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Forensic Science and Sexual Offences


by Sue Carney, Consultant Forensic Scientist, Ethos Forensics. Interpretation of the forensic findings in sexual offence cases are amongst the most complex and difficult interpretations faced by forensic scientists. What are the forensic issues in sexual assault and rape cases? And, why is it so difficult to achieve successful prosecution of rapists?


Following news of huge backlogs in U.S. rape kit processing, I was recently asked, for an article in Cosmopolitan, whether the situation in the UK was, or could ever be similar. Thankfully, the short version of my answer was, no. In my view, we are years ahead of the U.S. in terms of the effectiveness of our investigations of sexual offences. The UK has a wealth of specially-trained police officers to deal with sexual assault victims in the most efficient, yet sensitive way. We have regional sexual assault referral centres, staffed by forensically trained medical staff and rape counsellors. The UK forensic science provision offers specialist services for dealing with sexual offences casework. But, it hasn’t always been this way and despite today’s enlightened processes for dealing with sex crime, there are still improvements to be made.

In early 2013, the Ministry of Justice, the Home Office and the Office for National Statistics published An Overview of Sexual Offending in England and Wales. Crime figures cite an average of eighty-five thousand women who are raped each year and four hundred thousand subjected to sexual assault. These are the reported figures. It may be impossible to gauge the levels of unreported sexual assaults. This 2013 overview follows the 2010 Stern Report, which recommended improvements to policy and procedure in dealing with sexual offences, as have previous reports including the HMIC/HMCPSI joint investigation into the investigation and prosecution of rape cases of 2002, and the Cross Government Action Plan on Sexual Violence and Abuse of 2007. The mere existence of these and other investigations highlights the difficulties in successful prosecution of alleged rape and sexual assault. The difficulties are numerous.

The investigation and subsequent prosecution of sexual offences involves the co-operation of several agencies. Focusing on the forensic aspects of sexual offence cases, police officers need to be aware of timescales and contamination risks in order to minimise the loss of crucial evidence; forensic medical examiners need a current knowledge of forensic science in order to recover the most appropriate evidence from the victim and suspect; forensic scientists must be sexual assault specialists in order to carry out such complex interpretations; and prosecutors require a detailed knowledge of the significance and limitations of forensic evidence.

In the majority of circumstances, forensic evidence is time-dependent. This is particularly true of sexual offences casework. Biological evidence on the body or
inside a body orifice persists for a limited time, and loss is accelerated by activities such as washing or bleeding due to injury or menstruation. It is vital that
the forensic medical examinations of both complainant and suspect, if there is one, are carried out at the earliest opportunity to maximise the - recovery of trace evidence. The Faculty of Forensic & Legal Medicine of the Royal College of Physicians publish guidelines for the collection of forensic specimens in sexual assaults, in conjunction with the UK Forensic Service Providers and the Association of Chief Police Officers. These are updated regularly, based on current best practice, and offer some degree of standardisation, although variations in procedure and the provision of appropriate sexual assault referral centres still remain across the UK. It is not reassuring to perceive the potential outcome of a sexual assault investigation as little more than a post code lottery, but there are other factors at work here.

Much of the information gleaned at the complainant’s forensic medical examination is invaluable in determining the most appropriate forensic strategy in the case. It is the forensic scientist, in conjunction with the police force’s forensic submissions or scientific support department who will make these decisions.

Information at the time of forensic strategy setting is key in assessing the potential outcomes of the laboratory examination of exhibits and indeed, in determining whether the proposed examinations are worthwhile. In order to assess a case in this manner, the forensic expert uses an interpretive framework
based on three principles:

1. Interpreting the evidence in the light of the information associated with the case. We refer to this as the conditioning information.

2. Considering the evidence in terms of at least two competing scenarios or versions of events representing the views of the prosecution and the defence. And,

3. Considering the likelihood of the evidence rather than the likelihood of the versions of events. This approach leads to an evidential interpretation that is balanced, logical, robust and transparent. 

Forensic experts have long since realised that their remit must never extend to the provision of expert opinion on matters of guilt or innocence of a defendant. To offer such opinion would commit the ‘prosecutor’s fallacy’ and forensic experts know that the question of guilt versus innocence is purely the realm of the jury. However, this doesn’t prevent the the common misconception that forensic experts deal in ‘proof’. They do not!

Forensic evidence provides additional information to be used by jury members to update their opinion of guilt or innocence by taking the value of the forensic
evidence into account. The forensic expert presents their evidence in the form of a level of support for a particular view and very rarely, if ever, as conclusive
proof. Rulings of The Court of Appeal in some of the earliest cases using DNA evidence, such as that relating to Regina versus Doheny and Adams ([1996] EWCA Crim 728], clarify the role of the expert witness in such matters.

Forensic science interpretation addresses case issues using a ‘hierarchy of propositions’. For example, addressing ‘source level’ might consider issues of from
whom semen staining found in a sexual assault case has originated. ‘Activity level’ in the same case might seek to address the issue of whether the defendant
engaged in sexual activity with the complainant. In most sexual assault scenarios, addressing issues at activity level is of much more value to the criminal justice process. However, questions surrounding activity level require consideration of the issues surrounding the transfer and persistence of evidence: How and when was the semen deposited and does it relate to the alleged incident? 

The laboratory testing processes used in the identification of biological evidence have limitations that must be taken into account during the forensic interpretation of a sexual offence case. Many of the chemical tests for body fluids are not specific, giving rise to the possibility of false positive results. This is often compounded by the fact that some body fluid stains may not be readily visible and chemical testing is often used to screen exhibits in order to locate areas of staining.

The sensitivity of some body fluid tests is another issue to contend with. If biological evidence is dilute or mixed with another substance then some of the less
sensitive chemical tests may fail to detect it. The test for saliva is a case in point here. In addition to being a presumptive test, it is not particularly sensitive, and
the chemical reaction itself takes time to develop. These conditions, coupled with the fact that a small proportion of the population do not produce amylase, the enzyme in saliva that this chemical test reacts with, mean that forensic scientists must exercise caution in attributing particular results to the presence of saliva. Furthermore, an apparent absence of saliva can never be definitive, and as a general rule, an absence of evidence does not necessarily support a view that a particular activity did not take place, since there might be other legitimate reasons for that absence. This is especially true of rape scenarios. An absence of semen on intimate swabs does not refute the allegation, since it is possible that rape occurred without ejaculation, or that a condom was worn.

Forensic experts are well aware of the limitations of these tests and must take into account other factors such as the location and distribution of staining an potential for the presence of material from unrelated sources. Again, case conditioning information is key in addressing these concerns.

There are, of course, confirmatory tests available for some, but not all, body fluids. Some of these are complex and rarely conducted, such as the crystal tests used as confirmatory tests for blood. Since blood almost always has such a distinctive appearance, it is considered sufficient that if a stain looks like blood and gives a reaction to a presumptive test, then it can be interpreted as blood. Others are far more routine. For example, the identification of spermatozoa (sperm cells) by microscopy, is almost always definitive in establishing the presence of semen, since sperm are extremely unlikely to have originated from anything else. However, this is not without its complications.

Human sperm exist in a variety of subtly different morphologies and such variation in appearance may give rise to difficulties in identification. When viewing a microscopic sample, one is visualising a two-dimensional image of a three-dimensional structure. If a sperm cell is rotated within the field of view, it may not be instantly recognisable. This might be an issue if a sample contains very low numbers of sperm, due either to a delay in recovery of the sample or due to an individual having a low sperm count. It may be that, of those sperm observed, some are not readily identifiable.

Another consideration in a low sperm numbers scenario is the fact that sperm cells can be transferred to clothing items in the washing machine. Again, case
conditioning information is vital in the cautious interpretation of such findings. If an exhibit has been washed or was seized from a washing machine, the forensic expert must question the chance of transfer of sperm from a semen-stained item in the same wash before attributing those sperm cells to the presence of semen on the exhibit.

At the extreme end of the spectrum of limitations of body fluid testing, there are some body fluids for which there is no chemical test, presumptive or confirmatory, currently validated for forensic use. Vaginal secretions are a particularly relevant example. The existence of a reliable and specific test for vaginal cells would greatly enhance the degree of forensic interpretation possible in rape cases. Currently, the cells shed from the lining of the vagina, known as epithelial cells, can be visualised microscopically. But such cells are largely indistinguishable from epithelial cells from other body orifices. Saliva is a good source of epithelial cells from the mouth, and this, coupled with the fact that in some circumstances, vaginal secretions can indicate a false positive in the presumptive test for saliva, is inevitably problematic in interpreting the outcomes of penile swab tests in allegations including both oral and vaginal intercourse.

Current research at several academic institutions, most notably, the University of Huddersfield, seeks to address this issue using a system to characterise specific RNA molecules within various body fluids and tissue types. RNA is a nucleic acid, similar to DNA, that is produced by cells when genes are switched on. The rationale is that different tissues within the body, given their different functions, will have different requirements for actively used genes. Therefore, the pool of RNA molecules within a particular cell type should be specific for that particular tissue type. If this research can develop a specific test for each body fluid, especially vaginal cells, and such a test were relatively straightforward to conduct, then this would offer significant advances in the forensic interpretation of body fluid findings in sexual assault casework.

The issues in sexual offences casework have demonstrated the importance of case conditioning information for effective forensic interpretation. Forensic scientists usually have access to three main sources of information in a sexual offence case: The complainant’s interview transcript, the notes from the forensic medical examination and the statement or interview notes relating to the suspect. From these, the forensic scientist may be able to establish much of the conditioning information required for interpretation. However, some parts of the conditioning information may never be established simply because it is unavailable or unknown. For example, in a drug-facilitated sexual assault, the complainant may not have a clear recollection of the alleged sexual activity if she is intoxicated or unconscious. Whilst such cases may need to be dealt with in an investigative, rather than an evaluative manner, it is also clear that some flexibility must be built into the forensic interpretation of evaluative cases to allow for uncertainties in the conditioning information. This is another reason that forensic opinion is rarely provided as definitive proof.

Whilst information is vital for interpretation, is it possible to have too much information? Issues of cognitive bias in forensic science have been the topic of much debate of late. Forensic scientists must be impartial regardless of which ‘side’ of the trial process they are instructed by. Their duty is to the criminal justice system not to a particular police force or forensic provider. Is it possible that within the reams of extraneous information served up in sexual offence case scenarios, forensic experts are unknowingly making judgements based on factors such as the social activity of the parties involved, or the appearance of their clothing? Forensic scientists are well aware of the issues of cognitive bias but research indicates that awareness is not necessarily sufficient for prevention.

To summarise: Forensic evidence in a sexual offence is time-dependent. In order to provide the most useful interpretation, the forensic expert must consider the mechanisms of transfer and persistence of biological evidence. Their interpretation is heavily influenced by the circumstances of the case, especially the defendant’s version of events, and the limitations of current forensic test procedures.

Ethos Forensics have accrued many years of experience in the complex forensic interpretation of sexual offences casework and have worked on many hundreds of cases in this area. We would be delighted to assist defence Counsel by providing a comprehensive review and commentary on the limitations of the forensic evidence in your sexual offences case.

For a free initial consultation,
contact Sue Carney, Forensic Consultant,
on 07796 546 224 or
This email address is being protected from spambots. You need JavaScript enabled to view it.

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