries. Estimates of GRR from the subadult fertility formula were compared with estimates from Bocquet‐Appel and Masset's juvenile:adult ratio. Results indicate that the subadult fertility formula predicts GRR with consistent accuracy (R² = 0.98) and precision (± 1 offspring) in the model life tables, across diverse subadult age structures and demographic characteristics. The formula is useful for subadult populations with a proportion of perinates:subadults between 0.12 and 0.45. The adult component of the sample is not included in the analysis and thus the formula is similarly useful in cases where adults are under‐enumerated, or not. When applied to historical and archaeological populations, estimates for GRR are similar to previous estimates from the juvenile:adult ratio. Because crude birth rate and life expectancy at birth can be calculated from GRR using established fertility centred approaches to demography, the subadult fertility formula allows skeletal populations of diverse composition to be included in demographic research, essential for understanding of how mortality and fertility are affecting the morbidity profiles of subadult samples and for comparative bioarchaeological analyses. Copyright © 2010 John Wiley & Sons, Ltd.     

A comparison of different non-destructive methods of age estimation in skeletal material

The skeletal remains of unidentifiable persons, recovered in recent years from the churchyard adjoining the House of Correction in Oslo, included 91 instances in which the maxillae and/or mandibles had been preserved, and these were examined. The length of the apical translucent zone in unsectioned teeth and the amount of secondary dentine deposit have been regarded as two of the most reliable factors in odontological age estimation. This study has used two methods, each using one of these factors, on single-rooted teeth from, respectively, 78 and 76 individuals. The distribution of age at death, as estimated from the two dental methods and from anthropological criteria, was then compared with the distribution of age at death of 380 individuals recorded in the church register for the House of Correction. Age calculations from secondary dentine, measured indirectly on dental radiographs of premolars, and estimates based on anthropological criteria both seemed to approximate to the chronological age distribution better than age estimates based on the length of the apical translucent zone. Statistical analyses indicated that all three methods of age estimation were significantly different (p<0.05). When compared with the church register, each method assigned a lower percentage of individuals to the younger and older age-groups and a higher percentage to those in the middle. No statistical difference could be found between the age distribution from the church register and estimates from either anthropological criteria or dental radiographs; but when age estimates based on these two latter methods were compared, 39.5 per cent differed by more than 10 years.     

Developing stages of subacromial humeral‐impingement facets in the skeletal remains of two human populations

The humeral impingement disorder (HID) results from degenerative changes in the rotator cuff tendon which allows the humerus to press against the acromion and produce pressure facets on both the acromion and the greater humeral tubercle. A previous study of those below 55 years of age in the two populations defined a system of three morphological categories of subacromial features. The first two categories were postulated to be pre‐impingement while the third, distinguished by erosive pitting at the point of attachment of the cuff to the greater tubercle, was regarded as indicative of degenerative change in the cuff and, therefore, of HID. Extending the study to those over 55 years of age has now added two categories of more advanced HID change. Those on the right side, of dominant use, were always more severe, thus enabling a less severe Category 4 to be distinguished from Category 5, the chief feature of which was a raised facet which could be large or small. Confirmation of the earlier study's conclusions was achieved: the degenerative changes of Category 3 began to be common by 40 years of age in both sexes of the Scottish island people, living physically‐stressed lives; in the more wealthy and sedentary Londoners, this does not occur until 10 years later. Nevertheless, in both sexes in both populations, virtually all those over 65 years of age showed Category 4 or 5 change. It seemed likely that a higher proportion of the Londoners reached advanced ages than among the islanders and so had more time for the progressive disorder to show its full effects; for instance, there were 16 Londoners known to be over 80 years of age. Individuals of that age cannot be distinguished by methods for estimating age from those 15 years or so younger but it was regarded as unlikely that there were more than two or three octogenarians among the islanders. Copyright © 2000 John Wiley & Sons, Ltd.     

Age‐at‐Death Estimates from a Disarticulated,Fragmented and Commingled Archaeological Battlefield Assemblage

Estimation of age‐at‐death is an important and challenging part of any investigation of human remains. Extensive research has been undertaken on this subject as demographic information contributes much to bioarchaeological and forensic work. Disarticulated, fragmentary and commingled human remains occur from a wide range of spatial and temporal contexts, and estimation of age‐at‐death can be particularly challenging in these collections. This study evaluated the impact of preservation on techniques that might be applied and their relative utility, using human remains from the site of the Smith's Knoll associated with the Battle of Stoney Creek, a War of 1812 collection from Ontario Canada with some supporting documentary evidence on age‐at‐death. Features assessed were the pubic symphysis, auricular surface and epiphyseal fusion in the innominate. An age‐at‐death estimate was produced for 16 out of 19 individuals considered; the highest minimum number of individuals (MNI) was 24 (right radius) from 2701 identifiable fragments. The pubic symphysis made no contribution to construction of a demographic profile, but as has been suggested previously epiphyseal fusion was useful in this respect. Results demonstrate that previous statements regarding preservation of the auricular surface and its utility in fragmented and poorly preserved collections need to be carefully evaluated. Although 129 fragments of innominate were recorded just one had an auricular surface and post auricular area that could be fully assessed. Transition Analysis was easier to apply than techniques developed previously, and findings from this study suggest that use of the forensic prior distribution could assist in assessment of battlefield assemblages. These results raise some valuable points that need to be considered in any future attempts to improve age‐at‐death estimates using the auricular surface, and important questions regarding expectations for estimation of age‐at‐death in disarticulated, fragmented and commingled collections of human bone. Copyright © 2015 John Wiley & Sons, Ltd.     

Sample-specific (universal) metric approaches for determining the sex of immature human skeletal remains using permanent tooth dimensions

In this paper, three approaches for developing sample-specific sex determination methods of immature skeletal remains based on permanent tooth dimensions are proposed and tested using a sample of identified skeletons. The sample comprises adult and subadult individuals selected from the Lisbon documented skeletal collection, housed at the National Museum of Natural History in Lisbon, Portugal. Faciolingual and mesiodistal diameters were the tooth dimensions utilized. In the first approach, sex-specific logistic regression formulae based on adult tooth dimensions are developed and then used to determine the sex of the subadult sample. The second and third approaches are based on the sectioning point procedure, which uses the overall mean of a measurement (tooth diameter) collected from the sample as the discriminant criteria for determining the sex of the individuals in that sample. While in the second approach the adult overall mean of each dimension is used as the discriminant criteria for determining the sex of the subadults, in the third approach it is the subadult overall mean of each dimension that comprises the discriminant criteria that is applied back to the subadults for determining sex. Results show that the canines are the teeth with the highest sexual dimorphism and methods of sex determination based on canine dimensions provide correct allocation accuracies between 58.8% and 100% depending on the diameter and the approach that is being used. Canine faciolingual dimensions provide the best overall results. Combinations of measurements from the same and different teeth do not increase significantly the accuracy of the methods and approaches. Some of the problems of subadult sex determination methods based on adult tooth dimensions result from differing levels of sexual dimorphism between the adult and subadult segment of the sample. Mortality or cultural bias may increase or decrease the sexual dimorphism of the subadults compared to the adults. Small subadult samples utilized in this study may also raise questions regarding the accuracy of the three different sample-specific approaches. However, high consistency of results using the canine and different approaches, suggests that adult and subadult canine dimensions can be reliable sex discriminators of immature skeletal remains in archaeological samples. The major advantage of the approaches presented here is that they can be used to derive sample-specific methods and, therefore, eliminate the problem of applying morphological or metric methods to individuals originating from a population that differs from the one that contributed to the development of the method.     

Prediction Models for Age‐at‐Death Estimates for Calves,Using Unfused Epiphyses and Diaphyses

For cattle (Bos taurus), age estimations using dental criteria before the eruption of the first molar (3–8 months) have large error margins. This hampers archaeozoological investigation into perinatal mortality or the putative slaughtering of very young calves for milk exploitation. Previous ageing methods for subjuveniles have focused on the length of unfused bones, but it is rarely possible to use them because they are restricted to foetuses and because of the fragmentation of bones. This paper presents new age prediction models based on length, breadth and depth of post cranial bones produced from a dataset of modern calves (n = 27). This reference collection was compiled from material of known age at death, sex and breed from collections in Britain, France, Germany and Switzerland. Linear regression models were constructed using the modern data for age prediction, and these models were then successfully tested and assessed using a Middle Neolithic assemblage of complete calves' skeletons from Bourguignon‐Lès‐Morey, France. From the assessment, the astragalus and metapodials were determined to be the most reliable bones, and the femur was the worst. Measurements of the epiphyseal and distal elements and depth measurements were the most reliable. For ages before 12 months, these models can provide ±1 month age estimates. Copyright © 2013 John Wiley & Sons, Ltd.     

Measuring Root Dentine Translucency in Human Teeth of Varying Antiquity

Many workers have calibrated human root dentine transparency (RDT) as a linear regression on age. It is now regarded as a well-established means of estimating age at death in modern human material. Similar applications in archaeological material have not yet been developed. The aim of this study was to establish a standard protocol for measuring RDT which was derived from previous methods and which could be applied to teeth of unknown and varying antiquity. An initial study on two archaeological populations determined the choice of tooth to study and a second study, using expendable teeth (of unknown age and origin), evaluated various techniques of specimen preparation and examination. Findings from the pilot study indicated that the lower canine was the tooth of choice. From the second study it was observed that archaeological teeth could only be sectioned if they had first been infiltrated and embedded in methyl methacrylate. The optimal section thickness was found to be 150 μm and no benefit was gained by staining. Inter-observer reliability tests showed significant differences in repeated measures of RDT in intact teeth, which were not borne out when sectioned teeth were used. Intra-observer reliability was maintained for measurements in both intact and sectioned teeth. These findings have been used to establish a standard protocol for application to human teeth of any depositional phase to estimate the dental age at death of that individual.     

Cranial suture closure and its implications for age estimation

Three age estimation techniques using ectocranial and/or endocranial suture closure are tested on a sample of known age from Spitalfields, London in order to determine the value of cranial suture closure as an indicator of age at death. The three techniques are those proposed by Acsádi and Nemeskéri, Meindl and Lovejoy and Perizonius. Results indicate that the Acsádi and Nemeskéri technique, which is based on endocranial sutures, can be used to distinguish young and middle-aged individuals in the Spitalfields sample but gives no information for crania over the age of 50 years. Age estimation using the Meindl and Lovejoy and Perizonius (Old system) techniques, which use ectocranial sutures, was found to be subject to a number of complicating factors, of which sexual dimorphism in the rate and pattern of closure is the most significant. A method of estimating age at death based on both endocranial and ectocranial suture closure is developed on the basis of the Spitalfields sample. The technique attempts to overcome some of the problems associated with both intra- and interpopulation variation in cranial suture closure. For a truly accurate age-estimation technique based on cranial suture closure we would need to know more about the causes and functions of suture closure in human populations.     

Panel regression formulas for estimating stature and body mass from immature human skeletons: a statistical approach without reference to specific age estimates

Anthropologists require methods for accurately estimating stature and body mass from the human skeleton. Age-structured, generalized Least Squares (LS) regression formulas have been developed to predict stature from femoral length and to predict body mass in immature human remains using the width of the distal metaphysis, midshaft femoral geometry (J), and femoral head diameter. This paper tests the hypothesis that panel regression is an appropriate statistical method for regression modeling of longitudinal growth data, with longitudinal and cross-sectional effects on variance. Reference data were derived from the Denver Growth Study; panel regression was used to create one formula for estimating stature (for individuals 0.5–11.5 years old); two formulas for estimating body mass from the femur in infants and children (0.5–12.5 years old); and one formula for estimating body mass from the femoral head in older subadults (7–17.5 years old). The formulas were applied to an independent target sample of cadavers from Franklin County, Ohio and a large sample of immature individuals from diverse global populations. Results indicate panel regression formulas accurately estimate stature and body mass in immature skeletons, without reference to an independent estimate for age at death. Thus, using panel regression formulas to estimate stature and body mass in forensic and archaeological specimens may reduce second stage errors associated with inaccurate age estimates.     

Analysis of Neolithic human remains discovered in southern Italy

The authors describe the discovery of the remains of two unidentified skeletonised individuals in a small town located in southern Italy. The bodies were discovered while workers were preparing to lay an oil pipeline. The two individuals were found at a depth of 2 meters, and in very close proximity to one other. The recovery process of the skeletonised remains and their related findings, carried out by a team of forensic anthropologists and archaeologists, is described here.Archaeological examination determined that the remains date back to the 4th millennium B.C. Forensic anthropological and odontological examinations were performed to determine the biological profile of the skeletal remains by estimation of age and height, as well as the determination of sex. Age determination was performed by the Kerley and Ubelaker (1978, Revision in the microscopic method of estimating age at death in human cortical bone. Am. J. Phys. Anthropol., 49, 545–546) histological method using a 1 mm thick piece of bone tissue taken from the diaphyses of femur in both individuals. Dental age was estimated by examining root transparency. In order to determine the height of the individuals, various research methods based on the dimensional values of particular skeletal structures were applied. DNA analysis showed genotype differences of all the systems as compared to the haplotypes of present day subjects. This provided confirmation that the skeletal remains were from individuals of an ancient population (4th millennium B.C.). In addition, radiocarbon dating provided useful information as to the approximate period of death of the individuals. Interpretation was further enhanced by analysis of various bone fragments from each of the skeletons by high resolution mass spectrometry. 3D computerized imaging was used to analyse the patterns of skull fractures present, which resulted in supporting the hypothesis that the fractures were caused by stoning.     

The Meul Index: A non‐destructive method for estimating the cortical index in skeletal samples

The study of osteoporosis in past populations provides insight into the history and evolution of this condition. The Cortical Index is a relative measure of cortical bone often used for identifying individuals with age‐related bone loss associated with osteoporosis. Current methods of measuring the Cortical Index involve radiography or cutting actual sections of long bones at the mid‐shaft. We propose a new index, the Meul Index as means for estimating the Cortical Index without the use of radiography. Based on a preliminary study of 13 individuals consisting of 9 males, 3 females and one unknown sex, ranging in age from young to older adult, the Meul Index is highly correlated with the Cortical Index (r = 0.903). The proposed Meul Index provides a non‐destructive and inexpensive means for studying age‐related bone loss in prehistoric and historic populations represented by skeletal samples. Our research builds on the recent publication of Silva, A.M., Crubezy, E. and Cunha, E. 2008. Bone weight: new reference values based on a modern Portuguese identified skeletal collection. Int. J. Osteoarchaeol. DOI: 10.1002/oa.998. Copyright © 2009 John Wiley & Sons, Ltd.     

Identification of the Archaeological ‘Invisible Elderly’: An Approach Illustrated with an Anglo‐Saxon Example

The aim of this paper is to present a method to facilitate age‐at‐death estimation of older individuals (generally those aged 50+ years) in a representative cemetery sample. The purpose of disaggregating catch‐all categories, such as 50+ years, is to enable the exploration of the elderly (those in their 50s, 60s, 70s or 80s) in the context of mortuary archaeology, bioarchaeology and/or palaeopathology. The methodological steps include the following: (1) assessment of occlusal tooth wear in an Anglo‐Saxon cemetery sample from Worthy Park, UK; (2) seriation of the sample, from youngest to oldest, based on the degree of tooth wear; (3) selection of an ethnographically derived model (known mortality profile) by which seriated individuals in the Worthy Park sample could be reallocated to more realistic or appropriate age classes; (4) reallocation of individuals in the seriated Worthy Park sample to the model age classes. A Hadza, Tanzania, hunter‐gatherer profile was chosen to model the Worthy Park sample, although others are available. By using this model, some 66% of the entire adult sample, originally allocated to the single final age category of 45+ years, was distributed across four new age categories from the mid‐40s to mid‐70s. Relatively straightforward, this approach provides a way to identify those individuals, 50+ years old, not normally sensitive to traditional age‐at‐death estimation methodologies currently available. Copyright © 2014 John Wiley & Sons, Ltd.     

A dental study comparing age estimations of the human remains from the Swedish warship Vasa

The great Swedish warship Vasa capsized and sank in Stockholm harbour on her maiden voyage in 1628. The ship was raised from the seabed in 1961 and skeletal remains were recovered from at least 25 individuals, with teeth and jaws from 17 of them. The skeletal material was odontologically examined, including dental radiographs. Teeth lost both ante‐mortem and post‐mortem were recorded and variations in tooth anatomy noted. Acquired changes were recorded including enamel hypoplasia, attrition, dental caries and periodontal recession. Age estimations based on a separate odontological study, osteological changes and tooth attrition had been made in 1989. Ages were estimated additionally by three non‐destructive dental methods based on (a) the length of the apical translucent zone, (b) selected measurements both on intact teeth and from ratios on dental radiographs and (c) ratios of the length and width measurements on dental radiographs from selected teeth. Several sources of post‐mortem changes had been observed, among them vivianite in 33 teeth from four individuals. The results of the dental examination showed that few teeth had been lost ante‐mortem, there was a low caries rate and little loss of periodontal attachments. With few exceptions, there is fairly good agreement between the different methods of estimating age. Age estimates based on dental attrition were, however, significantly different from the other dental methods and age estimates calculated from apical translucency ought to be regarded with scepticism due to post‐mortem changes. The chronological ages of the victims will never be known, but age estimates based on all the age‐markers available will probably reach a fairly close approximation. Copyright © 1999 John Wiley & Sons, Ltd.     

Linear Enamel Hypoplasia and Age‐at‐Death at Medieval (11th–16th Centuries) St. Gregory's Priory and Cemetery,Canterbury, UK

Linear enamel hypoplasia (LEH) is a macroscopically detectable band‐like dental defect, which represents localized decrease in enamel thickness caused by some form of disruption to a child's health. Such dental deformations are utilized in osteoarchaeological research as permanent markers of childhood physiological stress and have been extensively studied in numerous ancient human populations. However, currently there is no such data for medieval populations from Canterbury, UK. Here, LEH is examined in the context of age‐at‐death in human burials from the medieval St. Gregory's Priory and adjacent cemetery (11^th–16^th centuries), Canterbury, UK. The cemetery and Priory burials represented lower (n = 30) and higher status (n = 19) social groups, respectively. Linear enamel hypoplastic defects were counted on mandibular and maxillary anterior permanent teeth (n = 374). The age and sex of each skeleton were estimated using standard methods. Differences in LEH counts, age‐at‐death, and LEH formation ages were sought between the two social groups. Results indicate significantly greater frequencies of LEH in the Cemetery (mean = 17.6) compared to the Priory (mean = 7.9; t = −3.03, df = 46, p = 0.002). Adult age‐at‐death was also significantly lower in the Cemetery (mean = 39.8 years) compared to the Priory burials (mean = 44.1 years; t = 2.275, df = 47, p = 0.013). Hypoplasia formation ages differed significantly between the Priory (mean = 2.49 years) and Cemetery (mean = 3.22 years; t = 2.076; df = 47; p = 0.034) individuals. Results indicate that childhood stress may reflect adult mortality in this sample, and that the wellbeing of individuals from diverse social backgrounds can be successfully assessed using LEH analyses. Results are discussed in terms of the multifactorial etiology of LEH, as well as weaning‐related LEH formation. Copyright © 2012 John Wiley & Sons, Ltd.     

Estimating Migration Flows from Birthplace‐Specific Population Stocks of Infants

When adequate data on migration are unavailable, demographers infer such data indirectly, usually by turning to residual methods of estimating net migration. This paper sets out and illustrates an inferential method that uses population totals in the first age group of birthplace‐specific counts of residents in each region of a multiregional system to indirectly infer the entire age schedule of directional age‐specific migration flows. Specifically, it uses an estimate of infant migration that is afforded by a count of infants enumerated in a region other than their region of birth to infer all other age‐specific migration flows. Since infants migrate with their parents, the migration propensities of both are correlated, and the general stability of the age profiles of migration schedules then allows the association to be extended to all other age groups.     

Eneolithic Cattle Burials from Aljmaš‐Podunavlje and Osijek‐Retfala,Croatia

Two single bovine burials and one mixed animal burial (containing bovine and canid skeletal remains) have been unearthed at two Baden culture sites, Aljmaš‐Podunavlje and Osijek‐Retfala located in Eastern Croatia. Zooarchaeological analysis attributed the faunal remains to domesticated cattle (Bos taurus) and in one case a dog (Canis familiaris). Almost complete and articulated skeletons of subadult or adult cows were found in all three examined features, while pit 59/60 from Aljmaš also contained a skull belonging to an adult‐domesticated cattle and a skeleton of a very young dog, 2–3 months old. All of the skeletons are well preserved and display no evidence of carcass processing indicating that the animals were buried intentionally. Consideration of the positions in which these animals were interred and their relationship with nearby deposits enables discussion of potential animal burial strategies and possible connections with certain ritual practices in which cows and in one case a dog played important roles. The described burials represent the southernmost known distribution of the rite discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

The pattern of spondylosis deformans in prehistoric samples from west‐central New Mexico

A Pre‐Columbian skeletal sample (n=42) from two ancestral Pueblo sites in the Rio Grande valley of west‐central New Mexico was examined for frequency and severity of spondylosis deformans (vertebral osteophytosis). No significant sex differences were detected. Degenerative changes in the collective sample are generally confined to no more than well‐defined horizontal lipping at the joint margins. Advanced proliferative osteophytic change is infrequent even in the oldest age category. Not surprisingly, the lumbar vertebrae were the most frequently and most severely involved vertebral segment for all three adult age cohorts defined. The cervical vertebrae were the least involved. This pattern generally conforms with observations made on other archaeological samples from west of the Mississippi River, but it contrasts with the general pattern of more extensive cervical involvement in Pre‐Columbian North American samples from the Eastern Woodlands. This possible east–west difference is hypothesized (Bridges, P.S. 1994. American Journal of Physical Anthropology 93: 83–93) to be related to differential burden bearing habits. Copyright © 2000 John Wiley & Sons, Ltd.