IN 1995, Steve Fortin pleaded guilty to a savage attack on a woman in Maine, in which he broke her nose, bit her, strangled her into unconsciousness and raped her. So when it was discovered that Melissa Padilla, a mother of four from New Jersey, had been found murdered with similar injuries years before, police linked Fortin to the case.
He was convicted, but the New Jersey Supreme Court later overturned the conviction on the grounds that linkage analysis – the technique of connecting serial crimes by similarities in the criminal’s behaviour – lacked sufficient scientific reliability and should not have been used in court testimony. Fortin was later retried without evidence from the linkage analysis expert and is serving a 30-year sentence for Padilla’s murder. The case confirmed what many had suspected: despite being used to generate leads for sex attacks and murder cases, linkage analysis actually had little evidence to back up its claims.
That could be changing. Several groups have developed linkage analysis software and are now carrying out the research needed to establish the technique as a more rigorous tool. The work challenges some of the common assumptions about patterns of criminal behaviour – that serial murderers will mutilate their victims’ bodies in the same way, for example. It also suggests that linkage analysis could be applied to a much broader range of crimes than at present.
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Linkage analysis differs from the “signature analysis” used by specialist police forces such as the FBI. “They’re looking for very, very specific behaviours, or combinations of specific behaviours, many of which are ritualistic and some of which are very bizarre,” says of Carleton University in Ottawa, Canada. These are quite rare, and tend to be confined to only the most disturbing cases. “We’d love it if an offender left the ace of spades after every rape or murder, but it’s just not like that,” says Adam Gregory, behavioural investigative advisor for the UK’s Serious Crime Analysis Squad (SCAS), which helps police to link seemingly motiveless murders, sexually motivated murders or rapes by strangers.
Linkage analysis works on similar principles, but can be applied to a wider variety of crimes. “The assumption is that a given offender will exhibit similar behaviours across their crimes, and that these will be relatively distinct from those of other offenders committing similar types of crimes,” says Bennell.
However, the success of the technique depends on the ability of police officers to recognise these similarities. And this is where the problem lies, because individual officers will form their own subjective impressions of crime situations. What’s needed is a consistent way of categorising and analysing behaviours, and weighing up the probability that they can be used to link crimes.
Bennell has been developing just such a system. It is a computer program that can read a database of crimes in which behavioural domains – such as the method of entering a victim’s property, or items stolen – have been detailed by the police. The program then comes up with a set of behavioural similarity scores for any two crimes, with one rating in each domain. “If the crimes have been committed by the same person you’ll often find higher levels of similarity in certain behavioural domains,” he explains.
The trick is identifying the behaviours to rely on. Some behavioural domains may not be particularly useful for linking purposes, because they are not exhibited in a consistent fashion across crimes. In other cases, consistency will exist, but the behaviours will not be distinct – for example, different burglars may steal very similar items and so property stolen cannot usually tell you if two crimes are linked.
However, after applying the system to a database of solved crimes, regression analysis can provide a weighting to similarity scores, according to how predictive of a link they actually are. This enabled Bennell’s team to work out which behaviours were most useful in linking serial burglaries. By combining the most predictive behavioural domains, the program produces a probability that two crimes were committed by the same offender. In the case of burglaries, for example, Bennell’s team found that a short distance between offences was far and away the most accurate predictor of a connection.
“The program produces a probability that two crimes were by the same person”
Several groups have since used the technique to evaluate the use of linkage analysis in car theft, commercial robbery and sex offences. A different statistical technique has also been used to look at behavioural patterns in serial murders. These studies are turning up a few surprises (see “Criminal behaviour patterns”).
In serial murders, for example, it seems that only the domain of planning and control – activities such as destroying evidence, and bringing gloves or rope to tie up the victim – could potentially be used to link serial crimes, but the method of body disposal, type of mutilation, sexual acts, theft and weapon use are less consistent between these crimes (Behavioral Sciences and the Law, vol 25, p 527).
In serial sex offences, the distance between attacks, methods of controlling the victim and methods of evading capture tend to be consistent between crimes by the same offender, but behaviours reflecting the attacker’s personality are less consistent. “The domains that are coming out as the best predictors are those that depend less on the situation,” says Jessica Woodhams at the University of Gloucestershire in Cheltenham, UK, who led the study and presented the results at a forensic psychology meeting at the University of Leicester, UK, last month. Details like bringing twine to tie up a victim are decided in advance, while what the criminal actually does to them may be influenced by how much the victim struggles, for example.
Bennell hopes the research will result in improved police databases capable of supporting this kind of analysis. “We envision a computer program that provides the police officer with a means for uploading data about specific crime-scene behaviours… likely right at the scene using a hand-held device,” says Bennell. The computer program would then combine and weight the behaviours, and suggest to the officer whether to treat the crime as a linked one or not.
He has some preliminary evidence that this could work. In a recent study, he asked a group of 40 university students and another group of 31 police professionals to look at the crime records for 38 burglaries and decide which were linked. Before taking part, half of the volunteers in both groups were told that the likelihood of two offences being committed by the same offender increases as the distance between the crimes decreases. While this “training” improved the accuracy of their decisions, none of the groups performed as well as the program. “Even when people are told to use information that is useful to them, sometimes they don’t have the ability to identify the correct links,” says Bennell.
Professional crime analysts are not completely convinced, however. Previous research has shown offenders only become consistent over time, says Gregory. “Most of the time we’re trying to link offenders’ first and second, or first and third, offences – a time when offenders will be at their least consistent.”
Bennell stresses that the system would simply be an additional tool in the crime-analyst’s kit. They will still “need to read the victim’s statement to understand the context” in which the crimes were committed, he says.
Criminal behaviour patterns
BURGLARIES
Consistent: distance between offences
Less consistent: method of entry; type of property; goods stolen
ROBBERIES
Consistent: distance between offences; control (working alone or in a team? Aggressive or calm? Did they threaten, restrain or harm witnesses?)
Less consistent: planning (did they bring gloves, wear a disguise, use an escape vehicle?); type of business targeted
SEXUAL OFFENCES
Consistent: distance between offences; escape (use a condom? Wear gloves or a disguise?); control (weapon use, physical restraint of the victim, how they communicated); sexual behaviours
Less consistent: style (behaviours that reflect the offender’s personality or offence style, but which are not necessary for the success of the attack)
CAR THEFT
Consistent: distance between offences; where cars were dumped
Less consistent: method of entering vehicle; how vehicle was started; condition vehicle was left in
MURDER
Consistent: planning and control (was the victim gagged? Evidence destroyed?)
Less consistent: how the body was disposed of; mutilation; sexual behaviour; weapon used
