6.1 Introduction

Physical anthropology and archaeology form the basis of the applied discipline of forensic anthropology. The foundation of physical anthropology in the United States can be traced back to anatomy departments, where anatomists extended their interest in soft tissue variation to the skeletal framework of the human form. The conventional anatomist’s talents lie largely in dissecting, identifying, and describing the range of human variation, but the anatomist as a researcher sets the foundation for questions about the evolutionary relationship between form and function. Early physical anthropologists expanded their anatomical training by studying variation in the context of evolutionary theory to formulate questions about human lifestyle and life history. The ability to interpret human skeletal biology in an evolutionary framework remains a trademark of physical anthropology today. This interpretive structure spans the time frame from the human fossil record to modern forensic casework. As an applied endeavor, however, theoretical foundations are often overlooked in forensic anthropology, and this has been difficult for the discipline to overcome.

This chapter seeks to rectify this issue by exploring the history and theoretical underpinnings of one aspect of forensic anthropology practice—skeletal age estimation. The beginnings of methodologies targeting the estimation of skeletal age stem from observation. Early anatomists involved in the dissection and processing of human cadavers began to document skeletal changes associated with development and degeneration. While skeletal maturation cannot be classified as a phenomenon, it is an observable, patterned process that has been described and interpreted in an evolutionary and theoretical context. Foundational theory encompasses a broad interpretive approach to the natural biologic processes of the world (Mayr, 2001; Quammen, 2006), while interpretive theory involves the derivation of hypotheses based on a set of observations and period of experimental testing that supports or refutes the original statements of the observer. Thus, the chronicling of age‐related changes exhibited in the human skeleton and the effort to estimate age based on described morphologies fall under the premise of interpretive theory. As with the broader discipline of forensic anthropology, observations lead to constructed interpretations based on context. The methodological approaches that have been produced in an effort to capture how the human skeletal form changes from birth to death encompass methodological theory in that the hypothesized method is evaluated in terms of accuracy. How well a model performs is dictated by its results. If an approach captures the true patterns or “phenomena” embodied in skeletal maturation and degeneration, then the precision and accuracy of the estimates will be reflected in efforts to validate the method on samples that were not used to develop the model.

The aim of this chapter is to provide a historical and theoretical context to the approaches practitioners use when estimating skeletal age. It is well known to any student or professional of physical anthropology that age estimation methods correlate known chronological ages of individuals in documented skeletal collections with morphological features of skeletal age indicators that include hallmarks of growth and development (i.e., epiphyseal union, tooth eruption) and degeneration (i.e., eburnation, osteophytic activity). The process of age estimation, from method development to implementation, combines inductive and deductive reasoning and much less thought is typically given to this process, unless one is involved in revising or creating a method. Researchers use inductive reasoning to develop hypotheses, conduct tests, and evaluate results to determine if predictions are supported or falsified (Boyd and Boyd, 2011). It should be recognized that well‐trained and highly experienced skeletal biologists do not enter into the method development process blindly. They recognize morphological patterns and deduce how these patterns may be related to the aging process, then use these data for method development. Once satisfactory methods are developed and tested, practitioners use them to estimate skeletal age of unknown individuals.

Skeletal age estimation too often is taught and practiced in the absence of theoretical context. Emphasis tends to be placed on application and deriving age estimates for reporting purposes without encouraging burgeoning academics to think about how the methods were developed initially and, perhaps more importantly, under what biological framework they are appropriate. As data sets are updated and expanded, statistical analyses become more sophisticated and computer packages are available to practitioners on a global basis; we must be cautious not to lose sight of the role of theory in informing practice and medicolegal application.

Furthermore, the discussion of the theory involved with age estimation in this chapter is easily generalized to analyses of the human skeleton concerning all aspects of the biological profile, pathology, and trauma. Analytical procedures for a forensic anthropology case report are constantly evaluated in terms of accuracy and truth, that is, if the timing and mode of a skeletal perimortem injury can be correctly interpreted, yet the understanding of why each approach exhibits any accuracy is not often addressed in the literature. Building an accurate depiction of one’s life and sometimes death from skeletal remains requires synthesizing a large amount of information and reducing it to a simple parameter using the theoretical premises of forensic anthropology (e.g., an age or stature range, sex or ancestry estimate, or an opinion about the cause or manner of death). The last few decades of analytical reflection have resulted in a shift to an emphasis on why predictive techniques work in the field of forensic anthropology and how and why analytical methods exist (Boyd and Boyd, 2011).

6.2 Skeletal age

To discuss the theoretical foundations for inferring biological age from skeletal characteristics, we must distinguish between chronological age at death and skeletal age at death. The former refers to a number calculated by subtracting the date of birth from the date of death. The latter refers to an age range that corresponds to a suite of morphological traits consistent with the majority of individuals at a given age. Therefore, chronological age at death is an integer (23, 35, 49, etc.), whereas skeletal age at death is necessarily an interval (35–45, 54–60, etc.). A number of factors affect skeletal age at death, including habitual activities, diet, congenital anomalies, injuries, pathologies, and genetic criteria such as sex and ancestry. Additionally, skeletal age is not a discrete variable, which can make accurate estimates immensely difficult. Teasing out these complicating factors from skeletal age indicators is the bread and butter of holistically trained forensic anthropologists who incorporate human skeletal biology, anatomy, culture, and life history into their analyses. Even with anthropological training, human variation dictates that exceptions exist as a response to a multitude of evolutionary processes, both at small and large temporal scales. Thus, although perceived as biological reality, the skeletal expression of traits is not always in perfect agreement with the methods developed to describe them. Well‐rounded training in anatomy, life history, and statistics helps forensic anthropologists identify population substructures and explain how and why outliers exist.

6.3 Historical context

The anatomist T. Wingate Todd began large‐scale skeletal age‐at‐death research in the late nineteenth century when he documented pubic symphyseal age changes in Western Reserve University anatomical donors used for gross anatomy dissection (Shapiro, 1939). Todd documented morphological patterns correlated with growth, development, and degeneration and established a phase‐based method for age‐at‐death estimation of skeletal remains. An examination of Hamann‐Todd collection skeletons, death records, and Dr. Todd’s notes reveals that Todd recognized the distinction between chronological and skeletal age at death, as intrinsic factors such as a life of hard labor or poor health caused inconsistencies between these categories. Some of the drawers in the collection contain handwritten notes commenting that the skeleton looks older or younger than the reported or “stated” age at death. Suchey and Katz (1998) astutely note significant problems with what is colloquially referred to as the “Todd collection.” Collection curators appeared to have a specific morphology in mind for each age group based on extensive observations of skeletal remains. Thus, the death record may state “23 years old,” but the anatomist felt that the skeleton was more consistent with a 30‐ or 35‐year‐old, and the latter “observed age” was entered into Todd collection records (hence the 5‐year interval age lumping noted in the collection demographics). A large number of unclaimed indigents from City Hospital, Lakeside Hospital, and Warrensville Hospital in Cleveland comprise the Todd collection, so an exact date of birth was often unknown. Lovejoy et al. (1985) note that only 3 of the over 3000 records in the Todd collection contain legal documentation of birth date. Ages listed in the files were estimated from soft tissue indicators during hospital autopsy, and Todd collection curators often noted discrepancies as great as 15–20 years between “stated age” at autopsy and “observed age,” the latter referring to skeletal age at death (Lovejoy et al., 1985). Nonetheless, a handful of age estimation methods used in forensic anthropology casework have been developed using individuals in this skeletal collection (Todd, 1920, 1921; Todd and Lyon, 1925; Lovejoy et al., 1985; Meindl and Lovejoy, 1985; Passalacqua, 2009).

Thus, while early anthropologists used their anatomical knowledge of skeletal variation to develop age estimation methods, their samples were fraught with an uncontrollable source of error that created a circular logic between method and application: skeletal age was used to determine the age of the individuals, and then these estimated ages were used to develop age estimation methods. In addition to identifying problems with the accuracy of the data, critiques also exist regarding the data analysis in Todd’s pubic symphyseal age estimation method. Namely, several valid issues have been identified that potentially impact the accuracy and theoretical validity of the method, including overly precise age ranges, lack of statistical analysis, and limited age distribution of the reference sample.

6.4 Forensic anthropology and evolutionary biology

Shortly after Todd completed his work on skeletal age estimation, evolutionary theory was redefined to include the genetic mechanisms responsible for population change. The “modern synthesis” united Darwin’s idea of natural selection and Mendel’s explanation of simple inheritance of traits with population genetics. The term was coined by Huxley (1942) in his book Evolution: The Modern Synthesis, which linked the work of naturalists, paleontologists, and geneticists and advanced our understanding of how population genetics affects anatomical variation in extinct and extant species. Dobzhansky (1937) showed that natural populations are not uniform and harbor genetic variants, while Mayr (1942) defined species by their ability to mate and produce viable offspring as opposed to delineating these taxonomic groups solely on the basis of morphological similarities. Mayr (1942) also argued that evolutionary pressures work on the entire organism and its genome rather than isolated genes. Wright (1949) provided the mathematical foundation for modern population genetics, and Simpson (1942) explained evolutionary patterns identified in the fossil record.

The researchers of this time period provided mathematical models for quantifying the degrees of mutation, recombination, and selection that operate within species to account for evolutionary change. The modern synthesis explained that microevolutionary changes within populations were responsible for the macroevolution observed in the fossil record. Anatomical variation began to be interpreted and studied in a new light. Morphological variants were no longer the object of descriptive and typological treatises, but instead served as fodder for complex microevolutionary processes that provided evidence of evolutionary relationships as well as population and life histories. In 1951, Washburn launched the “new physical anthropology” in which he proposed incorporating the modern synthesis into the interdisciplinary four‐field study of human biology and culture. He also predicted the dominant role that genes would eventually play in understanding and interpreting human evolution (Washburn, 1951). Washburn’s (1951) call for a holistic, interdisciplinary interpretation of human biology and behavior has impacted forensic anthropology training and practice since the formal inception of the field in the 1970s.

The modern synthesis was followed by the discovery of the molecular double helix structure of DNA (Watson and Crick, 1953) and evolutionary biologists began to (over)emphasize the role of genes in phenotypic expression. While natural selection undoubtedly influences evolution, it does not account for all observable evolutionary change; however, neither do genes (Müller and Newman, 2003; West‐Eberhard, 2003; Pigliucci, 2009). West‐Eberhard (2003) argues that genes and the environment exert equally important influences on phenotypic variability. She suggests that epigenetic factors are essential for maintaining developmental plasticity and account for the production of alternative phenotypes from a fixed genome. Developmental plasticity shapes evolution in that most phenotypic change begins with environmentally mediated developmental change. This marks a departure from the modern synthesis interpretation of the environment’s role in producing variation (e.g., that environmental factors do not guide the production of new variants, but rather genetic variation produced by mutations are either favorable or unfavorable with respect to natural selection). The environment is now viewed as “initiator of evolutionary novelties,” with genes playing a secondary role in phenotypic evolution (Pigliucci, 2001; West‐Eberhard, 2003).

West‐Eberhard’s influence on the interpretation of phenotypic plasticity resonated with skeletal biologists who have documented short‐term changes in skeletal form due to environmental factors. Secular changes in long bone length and proportions (Meadows and Jantz, 1995; Jantz and Jantz, 1999), cranial vault size and shape (Jantz and Meadows Jantz, 2000; Jantz, 2001), and skeletal maturation (Crowder and Austin, 2005; Langley‐Shirley and Jantz, 2010) have enticed forensic anthropology practitioners and researchers to examine the process of skeletal aging at a deeper level and to implement abductive reasoning to parse out how and why skeletal growth, development, and deterioration can differ in various environments (Langley et al., 2016). For example, anthropologists have drawn on research documenting epigenetic markers that affect behavior and biology to formulate their hypotheses about secular change in human skeletal form, which has affected the size and shape of the human skeleton in a relatively minute temporal span (Low et al., 2012; Guth et al., 2013; Dias and Ressler, 2014; Langley et al., 2016). Furthermore, rapid evolution in other organisms supports the possibility of single generation changes in phenotype in humans (Drake and Klingenberg, 2008; Herrel et al., 2008; Pergams and Lawler, 2009; Milot et al., 2011; Brown and Brown, 2013; Harris et al., 2013). It is therefore no longer advisable or considered best practice to use methods developed on nineteenth and early twentieth‐century populations to derive a biological profile for modern forensic cases.

This realization has led to a surge of research to develop population‐specific standards for estimating the parameters of the biological profile and countless validation studies of these methods. Modern donor collections (e.g., Maxwell Museum Documented Skeletal Collection, William M. Bass Donated Skeletal Collection, and Texas State University Donated Skeletal Collection) were started to address the need for methods based on forensically‐relevant samples instead of skeletal collections with birth years dating to roughly a century before the current population (Hamann‐Todd Osteological Collection and Robert J. Terry Anatomical Collection) or, worse yet, prehistoric archaeological samples.

A legitimate concern of the latter was raised by Wood et al. (1992) in their discussion of the “osteological paradox.” The authors explain that using the archaeological record to interpret past populations does not result in an accurate depiction of overall health or represent fertility and mortality rates as skeletal remains only represent the individuals that died. As such, the percentage of the population that survived is not accounted for and can skew conclusions about the demographic and epidemiological state of a civilization. While the paradox refers to inferences about health in prehistoric populations, the article illustrates how the age distribution and frequency of pathology in reference samples may not be representative of the target population. Statistical models are built using observations and can be used to estimate parameters of an independent set of observations. However, if the two sets of observations are different enough, the model will perform poorly. This is true when discussing reference and target populations in age estimation, as there are many evolutionary factors that can affect growth and development of an organism. For example, poor health and nutrition that are often correlated with lower socioeconomic status have been shown to retard the speed of skeletal growth and dental eruption in children, which is more tightly regulated by genes than is degeneration and senescence (Garn et al., 1973).

6.5 Potential solutions to the problem of age estimation

One way that researchers have addressed the issue of skewed age distributions of modern donor populations is by partnering with medical examiner’s and coroner’s offices to expand modern samples (e.g., Broward County Medical Examiner’s Office, Los Angeles County Medical Examiner’s Office, Maricopa County Forensic Science Center, and Montana State Crime Lab) (Işcan et al., 1984; Işcan et al., 1985; Brooks and Suchey, 1990; Hartnett, 2010a, b; Dudzik and Langley, 2015). Research on international populations has also been undertaken to provide standards for international casework. For example, reliability of skeletal aging methods was questioned in trials prosecuted by the International Criminal Tribunal for the Former Yugoslavia (ICTY), leading to a body of work producing age, sex, and stature estimation criteria for the Balkan population (Berg, 2008; Jantz et al., 2008; Prince and Konigsberg, 2008; DiGangi et al., 2009).

The comprehensive theoretical framework provided by anatomy and evolutionary theory has identified a host of factors that can affect how, on average, an individual in a population matures into an adult organism and then slowly degenerates. These variables include diet, disease, biomechanical loading, and geographical location. However, many aging methods commonly employed in laboratories lack a rigorous statistical framework. In many studies, researchers seriated samples using a suite of morphological traits to constitute a phase, and then age ranges of individuals within each phase were analyzed to provide raw age ranges and/or summary statistics for each range. This is apparent in the work of Todd (1920, 1921), Lovejoy et al. (1985), Işcan et al. (1984, 1985), Katz and Suchey (1986), and Brooks and Suchey (1990) and has persisted in more recent publications as well (Osborne et al., 2004; Hartnett, 2010a, b). Phase methods lump several morphological indicators of age into a single category with the assumption that each trait progresses in conjunction with the others at the same rate.

Researchers have questioned the biological reality of this assumption and proposed using simpler component scoring systems to address this issue (DiGangi et al., 2009; Passalacqua and Uhl, 2009 Algee‐Hewitt, 2011; Dudzik and Langley, 2015; Shirley and Ramirez Montes, 2015). Component scoring is nothing new to age estimation (McKern and Stewart, 1957; Gilbert and McKern, 1973), but the data are more difficult to analyze statistically, so researchers traditionally have favored phase‐based methods. One attractive feature of component systems is that they are easy to apply and do not leave the practitioner deciding between two sometimes closely related phases. Another benefit is that they can be applied more reliably than phase methods regardless of observer experience (Shirley and Ramirez Montes, 2015). Component methods have been developed for the pubic symphysis (McKern and Stewart, 1957; Gilbert and McKern, 1973; Dudzik and Langley, 2015), auricular surface (Buckberry and Chamberlain, 2002; Igarashi et al., 2005), sacral auricular surface (Passalacqua, 2009), cranial sutures (Meindl and Lovejoy, 1985), and acetabulum (Rissech et al., 2006; Calce, 2012). In all of these, traits are treated as independent variables. Research has shown that component scoring with three or less states of expression for each variable results in good interobserver error agreement. Systems with a multitude of categorical tiers can result in significant levels of disagreement between observers (Shirley and Ramirez Montes, 2015). Component‐based systems form the basis of the multifactorial transition analysis method and the accompanying ADBOU software (Boldsen et al., 2002). Multifactorial aging methods use multiple skeletal indicators to arrive at an age range estimation. Criticisms of earlier approaches describe the use of single indicators (such as the pubic symphysis) or a combination of areas of the skeleton to deduce age in a nonsystematic way (Lovejoy et al., 1985). Multifactorial methods have shown higher accuracies in some instances; however, results are variable in regard to the method used and the average target age range to be estimated (Franklin, 2010).

Simply employing a component system does not address the issues with statistical rigor in many age estimation methods. An important caveat to consider is age mimicry bias, in which estimated age will be biased toward the age‐at‐death distribution of the reference sample (Bocquet‐Appel and Masset, 1982; Konigsberg and Frankenberg, 1992, 1994; Hoppa and Vaupel, 2002). Bayesian statistics have been suggested as a means of overcoming the limitations of reference sample age distributions and age mimicry bias. By choosing relevant samples, estimates are built using probabilities that an unknown is representative of whichever population was used to build the model. Inherently related to this approach is transition analysis, in which the age at which an individual moves across an age threshold is targeted (Boldsen et al., 2002). These methods have been slow to be adopted, perhaps due to computational intensity and selection of appropriate priors. The ADBOU software package offers a solution to practitioners in that it does the computing and the user can select a forensic, archaeological, or uniform prior. The software is based on the pubic symphysis, auricular surface, and cranial sutures and can also handle missing data.

With new data, more rigorous statistical approaches and computer software programs make their way into age estimation. Some question if the practice will become too automated and removed from its theoretical roots. But the fact remains that forensic anthropologists do not work in a vacuum, blindly using scoring systems and software to obtain an age estimate and produce a biological profile. We incorporate information about lifestyle, health, and habitual activities that we derive from our observations of the entire skeleton because we are holistically trained. It would take a concerted effort to score the pubic symphysis and auricular surface and ignore lipping on the intervertebral and zygapophyseal joints as we gather data for our age estimate. And it may be a grave oversight to ignore indications that skeletal remains look older or younger than established methods reveal. Forensic anthropologists do not argue that humans are the product of biocultural evolution, but they are torn between making scientifically unsupported assessments of biological parameters and the strict demands of a medicolegal system with legal precedents governing our analyses, case notes, and reports (Heilbronner, 2011; General Electric Co v. Joiner, 1997; Kumho Tire Company, Ltd. v. Carmichael, 1999; Melendez‐Diaz v. Massachusetts, 2009). So, how do forensic anthropologists provide age estimates that stand up to the rigors and ethical demands of the legal system yet remain sensitive to their holistic training?

The answer is not simple, but it must include quantification of the subjective traits that potentially influence our age estimates from other methods (Milner and Boldsen, 2012). If forensic anthropologists are to utilize the holistic toolkit that comes from multidisciplinary training and a biocultural interpretation of the human skeleton in the current legal environment (see National Research Council, 2009), we must provide a thorough analytical basis for our osteological conclusions. Researchers are beginning to make a headway in this direction (Milner, 2010; Milner and Boldsen, 2012; Milner et al., 2016). Milner and Boldsen (2012) have shown that our experienced‐based assessments are often more valuable than the standard set of anatomical structures typically used for age estimation. Clearly, there is something to be said for the role of the expert in any field (see Federal Rules of Evidence, 1975), but today’s experts must learn to operate in a less forgiving legal system than the one 50 years ago.

6.6 Final comments

Montagu’s (1941) publication eloquently describes the intertwined history between anatomy and anthropology and emphasizes that as researchers, we cannot be reductionists and hence cannot subsume the larger picture, in this case age distributions, into a few traits and tables of summary statistics. We bring a holistic biocultural approach to the table. This chapter advocates that practitioners, researchers, students, and educators incorporate theory and history into application. The foundations of evolutionary theory must not be omitted from the formation of forensically applicable methods or ignored in the education of students of the discipline. In addition, estimation of the biological profile requires an understanding of how skeletal and soft tissue morphologies of modern humans reflect both current and past microevolutionary processes. Understanding human evolution demands a working knowledge of the development and function of the skeleton and the related soft tissues. Thus, within the context of research, education, and forensic application, this chapter promotes the idea that formal training in anatomy, evolutionary theory, and statistics as well as a working knowledge of history and theory in forensic anthropology is imperative to move the discipline forward.

Today’s solutions involve the use of more indicators that capture growth and degenerative patterns, multifactorial methods, more appropriate statistics, and software programs with easy to use interfaces to handle complicated multifactorial probability‐based methods (such as ADBOU). Moreover, strict criteria now exist for method development that satisfy the scientific rigor required by the medicolegal community and, ultimately, the ethical demands of the criminal justice system.

While most programs of study require formal training in statistics and evolutionary theory, many do not require a full dissection course in gross anatomy. In fact, it is possible to earn a PhD in physical anthropology without so much as stepping foot into an anatomy laboratory in spite of the fact that physical anthropology is an extension of anatomy. As Montagu (1941) explains, the alphabet of physical anthropology is morphology, and students of physical anthropology must first be taught this alphabet before they can read, write, and speak the language of physical anthropology. We urge administrators and PhD advisors to mandate this essential component of physical anthropology training that was regrettably lost when physical anthropology departments made the transition from anatomy into departments of cultural anthropology and archaeology. We do not debate the benefits of this evolution of our discipline, but we lament that part of our foundation was somehow lost in the shuffle. Montagu (1941) brought this to the attention of the physical anthropology community in his American Journal of Physical Anthropology article “Physical Anthropology and Anatomy”:

…the calipers are brought out to carry on from the point at which the scalpel can go no further. But that does not mean that one forgets about the scalpel as soon as one has taken up the calipers. That this has occurred in a contemporary phase of the development of physical anthropology we all of us know, and most of us, I believe, regret. There are some physical anthropologists who have never so much as touched a scalpel, but have gone right on to the calipers. Such men, however competent they may be at their work, can rarely succeed in becoming anything more than good technicians. At best they can accurately record end‐effects, but they can never have any real understanding of the manner in which those end effects are brought about. In all this, as far as it goes, there is no harm; the place of the technician in any science is an important one, and a large part of the work of most scientists is that of the technician. The harm arises only when such technicians take it upon themselves to pronounce judgments upon the nature of the conditions underlying morphological facts with which they have, at best, no more than a bowing acquaintance. Such ungrounded pronouncements can only serve to bring physical anthropology into disrepute among scientists, and can do no one any good (pp. 261–262).

Ultimately, we must continue to use the theoretical constructs of anatomy and evolutionary theory to inform practice and research and to train future forensic anthropologists. Formal training in anatomy, evolutionary theory, and quantitative methods, as well as a working knowledge of history and anthropological theory, will move the discipline forward and concomitantly preserve our unique heritage.

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