Criminal Specialization

Christopher J Sullivan. 21st Century Criminology: A Reference Handbook. Editor: J Mitchell Miller. 2009. Sage Publications.

Since early in the study of crime, criminologists have been interested in the taxonomic identification of offenders. This interest dates as far back as the research of Lombroso, which attempted to define “born criminals” based on certain individual physical and psychological features (Gibbons, 1988). More recently, the study of criminal careers has brought renewed interest to whether, and to what degree, criminal offenders might concentrate, or “specialize,” in particular offense types. This gives rise to questions as to whether certain offenders engage predominantly, or exclusively, in drug offenses only, sex offenses, violent offenses, and so on. In short, research on specialization asks, can offenders be placed in distinct categories defined by their focus on particular types of crimes?

The answer to the specialization question has important implications for both criminological theory and criminal justice practice. Consequently, criminologists have made a number of explanatory propositions about the potential for specialization in offending, and empirical research has attempted to uncover its presence and nature. If there is no offense specialization, then criminologists would be wise to adhere to general explanations of crime, such as Gottfredson and Hirschi’s (1990) self-control theory, and policymakers and justice officials would be well served to treat all offenders in a similar manner based on their frequency of offending. On the other hand, findings of specialization suggest it would be prudent to consider more specific explanations of criminal behavior, such as differential opportunity theory (Cloward & Ohlin, 1960) or Moffit’s (1993) developmental taxonomy of antisocial behavior. If notions of specialization in offending are sustained, policymakers and practitioners might then look to make use of such information in developing intervention strategies and sanction structures.

Paternoster and colleagues (Paternoster, Brame, Piquero, Mazerolle, & Dean, 1998) define specialization as “the extent to which an offender tends to repeat the same specific offense or offense type on successive criminal events” (p. 133). According to Osgood and Schreck (2007), specialization reflects systematic individual differences in forms of criminal behavior engaged in by offenders. The concept of specialization may imply similarities in consecutive offenses or span an offender’s entire career (Miethe, Olson, & Mitchell, 2007). While specialization is often defined as the tendency to repeat specific types of offending, often in a direct, sequential form (see Kempf-Leonard, 1987), some believe this is too strict a definition (Francis, Soothill, & Fligelstone, 2004). Earlier studies focused somewhat more on offense-switching in consecutive criminal events or arrests, while more recent work tends to look at a particular period in time to assess the degree to which individuals exhibit a tendency to commit certain types of offenses during that time frame. Overall, specialization refers to a concentration in particular types of offenses. Clearly, the manner in which specialization is defined has important implications for theory and empirical findings.


Some of criminology’s major theories make assumptions, directly or indirectly, about specialization. These statements about specialization and generality in offending patterns derive from the positions that theories take on both criminals and criminal events. In contemporary criminology, two separate bodies of theory hold viewpoints on the existence and nature of specialization in offending. On the one hand, general theories that make few distinctions among offenders in terms of type and the explanation of behavior suggest that offenders do not specialize in particular offenses. Alternatively, taxonomic perspectives of criminal careers that make distinctions among offenders are more accepting of the idea that there may be qualitative differences in offending patterns.

In the general theory of crime, offending is viewed as only one of a host of activities undertaken in pursuit of short-term pleasure or gain at the expense of potential long-term consequences (Gottfredson & Hirschi, 1990). One of its fundamental propositions is that offenders have low levels of self-control, and this leads them to engage in a variety of criminal offenses. Thus, the theory suggests that there should be no discernable patterns of offenses. Criminal acts are seen as providing the potential immediate gains that will be attractive to those who have lower levels of self-control (Britt, 1994). Therefore, in the view of these theorists, offenders are unlikely to demonstrate specialization as this short-term, pleasure-seeking orientation will manifest itself in a variety of distinct deviant activities across different situations and shifting life circumstances. An offender who commits auto theft one week might get involved with a physical fight the next or commit another auto theft. The specific nature of the deviant behavior is likely to arise mainly out of the opportunities presented to the offender and will not reflect a coherent pattern of offending within a particular type.

The criminal career paradigm, outlined by Blumstein and colleagues (Blumstein, Cohen, Roth, & Visher, 1986), suggests that, in addition to onset, continuance, and desistance in offending, the qualitative issue of offense type is also worthy of focus. In short, this framework focuses on individual-level variation in offending over time, arguing that such patterns are worthy of empirical investigation and, potentially, theoretical explanation. The issue of specialization represents one such area of study. Criminal career researchers believe that there may be certain groups of offenders who warrant distinct behavioral explanations and may require some sort of specialized treatment or intervention. For example, there may be a group of serious, generalized offenders who engage in a lifelong pattern of antisocial behavior and will engage in a variety of deviant activities. This reflects the small group of serious, chronic offenders that are often identified in studies focused on criminal behavior in general populations (e.g., Wolfgang’s Philadelphia Birth Cohort Study; see Wolfgang, Figlio, & Sellin, 1972). On the other hand, it is possible that certain individuals tend to engage in less serious deviant behavior during adolescence only and desist from that behavior as they near adulthood. This is the premise of Moffitt’s (1993) taxonomy of antisocial behavior, which provides an example of a theory that outlines subgroups of offenders that may be distinct in terms of the manner and origin of their behavior. For example, Moffitt’s adolescent-limited offenders may specialize in relatively more benign forms of antisocial behavior (e.g., status offenses, substance use), while life-course persistent offenders commit an array of offenses over a longer period of time.

The criminal career view of distinctions in offender type has been criticized by proponents of more general views of crime and criminals. Such criticism extends even to the use of the term career, which implies some coherent pattern of behavior (Gibbons, 1988). Osgood (2005) summarizes this discussion in terms of the emphasis on generality and explanatory simplicity as a means of advancing understanding versus the criminal career alternative that looks at a series of specific benchmarks (e.g., onset, persistence) and considers individual differences in those properties. Specialization and versatility in offending represent one such parameter. The discussion also presents an issue for further consideration in terms of understanding the nature of criminal behavior. Consequently, the question of specialization serves as a touchstone for criminologists who hold different views regarding the nature and etiology of criminal behavior.

Empirical Evidence

As noted above, some key theoretical propositions in criminological research assert different degrees of specialization in offending. This has given rise to a fair amount of empirical research seeking to examine the presence or absence of such properties of offending. These studies have drawn on a variety of methods, and in many cases their differences may be partly attributable to alternative approaches to measurement or analysis.

Most studies tend to find only marginal levels of specialization in offending. Gibbons (1988) suggests that the typological schemes offered by criminologists and practitioners tend to lack support when attempts are made to identify cases that provide a “real world” match to their expectations. In general, the literature on this aspect of the criminal career tends to indicate fairly low levels of specialization among offenders (Piquero, Farrington, & Blumstein, 2003). Relevant early research, for example, found that having information about the number and type of an offender’s previous crimes was not a good predictor of later offenses (Wolfgang et al., 1972).

Farrington, Snyder, and Finnegan (1988) studied the question of specialization in a sample of juvenile offender court records. They found an average coefficient value of .10, which was statistically significant, but quite small relative to the value of 1.0 that would be expected in the case of complete specialization in offending. They also compared the actual number of specialists for each offense to that expected by chance and found that only about 20% could be designated as specialists. While identifying somewhat higher levels of specialization in an adult sample, coefficient values identified by Blumstein et al. (1988) were all below .50 and were generally closer to .20. As in these studies, Paternoster et al. (1998) identified fairly low levels of specialization in juvenile and adults when they analyzed crime patterns for a sample of British offenders.

Using the Philadelphia Birth Cohort data set, KempfLeonard (1987) examined five measures of specialization (e.g., specialization coefficient, transition probabilities) and found that, in general, offenders exhibited low levels of specialized criminal activity. Piquero and colleagues (Piquero, Paternoster, Brame, Mazerolle, & Dean, 1999) and Mazerolle and colleagues (Mazerolle, Brame, Paternoster, Piquero, & Dean, 2000) examined the relationship between specialization and versatility of offending and key covariates (e.g., age of onset, gender). Mazerolle et al. calculated diversity index values across three crime types and found observed values ranging from about .28 to .50, depending on the number of offenses, suggesting fairly diverse offending profiles. Piquero et al. utilized several measures (e.g., diversity index, transition matrices) and similarly found fairly low degrees of specialization. For example, the specialization coefficients were below .35 in all cases (with 1.0 representing complete specialization). Focusing on violent specialization, Piquero (2000) likewise found no discernable evidence that certain offenders tend to concentrate on such offenses and suggested that individual volume of offending is the primary correlate of whether offenders are violent or nonviolent.

Some recent evidence suggests that the question of specialization in offending may be more nuanced than first thought. For example, in work drawing on ethnographic accounts, Shover (1996) found that, in contrast to specialization over the entire criminal career, offenders make shortterm adjustments in offense patterns that suggest some degree of temporary specialization. Sullivan, McGloin, Pratt, and Piquero (2006) report a similar finding with the diversity index in a sample of incarcerated felons when studying short pockets of time (e.g., months and years). Francis et al. (2004) also found evidence of offender “types” when studying longitudinal crime patterns in a U.K. offender sample. Like Shover, both of these studies also suggested that criminals may shift their offending preferences over time, which would aggregate to versatility over a career.

Using a novel measurement and analytic approach, Osgood and Schreck (2007) report clear and consistent evidence of specialization in violent offending. Their study incorporated self-reported delinquency measures across three prominent studies of juvenile delinquency (e.g., Monitoring the Future). They also found that the propensity to specialize in violent behavior was relatively consistent over time and was systematically related to some expected correlates (e.g., risk seeking). Thus, they assert fairly strong evidence of specialization using their approach.

It is also important to consider different correlates of offending patterns that might be important in connecting this aspect of the criminal career to theoretical propositions and policy considerations. Piquero et al. (1999) and Mazerolle et al. (2000), for instance, attempted to tie the degree of specialization to important aspects of the broader criminal career. Lynam, Piquero, and Moffitt (2004) looked at specialized offender groups and found some differences in the extent of their respective developmental risk factors (e.g., childhood conduct).

Although these findings do suggest some degree of specialization in offending, it is important that they are viewed in the context of a larger group of studies that have identified offenders who tend to commit a variety of different criminal acts over their careers. Britt (1994) suggests that researchers sometimes lose sight of the fact that even studies identifying purported specialized offenders tend to suggest that they represent a fairly small portion of the overall pool. So, in considering empirical results, it is important to keep the findings of generality in criminal careers in mind but also consider potential segments of that career where there might be specialized pockets of offending.


As research on the question of specialization and diversity in offending has grown, so too have discussions about how it should best be studied and measured from a methodological perspective. The question of specialization is one of a few areas related to criminal career research that has generated a fair deal of disagreement among criminologists. This debate has often played out in arguments about how best to measure and analyze specialized or diverse patterns of offending. Consequently, several different measurement and analytic approaches have been proposed and utilized in the study of this property of the criminal career.


Some early specialization research relied on the use of transition matrices, which are aggregate measures indicating how often an offender “switches” from crime to crime over the course of a criminal career (Blumstein, Cohen, Das, & Moitra, 1988). Such studies provide a sense of the average sample probability that individuals would switch offenses or maintain the same type across a series of arrests. Although one can examine a number of transitions, this method has been criticized as it may give too much weight to consecutive offenses at the expense of a broader sense of offending patterns. For example, if a substantial proportion of an offender’s arrests are for the same crime type but they are not ordered, this approach might still suggest a generalized offending pattern. Alternatively, an offender with a smaller proportion of arrests for the same crime, occurring consecutively, would demonstrate a greater degree of specialization on that measure.

Farrington (1986; Farrington et al., 1988) introduced a single summary measure drawn from transition matrices called the forward specialization coefficient (FSC) (see also Paternoster et al., 1998). This measure extends the traditional transition matrix approach to include observed and expected frequencies in an offense array so that the pattern identified in the data can be benchmarked against what one would expect if there were no relationship between consecutive offenses. Using these basic elements, one can then calculate an estimate that reflects cases where there is no specialization in offending (i.e., the observed values perfectly match those expected when there is no relationship between consecutive offenses) ranging to those with complete specialization (i.e., offense two is the same as offense one in all cases). While this approach provides a concise summary, it has also been criticized because its interpretation is somewhat murky and it lacks statistical properties that would open it up to further analysis (Britt, 1996).

Another criticism of both the transition matrix and specialization coefficient approach is that they are aggregate measures of offending patterns. As such, their interpretations are based on the offense patterns at the group level, not for individual offenders. As a result, attempts to make statements about individual patterns of specialization or versatility may lead to mistakes in the attribution of results to individuals versus groups (Piquero et al., 1999). This may be problematic since the criminal career framework (Blumstein et al., 1986), which in part prompted renewed interest in these questions, generally focuses on individual patterns of offending, not on aggregate crime rates. So, the transition matrix, and those measures derived from an aggregate array of offenses, may get away from the foundation of criminal careers research, which tends to seek out more individual-centered explanations for the nature and etiology of offending.

Partly as a result of this limitation, some have used the “diversity index,” which provides an estimate of the likelihood that any two offenses selected from an individual’s offending profile will differ (Mazerolle et al., 2000). The total number of offense types included in the measure creates an upper boundary on its value, and the lower value lies at zero. A diversity index value of zero indicates that the offender engages in complete specialization (i.e., all offenses come from the same category). As the value of the index increases toward its upper limit, the offender is identified as having committed a greater variety of offenses (i.e., covers more categories). One of the key features of the diversity index is that it measures offense specialization at the individual level (Mazerolle et al., 2000). So, relative to other measures, the diversity index approach is a closer fit to criminal career research and life course criminology. Also, unlike transition matrices, the order of offenses does not figure into its calculation, so the same offense type need not occur successively to denote specialization. Rather, it considers the pattern of offenses in the particular time period as a whole and then makes an assessment regarding their mix (or lack thereof). It can also be incorporated in further statistical analysis of individuals. Still, this measure may be influenced by the number and type of offenses included in its calculation and also the observed time window (Sullivan et al., 2006).

Analytic Approaches

Osgood and Schreck (2007) outlined a number of characteristics necessary for the appropriate measurement and analysis of specialization in offending. According to these authors, key facets in the measurement of specialization include a focus on the type of crimes committed rather than their ordering, definitions of specialization at the individual level, separation of specialization from the more general frequency of offending, and consideration of an individual’s pattern of offending in relation to the prevalence with which particular offenses are committed in the sample as a whole. These suggestions were drawn from a review of the methodological difficulties apparent in previous studies of specialization.

Working from this foundation, Osgood and Schreck (2007) used a statistical modeling approach that nests individual offense patterns within individuals to determine whether or not they engage in more offenses of a particular type (violent vs. nonviolent) than would be expected by chance alone. In this approach, specialization is captured by an individual’s relative balance of violent and nonviolent offenses. This method also allows researchers to account for other factors, such as the offender’s overall frequency of criminal activity, which may be important in the study of specialization.

Another method of assessing specialization, adopted in some inquiries, involves statistically identifying individuals who fall into relatively distinct categories of offending. One statistical modeling approach that can provide some evidence of both prevalence and type of offending patterns is latent class analysis. This method draws on observed offender response patterns to conditionally place individuals into classes based on specific offending types. In this procedure, an offender is asked (or his or her record might be checked) whether he or she has committed any of an array of offenses, and a statistical model is then used to place the offender in a class based on those responses. For example, Britt (1994) found that individuals in the Seattle Youth Study were best represented by two groups, which largely reflected those who committed delinquent acts and those who did not. This suggests a lack of specialization, providing support for general theories of offending. More recently, Francis et al. (2004) undertook a similar type of analysis with a wider array of offense categories and found distinct classes such as shoplifting offenders and fraud/general theft offenders. Where measures like the diversity index and forward specialization coefficient demonstrate the level of observed specialization, these latent class models can help researchers to determine the nature of the offense type clusters that may be present in the data. These models also offer extensions that allow researchers to discern whether and how offenders may move or transition across offending types over time. Knowledge from such an approach can help in understanding findings of specialization at fixed points in time in the context of versatility over the longer criminal career.

Time Window

While several studies have shifted away from aggregate, population-level measures of offending specialization, they still tend to aggregate offending careers across a number of years. Sometimes studies pool offenses across more than a decade’s worth of time. Therefore, despite the use of measures that tap into individualized offense patterns, the findings that emerge from studies of specialization must be considered in relation to the time frame in which the data are viewed. One recent study, for example, found that the observed level of specialization shifted gradually as the time window was lengthened (Sullivan et al., 2006). This point is also partly supported by Osgood and Schreck’s (2007) finding of lower stability in individuals’ likelihood of violent specialization as the range of time in focus grew longer.

As mentioned above, operational definitions concerning specialization vary from study to study in terms of the use of time frames and also the order of offenses. Clearly, the degree to which the length of offending career is aggregated or disaggregated in a particular study may impact the degree of specialization or diversity observed. It might also be necessary to use longer time intervals in order to obtain enough volume in individual offending to make an assessment about the presence or level of specialization. As a result, it is important to contextualize findings of specialization and diversity in their observed time window and consider whether a full portrait of the criminal career is desired or a segmented view of some portion of that career is more relevant. Differences in the time window studied will inherently impact findings, so it is important to take note of how specialization is being assessed in a particular study so that it can be properly considered in relation to theory and practice.

Data Source

Offending has traditionally been measured in two main ways: self-reports and official records. Both have identified strengths and weaknesses for understanding the general prevalence of offending, which also extend to the study of criminal specialization. While official records may allow for the ordering of particular offenses and facilitate transition matrix-based analysis of offense patterns (Farrington et al., 1988), they also have shortcomings in terms of understanding the full scope of offending. It is well understood that official records capture only a fraction of the overall offenses committed by an individual. In the case of the study of specialization, this has important implications as it may distort the observed pattern of criminal activity toward more serious offense types and not fully capture the array of crimes engaged in by individuals. For example, Lynam et al. (2004) compared levels of specialization in violent offenses across official records and self-reports. Specifically, they examined the observed distribution of violent acts relative to what would be expected based on the overall distribution of offenses to determine whether their values differed, thus indicating specialization. They found evidence of violent specialization in selfreports, but not in official records.

Osgood and Schreck (2007) suggest that self-report information is preferred to the use of official records in studying specialization because it provides more depth of coverage of individual offense patterns and is less susceptible to biases of the type mentioned above. Still, there may be some problems with the use of self-reported offending as a foundation for understanding specialization. Bursik (1980) notes, for example, that self-report data may carry a specific form of risk in the context of such research. He argues that an offender who commits fewer offenses is likely to have better recall regarding their precise nature than a high-rate offender. This results in estimates of offending patterns that may be systematically biased. Clearly, for both official records and self-report methods, any potential systematic inclusion/exclusion of particular offense types might distort findings related to the question of specialization. So, as with offending prevalence in general, it is important to consider the source of the data in making sense of estimates of offending diversity and specialization.

Underlying Classification of Offenses

In addition to the methodological considerations noted above, the categorization of offenses that underlies a particular measure of specialization has important implications for research findings. Miethe et al. (2007) point out that the potential for finding specialization will decrease as the array of offenses included in its calculation increases. This owes to the fact that finer classification of offense types inherently reduces the likelihood that the same offense(s) would be observed on multiple occasions. Often, researchers investigate specialization based on some configuration of the three overarching crime types—violence, property, and drugs (see Mazerolle et al., 2000). Studies relying on broad “violence,” “property,” and “other” categorizations of crime might generate less observed diversity than those that fully distinguish among offenses that fall within these wider umbrellas (e.g., auto theft, larceny, and fraud for “property”). Also, the manner in which offenses might fit together empirically may be different from initial researcher and practitioner perceptions about which types are similar.


Despite some questions from theorists and empirical researchers, the categorization of offenders nonetheless retains some appeal, particularly among policymakers, practitioners, and the general public (see Lieb, Quinsey, & Berliner, 1998; Matson & Lieb, 1997). Differential incarceration and treatment plans, for instance, often assume some offender types, whether in terms of specific groups like sex offenders or the general consideration of violent/nonviolent behavior. Legislation directed against particular subgroups of offenders may be enacted when public outcry regarding a well-known case is high. Simon (1997) points out that few outside the research community understand that offender specialization is a relatively rare occurrence. Consequently, an understanding of the degree to which offenders specialize can inform debates about key aspects of justice processing and crime prevention (Blumstein et al., 1986). For example, specific sanctions and treatment for drug offenders may be more palatable (and effective) if it can be demonstrated that such offenders tend not to engage in other offenses that pose a greater threat to public safety.

Clearly, attaching generalized labels to offenders offers practitioners and policymakers an opportunity to simplify decision making because officials can develop a template for dealing with a particular group and then apply that approach to those who fall in that classification. At the same time, in practice, the viability of this perspective does in part hinge on the reality of whether or not such types, or at least concentrated behavioral patterns, truly exist. Thus, this becomes an important information point in balancing the cost and benefit of particular forms of justice response to such offenders. Although there are others (e.g., domestic violence offenders) (Simon, 1997), two groups that have been a specific focus of policy-related discussion of specialization are drug offenders and sex offenders.

Drug Offenders

The question of sentencing for low-level drug offenders has been a matter of some debate. Very little empirical literature has examined this question in relation to its specialization assumption, however. DeLisi (2003) addresses the argument of some who contend that there is a group of “drug offenders” who are generally nonviolent and do not pose much of a threat to public safety as they are only users. The held belief is that imprisoning these individuals represents an overly harsh and wasteful reaction on the part of society. DeLisi examined the criminal arrest histories of 500 detained offenders to assess whether their offending careers reflect specialization in drug offenses or generalized criminal behavior. He found that drug offenders, characterized as having a history of imprisonment for drug use/possession, tend to engage in a variety of offenses over their careers and were actually more likely than nondrug offenders to engage in violent, property, and nuisance offenses. Based on these findings, DeLisi casts doubt on the contention that drug offenders are specialized and therefore should be treated with leniency because they pose little risk to society. Unfortunately, work in this area is quite limited, so it is important that the issue is studied further in the context of specialization. Still, the issue of response to drug offenders provides one example of the important policy discussions inherent in the study of specialization and versatility in criminal behavior.

Sex Offenders

Due in large part to its policy implications, specialization among sex offenders has received a great deal of attention (e.g., Lussier, 2005; Miethe et al., 2007; Simon, 1997). This comes at least in part because there has been a move toward specialized sanctions and treatment for sex offenders (Simon, 1997). For example, Megan’s Law provides for public notification regarding known sex offenders in the community, and civil commitment procedures permit authorities to house these offenders in secure, residential treatment facilities beyond the completion of their criminal sentence if they are still deemed to be a threat to the community. Both approaches imply some degree of specialization.

Specialization studies focused on sex offenders seek to better understand whether this particular group of offenders differs from others in ways that justify considering them as a distinct group for the purposes of treatment and sanctions. Studies by Lussier (2005) and Simon (1997) looked at this question specifically. Using the proliferation of measures aimed specifically at sex offenders as a pretext, Lussier undertook a comprehensive review of the literature related to specialized and generalized offending among that group. Based on studies that compare recidivism rates for sex offenders to those of nonsex offenders, he concluded that sexual crime does not engender greater levels of specialization than other crimes. Other studies did find, however, that there were differences in degree of specialization between offenders who victimized children compared to those whose offenses were perpetrated against adult women. Overall, Lussier reported mixed findings with regard to specialization and generality in offending but states that sex offenders do not appear to limit themselves strictly to that area of criminal behavior. Simon (1997) came to a similar conclusion in a review of the literature on domestic violence offenders, sex offenders, and general population criminals. In looking at sex offenders specifically, she indicates that findings of generality in criminal histories are often implicit even in studies that treat them as a distinct group.

Drawing on a similar premise as Lussier (2005), Miethe and colleagues (2007) examined the offending patterns of a large sample of sex offenders released from prison in the mid-1990s. They found that, in comparison to other offender groups, sex offenders exhibited lower probabilities of committing the same offense for adjacent arrests. Furthermore, they identified offending versatility regardless of whether they examined raw probabilities of repeated sex offenses, the forward specialization coefficient, or the diversity index. Thus, the sex offenders in this large, nationally representative sample did not appear to be “persistent specialists” as is often assumed. Studies of the type mentioned here further emphasize the importance of fully considering the available evidence regarding potential specialization of offending in determining whether policies aimed toward specific groups of offenders are likely to be successful.

Future Directions

In the future, further consideration of specialization of diversity and specialization of offending is needed to inform criminological theory and public policy and practice. Presently, much of the research regarding criminal specialization is descriptive in nature and does not necessarily consider the context of contemporary theory and practice. On the theoretical side, it is important to consider this characteristic of the criminal career in the context of emerging integrated perspectives. For example, identification of specialized offending patterns in shorter segments of the life course may provide important insights regarding situational influence on criminal behavior. At the same time, stringing shorter portions of the life course together will likely show an aggregate pattern of versatility. The process of merging those segments may provide greater insight into the trajectory of offending careers by identifying the social circumstances in which an offender might limit or expand his or her repertoire of offenses.

In the case of sex offenders, Lussier (2005) notes that limitations in the existing literature may help explain the disparate findings observed in his review. This is also likely the case in more general considerations of specialization and diversity (e.g., Osgood & Schreck, 2007; Sullivan et al., 2006). Lussier criticizes existing studies for not fully capturing the key theoretical issues surrounding specialization in offending. Specifically, researchers have not focused enough on within-individual change across the criminal career in their consideration of the issue of specialization. He suggests that use of the knowledge drawn from the foundation of the criminal career framework is an important aspect of understanding findings that demonstrate both generality and specialization in sex offender careers (see also McGloin, Sullivan, Piquero, & Pratt, 2007; Sullivan et al., 2006). Such a focus may help to reconcile patterns of short-term specialization with the more abundant findings of versatility in criminal careers.

As with much research in criminology, and the social sciences generally, it is important to assess substantive findings in the context of the samples and methods used in specialization studies. A variety of approaches have been used in order to assess the presence and nature of this property of criminal careers. The use of different measures, analytic approaches, and data sources has caused some difficulty in terms of fully understanding the level of specialization and its meaning for theory and policy. Gradually, emergent methods have begun to deal with some of the criticisms raised about the existing evidence. For example, Osgood and Schreck (2007) identified a variety of guidelines for studying specialization that may serve as a foundation for future research. It is important that findings from studies of specialization are viewed in relation to their operational definitions, time window, data source, the number and type of offenses used, and the analytic procedure.

Policymakers and practitioners make implicit assumptions about categorizations of offenders that allow them to execute prevention, processing, sentencing, placement, and treatment planning. These assumptions allow for simplified decision making, but they may also limit the ability to deal with offending problems in the long term. For instance, treating a versatile offender who just happened to commit a sex offense using a modality designed around assumptions regarding the psychological makeup of “sex offenders” may contribute to a squandering of already limited justice resources. Similarly, a policy that treats drug offenders as a pure category and deals with them as if they are specialized offenders may compromise public safety if those underlying assumptions turn out to be false. As Miethe et al. (2007) point out, if policies and treatment approaches for criminal offenders are based on false assumptions regarding specialized behavior patterns, they may be destined for failure before they are even initiated. Clearly, in the future it is important that policies and practices built on assumptions of specialization be carefully considered in relation to information obtained from actual offenders. In addition, theory and existing empirical findings should be utilized to explain why particular types of offenders might be more or less likely to specialize.


The notion that offenders can be categorized in some fashion has been around for as long as criminal behavior has been studied. The potential for specialization continues to be a question of some importance in contemporary criminological theory and justice policy. The ongoing study of criminal careers has led to a great deal of further research in this area. Still, many scholars believe that offenders commit criminal acts based on desire for short-term pleasure and do not regularly engage in specific patterns of offenses. A variety of research methods and analytic approaches have been used to examine this research topic; some focus on offense patterns across groups of offenders, and others attempt to assess specialization and diversity of offending at the individual level. Empirical research tends to suggest that the majority of offenders generalize over the course of their active criminal careers. This raises some questions about theories, policies, and treatment modalities that assume offender types.

Some recent evidence, however, suggests that offenders may specialize during short time periods of their overall careers. Therefore, it is important that further research on specialization accommodate the fact that most offenders will commit a variety of different types of crime over their careers, but the specific nature of those activities may be tied to particular life events and situations. Emerging measurement strategies and analytic techniques offer some opportunity to better understand this property of criminal careers. Incorporating these approaches with greater emphasis on theoretical understanding has the potential to advance knowledge of the degree to which criminal specialization exists and its origins. This area of criminological study has some important implications for how offenders are sanctioned and treated by the justice system as well and should be pursued further to inform responses to crime.