Data collected from actual police files on 54 American serial killers is presented. The study applied concepts from environmental psychology along with multidimensional scaling techniques to analyse the spatial distribution of 54 American serial killers. In addition, this is the first empirical study to measure the angles of each crime location as seen from the offenders' home/bases.
The research hypothesises are:
The urban migration patterns of American serial killers are sectorally biased toward locations significant to the killers and are wedge-shaped. The angles of crimes, geographically, as seen from the serial killers' home/bases, will be less than 180 degrees.
On analogy from the apparent dark space in the Milky Way, the term coal-sack effect (CSE) is applied to areas immediately around the killers' home/bases where no crimes have been committed. This is termed a buffer zone in geography. A coal-sack effect will exist for the victims' body dump locations, but not for the victims' abduction sites.
The further American serial killers travel from familiar environments, the frequency of crimes increase. Distance decay theory suggest that the further offenders travel away from familiar environments the frequency of crimes decline rapidly. The concept of distance decay has its foundation in the theory of least-effort principle. The least-effort principle means that when multiple destinations of equal desirability are available, all else being equal, the closest one will be chosen. The least-effort principle will influence where victims are geographically targeted, but not where victims' bodies are disposed.
Hypothesis one: The descriptive and multidimensional scaling results on 54 American serial killers found that their spatial distribution forms wedge-shape patterns similar to consumer shopping patterns, not circular as current geography of crime literature suggest. To further test the research findings, a computer programme was written to measure the angles of the crime locations as seen from the killers' home/bases. The first angles were measured counter-clockwise using the two furthermost crime locations from each series. The second measurements were the acute (shortest) angles between each of the crime locations as seen from the killers' home/bases. The findings support hypothesis one that the distribution patterns are wedge-shaped. The counter clockwise measurements for 40 of the serial killers had a mean of 40 degrees to 60 degrees. The acute angles measurements had a mean of 30 to 40 degrees. Measuring the angles further supported the hypothesis that the distribution patterns were in fact wedge-shaped. The wedge-shaped theory is defined as a sectoral mental map and similar migration patterns have been observed in consumer shopping patterns. This new theory has major implications for police investigations.
The research found that when serial killers make environmental choices involving locational considerations, their mental maps are used as preferences in determining which areas to forage for potential victims and dispose of their bodies. When a change of hunting areas or body dump sites are sought, the killers will be confined by their mental map derived from daily and weekly travel patterns.
Hypothesis two: The results found that a coal-sack effect exist for the victims' body dump sites, but not for the victims' abduction sites. The findings do not support the literature on buffer zones which suggests that in many instances there will be an area around an offender's home/base that they perceive as being of high risk for offending, and where less offences are likely to occurs. Hypothesis two was confirmed.
Hypothesis three: The distance decay theory suggest that the further criminals travel out from their home/bases the frequency of offences decline rapidly. No evidence for the distance decay theory was found in this study. In fact just the opposite occurred. Serial killers travelled outwards with increasing increments from their home/bases to commit their crimes. The further distances killers travelled the frequency of killings increased. The killers' environmental range starts only to decrease later in the series. Similarities were found with the victims' body dump sites. It is only with body dumps later in the series that a gradual decease occurred in the distances travelled from the killers' home/bases. The findings support hypothesis three.
The geographical patterns of American serial killers suggests that we are not dealing with economic killers, with their perfect knowledge, but real individuals with imperfect knowledge and a predilection for satisfying behaviour.
The research is applicable to researchers and police investigations. Some examples of the research benefits are outlined below.
Suspect prioritisation: All serial murder investigations involve a lengthy list of suspects. Sometimes the list can have up to and as many as two thousand suspects. For the police trying to narrow down the suspect list, in an attempt to apprehend a serial killer, is tedious and difficult. Results obtained from this research will assist police investigators in prioritising suspects and linking cases, which could result in an early arrest.
Patrol situation: The geographical analysis in this research will aid police in determining the most probable areas associated with the offender. The research findings show that crime patterns are wedge-shaped, not circular. The research will be particularly effective in determining the killers' geographical areas. Police can give priority to the areas within the triangle by using such methods as canvassing efforts, area searchers, community co-operation and media campaigns.
Multi-agency task force: Often times during a serial murder investigation task forces are formed with the hopes of linking cases and arresting a suspect. Geographical profiling can assist in these tasks through helping police prioritisation of street addresses and postal codes. Knowing from research, for example, that a serial killer's geographical range spreads further outward as their crimes become more violent could assist police in predicating where the next victim might be targeted.