The traditional approach taken by police investigators to making inferences is the one that has always been characterised in crime fiction as deduction. This is the process of reasoning from commonly known principles. For example, if a walking stick is found with strong, large teeth marks on it then the it may be reasoned that this was most likely caused by a large dog that carried the stick (as Sherlock Holmes reasons in The Hound of the Baskervilles). A subtler piece of reasoning may come from the knowledge that an offender had long nails on his right hand but short ones on his left. This is a pattern favoured by some guitarists and so it may be assumed that the offender was a serious guitar player.
However, as attractive as such deductions are in fiction they are a very poor basis for developing robust inferences in real-life crime. They are vulnerable to the knowledge and reasoning ability of the deducer and the particular features that they notice. Even more importantly, they may be worthless. It turns out that many trades give rise to people having longer nails on one hand than the other and so the inference of a guitarist could be very misleading. A dog may have bitten a walking stick in situations other than carrying, and so may not be directly associated with the owner of the walking stick.
In order to determine what the salient aspects of an offence are and how they may be validly related to useful investigative inferences, it is necessary to collect information across a range of cases and to test hypotheses about the actual co-occurrence of various features. This is the process of inductive reasoning that is at the heart of empirical science. Investigative psychologists have consequently been active in conducting a wide range of empirical studies aimed at providing objective bases for investigative inferences. These are studies that have been characterised by Canter (1995b) as attempts to solve the set of equations that link the Actions that occur during the offence, including when and where it happens and to whom, to the Characteristics of the offender, including the offender's criminal history, background and relationships to others. These have become known as the A v C equations, or the 'profiling equations', where A are the Actions related to the crime and C are the Characteristics of typical offenders for such crimes.
Studies of these equations have given rise to the identification of a number of aspects of criminal behaviour that are crucial to any models of inference for effective use in investigations. One recurring conceptual basis for these models can be seen as an elaboration of routine activity theory in which it is hypothesised that offenders will show some consistency between the nature of their crimes and other characteristics they exhibit in other situations. This is rather different from the many psychological models that attempt to explain criminality as being a product of psychological deficiencies (Farrington, 1998). The inference models used for profiling are less concerned with the prediction of criminality than with unravelling the structure it takes and how that structure connects with features of the offender that will be of interest during an investigation.
Rather than being concerned with particular individual clues, as would be typical of detective fiction, these inference models operate at the thematic level. This approach recognises that any one criminal action may be unreliably recorded or may not happen because of situational factors. But a group of actions that together indicate some dominant aspect of the offender's style may be strongly related to some important characteristic of the offender. Davies, Wittebrod and Jackson (1997) showed the power of this thematic approach. They demonstrated, from their analysis of 210 rapes, that if the offender took precautions not to leave fingerprints, stole from the victim, forced entry and had imbibed alcohol, then there was a very high probability, above 90%, that the offender had prior convictions for burglary.
Unfortunately Davies et al. (1997) do not provide a detailed structural analysis of the relationships between all the activities that they considered. They used a logistic regression that searches through the data to find the best matches, so that low-level relationships that may add up to provide a stronger picture than any individual indicator, generating an overall picture, may be ignored. Actions brought together by Davies et al. (1997) to predict prior burglary indicate an offender who is determined to commit the crime and get away with it, treating the victim as a resource or 'object' rather than a significant person. When seen in this light other aspects of the assault may be recognised as relevant beyond the limited indicators thrown up by the logistic regression.
Salfati and Canter (1999) examined all the actions together with the offenders' characteristics in their study of 82 stranger homicides. Their analysis did reveal consistency in the themes across actions and characteristics. As with Davies et al.'s (1997) study the clearest associations of criminal actions were with previous offence history. Those murderers who stole non-identifiable property, who were careful not to leave forensic evidence and who hid or transported the victim's body, were more likely to have had a custodial sentence, but interestingly were also more likely to have served in the army.
The most developed exploration of thematic inference hypotheses is Canter and Fritzon's (1998) study of arsonists. They developed scales to measure four themes in the actions of arsonists derived from their action system model. They developed a further four scales to measure themes in the background characteristics of the 175 solved arson cases they studied. Their table, relating measures on all four background scales to all four action scales, showed that the strongest statistically significant correlations were, as predicted, between actions and characteristics that exhibited similar themes, and lowest between those that did not.
These studies of inference are therefore slowly beginning to provide a basis for a more general theory of offender consistency. But they suffer from dealing with the criminal as an individual independently of the social or organisational context in which he or she operates. As Canter and Alison (1999c) have argued, the social processes that underlie groups, teams and networks of criminals, can reveal much about the consistencies in criminal behaviour and the themes that provide their foundation. A clear example of this is the study by Wilson and Donald (1999) looking at the different roles that are taken by teams of 'hit and run' burglars. They demonstrated, for example, that the offender who was given the task of driving the get-away vehicle was most often likely to have a previous conviction for a vehicle-related crime. In contrast, the criminal assigned the task of keeping members of the public at bay, or controlling others who might interfere with their crime, the 'heavy', was most likely to have a previous conviction for some form of violence offence.
These results of consistency between social role and other forms of criminal endeavour are thus in keeping with the general thematic framework that is emerging through the studies of actual actions in a crime. They lend support to a general model of criminal activity that recognises the specific role that criminality plays in the life of the offender. It further supports the perspective that for the sorts of offenders considered in the studies cited, the style of criminality is an integral, natural part of the criminal's general lifestyle, not some special, atypical aspect of it.
As mentioned above when discussing the information at the heart of Investigative Psychology, one important aspect of these models is that the variables on which they can draw are limited to those of utility to police investigations. This implies that the A variables are restricted to those known prior to any suspect being identified. The C variables are limited to those on which the police can act. So an offender's personality characteristics, detailed measures of intelligence, attitudes and fantasies are all of less utility than information about where the person might be living, his or her criminal history or domestic circumstances.
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