otherwise it drops to 1.6. This appears to suggest a desire on the part of offenders to distance the remains of victims encountered relatively close to home.

While only 61% of all sites were known to police, this figure increased to 67% for body dump sites and 70% for encounter sites. These two location types are the most important in geographic profiling, and it is usually sufficient to know one or the other for the purposes of a criminal geographic targeting analysis (see Table 10.7). Approximately 12% of the crime sites were within the killer’s residence. Such locations are likely unknown to police.

Multiple responses were allowed for site description, and 238 encounter site types, 144 body dump site types, and 476 total site types were recorded. Streets and residences are the most common crime locations. The “other” classification typically refers to deserted lots or waste ground. Residential and commercial land use predominated. The majority of incidents occurred in outside public places, followed by inside private places. Serial murderers prefer to travel by vehicle.

9.4.4Crime Parsing

A crime is often treated as having a single location, but depending upon crime type there may be various sites connected to a single offence. These have different meanings to the offender and, consequently, distinctive choice properties. For serial murder these location types include victim encounter, attack, murder, and body dump sites. For serial rape they include victim encounter, attack, rape, and victim release sites. Serial arson normally involves only one location, the fire setting site. While these particular actions can all occur in a single place, the majority of cases involve two or more locations.

Eight possible combinations, called crime location sets, result from the four different murder site types. Breaking an offence down into its crime location set is referred to as crime parsing. While the specific location set for a given crime is a function of victim selection and encounter site characteristics, it also implies something about the offender’s mobility, search strategy, and level of organization. Generally, the greater the organization and mobility of an offender, the greater the potential complexity (i.e., the more separate locations) of the crime location set.

Table 9.7 presents the percentage breakdown for the eight crime location sets. Movement between victim encounter (E), attack (A), murder (M), and body dump (D) sites are represented by an arrow (→). For example, EAM→D indicates that the encounter, attack, and murder sites were in the same place, but the body dump site was in a different location.

There was a high level of geographic consistency in the M.O. of this sample as most offenders repeatedly employed the same crime location set. Approximately 85% of the total number of serial murder victims (n = 178)

© 2000 by CRC Press LLC

Table 9.7 Crime Location Sets

fell into the most common crime location set used by their killer, and 96% into either the first or second most common crime location set. This implies that crime location set might be used as an assessment characteristic for the linking of serial offences.59

An understanding of consistency, change, and progression in offender behaviour is an important principle of linkage analysis. Warren et al. (1995) found about half of the 119 quantified behaviours they examined from serial rape crime scenes remained consistent over time. Other offender behaviours either showed progression (e.g., degree of planning, protection of identity, and use of bindings), or exhibited inconsistent change. They suggest the pathological aspects are more constant. “The idea of consistency and change represents an important area of serial crime: in theory, it helps to define relevant dimensions of classificatory paradigms, which can inform investigative efforts to link crimes perpetrated by the same offender” (p. 255). Measures of consistency and change in offender geographic behaviour assist in this effort (see, for example, Dettlinger & Prugh, 1983).

9.4.5Clusters

Short-term spatial selectivity has been observed in the hunting behaviour of animals and certain predators repeatedly visit the same forage sites (Smith, 1974a, 1974b; Smith & Sweatman, 1974). Geotropism is also found in serial killers, many of whom return to favoured sites to hunt victims, or dispose of bodies in cluster dumps or forest “graveyards” (Newton, 1992). Beyond convenience, these private “totem places” may also be significant to the offender’s fantasies.

59 The design of ViCLAS incorporates certain geographic profiling principles, and queries can be based on similarities in crime location sets in conjunction with other search criteria.

© 2000 by CRC Press LLC

The Yorkshire Ripper and the Son of Sam were both known to revisit the areas of their previous crimes in pursuit of new victims (Ressler & Shachtman, 1992).

When we went to New York to talk to the ‘Son of Sam,’ David Berkowitz,” says [FBI Special Agent] Robert K. Ressler … “he told us that on the nights when he couldn’t find a victim to kill he would go back to the scene of an old crime to relive the crime and to fantasize about it. Now that’s a heck of a piece of information to store somewhere to see whether other offenders do the same thing. (Porter, 1983, pp. 49–50)

LeBeau (1985) notes “the proclivity of chronic serial offenders to use repeatedly the same geographic and ecological space ... The geographical and ecological patterning of the serial offender may be tangible information which is discerned by police investigators and utilized in apprehension” (p. 397). He determined through nearest neighbour analysis that the mean distance between crime scenes for chronic serial rapists in San Diego varied from 0.12 to 0.85 miles, averaging 0.35 miles (1986). Büchler and Leineweber (1991) observe bank robbers in Germany follow similar patterns of escape, information which may help police connect crimes. Davies and Dale (1995b) found several cases of geographic “backtracking” in their study of British rapists. They propose national access to intelligence information on former crime sites, and people and places of significance to an offender is an important investigative resource. Serial killer Monte Rissell was apprehended only after he returned to his former rape sites in Alexandria, Virginia (Ressler & Shachtman, 1992).

An analysis of the pattern of crime sites in the SFU serial murder study showed a tendency towards aggregation (59% of R scale values were smaller than 1). One of the main influences on divergence from randomness is target backcloth; not surprisingly, a lack of uniformity leads to clustering. Another factor appears to be opportunity. If an offender was successful in a particular neighbourhood once, then why not again. Other potential targets may have been noticed and remembered during the commission of the first crime. In many ways, these influences are similar to those documented in repeat victimization studies of property crime (Farrell & Pease, 1993; Pease & Laycock, 1996). Fantasy also plays a role in drawing an offender back to a particular location. A crime site situated close to a previous offence is referred to as a contagion location, and is generally regarded for the purposes of geographic profiling as a nonindependent event.

© 2000 by CRC Press LLC

9.4.6Trip Distance Increase

While it has been suggested individual offender crime trips increase in distance over time, there has been little empirical testing of this hypothesis. In an attempt to see what influence, if any, time had on the journey to crime, distances travelled by serial murderers, both individual and collective analyses were conducted. This approach is limited, however, as any particular journey to crime may not have originated from the offender’s residence. Also, a killer could have been responsible for unknown murders, in addition to preceding crimes such as burglary, robbery, or sexual assault. And some offenders moved residence in the midst of their murder series, complicating journey- to-crime comparisons.

The Manhattan distance from offender residence to location of victim encounter was measured for every known crime in each killer’s series. Where two or more victims were attacked during the same day, only the first crime was counted. The offences were chronologically ordered, outliers excluded, and significant gaps ignored. Distance from offender residence to encounter site was plotted against crime number (see Rossmo, 1995a).

Table 9.8 summarizes these results. Half of the serial murder cases showed a significant increase (defined as a slope with an R2 > 0.300), while the other half exhibited no significant change; none showed a significant decrease in crime trip distance. Average journeys varied considerably by offender (ranging from 1 to 40 km), therefore slopes were compared to each killer’s mean crime trip distance. This figure represents the incremental increase in crime trip distance expressed as a proportion of the mean crime trip distance. The overall mean percentage increase is 16% (standard deviation = 17).

Table 9.8 Crime Trip Distance Increase

Figure 9.5 shows crime trip distance changes in the aggregate. Wide variations in hunting range distort the pooled data, therefore the unweighted mean natural logarithm of crime trip distance was plotted against offence number. Only those cases with a minimum of five crime trips were included in the analysis. When a linear trend line is fitted to the data, a significant increase over time is observed (R2 = 0.519). A better fit is found, however, with a third-order polynomial expression (R2 = 0.880), as shown in Figure 9.5.

© 2000 by CRC Press LLC

Figure 9.5 Mean logarithm of crime trip distance over time.

This raises an important issue. The above analysis assumes that increases over time in crime trip distance are linear, an assumption that may not be warranted. If journey-to-crime distance grows proportionately (e.g., if a given crime trip is, on average, 10% longer than the previous trip), then the relationship is best expressed through a power curve. Distances might also increase in steps or after significant thresholds (as suggested by Figure 9.5). Such a growth process could result from an offender first exploring directional alternatives before increasing crime trip distance. The exact nature of the relationship requires further research, preferably with larger data sets of offenders responsible for lengthier series of crimes.

The FBI maxim that the first crime in a series is the one closest to the offender’s residence was tested. Of the serial murder cases in the SFU research, 50% involved at least one crime trip distance of a mile or less (Chase, Olson, Dahmer, Brady, Brudos, and Collins). First offence was closest in 41% of the cases, and distance to first crime averaged 40% of mean distance (standard deviation = 42). The FBI belief in offender residence proximity to first offence appears to be a reasonable though not universal rule, consistent with the increase of crime trip distance over time. It was found to apply in just under half of the cases.

The argument that journey-to-crime distances increase over time is based on the supposition that offenders learn from experience and increase their spatial knowledge accordingly. Warren et al. (1995) observed that serial rapists who travel greater distances appear to have longer criminal careers; they also found distance indicators to be significantly related to time between successive offences.

© 2000 by CRC Press LLC

Figure 9.6 Crime trip distance mean standard deviation over time.

Expansion in hunting region, however, does not necessarily result in victims being selected exclusively from the perimeter of the search area. It does indicate that increasingly large target areas are available for the offender to hunt within. While this implies crime trip distances lengthen over time (albeit at a rate slower than the growth of the hunting area), it also suggests an increase in the variation of crime journeys. To examine this possibility, the standard deviation for crime trip distance was calculated in each serial murder case for every crime trip after the first. When the mean standard deviations for all cases are plotted against offence number (Figure 9.6), a significant increase is observed over time (R2 = 0.790). This suggests that as a serial murderer’s career progresses, his or her crime trips may increase in both distance and variation.

Fully half of the cases in the study, however, showed no significant increase in journey-to-crime distance over time. Indeed, the nearness and least-effort principles, and the tendency for crime locations to cluster, mitigate against expansion. Assuming learning underlies crime trip growth, why does an offender need to expand his or her spatial knowledge? The most obvious reasons include greater victim search opportunities and lowered risk of police detection. Previous crime sites become tarnished and unattractive because of the risk of increased community vigilance, as shown by the concern of child killer Westley Allan Dodd over losing his hunting ground.60 Displacement, occasioned by police intervention or community response, was responsible on several occasions for the shift or expansion of a serial murderer’s hunting area (e.g., Bianchi, Sutcliffe, Dahmer, Ramirez, and Berkowitz).

© 2000 by CRC Press LLC

Increase in size of hunting area is most likely to occur in cases where an offender has the capability and need to learn, and will benefit from doing so. In this context, those with the capability to learn include organized mobile offenders who possess well-developed mental maps. Offenders with a need to learn include those whose hunting style, body disposal methods, or offence timing result in their crimes being linked, generating community fear and a significant police response. Offenders who benefit from learning include those who hunt close to home or prefer victim types that are available in several different areas (i.e., there is a uniform target backcloth).

The case of Joel Rifkin is a good example of an offender with little need to alter his crime trip distances, which were a consistent 37 kilometres from residence to victim encounter site. Rifkin was sane, drove an automobile, and was familiar with large areas of New York and Long Island. His victims were street prostitutes and their strangled bodies were dumped, often in steel drums, at remote sites that spanned thousands of square miles. Consequently, the murders were not linked and the police failed to realize that a serial killer was operating in their jurisdiction. Rifkin also lived a significant distance from the red-light district in Lower Manhattan where he picked up most of his victims.

60 In times of short food supply, the nomadic Naskapi Indians employed scapulimancy – a technique in which a heat-cracked caribou shoulder is used as a map – to help them find game on the Labrador plateau. Anthropologists have theorized that this divination process produced effective results because of its randomizing effect, preventing grassland and tundra areas from being overhunted (Moore, 1957).

© 2000 by CRC Press LLC