DNA evidence has largely been viewed as the ‘gold standard’ of forensic science, offering a seemingly solid means of linking individuals to crime scenes and, in more recent years, exonerating those wrongfully convicted. Whereas successful DNA analysis previously required a visible biological contribution, for instance a drop of blood, new advances in DNA technology have allowed for profiles to be produced from just a few dozen cells (you may have heard the term ‘Touch DNA’ be used to describe this). But as DNA technology has advanced, the improved sensitivity of DNA analysis techniques has become something of a double-edged sword, with concerns being raised over DNA analysis being too sensitive.

Imagine a scenario. A man and a woman are having an innocuous conversation. Some physical contact happens, perhaps the touching of hands or the brush of a cheek. The woman later experiences a sexual assault and an investigation ensues, an investigation in which DNA evidence is likely to play a pivotal role. During a sexual assault investigation, it is likely that swabs may be taken of a suspect’s clothing and genitals, specifically aiming to detect any of the victim’s DNA. This may particularly be the case if no semen has been detected, necessitating any other means of establishing whether or not sexual contact may have occurred.

But is it possible for a person’s DNA to be inadvertently transferred to the clothing or body of another person through innocent contact, only to later wrongfully incriminate that person? Research recently published in Science & Justice aimed to provide some insight into this question.

The aim of the study was to determine the frequency and amount of DNA transferred from a female to a male’s underwear and genitals following a non-intimate social contact situation. Using a staged scenario in which a male and female are interacting, the male participant was asked to touch the female’s face for 2 minutes and then hold her hands for 3 minutes whilst maintaining a conversation. This exchange provided the opportunity for the direct transfer of DNA from female to male. Following this exchange, the male participant went to the bathroom to simulate urination, offering the opportunity for secondary transfer of the female’s DNA to the male’s underwear and genitals. Other trials also introduced a 6-hour delay between the social contact and bathroom visit. Swabs were then taken of the man’s underwear and penis. In separate experimental trials, the same swabs were taken from male participants immediately following unprotected sexual intercourse to act as a comparison.

Following SGM Plus DNA profiling (routine at the time of the research), female DNA was found on the waistband of the underwear on only 5 occasions out of 30, on the penis in 4 out of 30 samples, and just once on the front panel of the underwear. In no other instances was female DNA detected. Unsurprisingly, this was even lower in trials implementing a 6-hour delay. In comparison to swabs taken from a male following sexual intercourse, transferred female DNA was detected in all samples and in larger amounts. So although the research demonstrated the possibility of the transfer of the female’s DNA to the male’s underwear and genitals (obviously somewhat incriminating if this occurred during a sexual assault investigation), the frequency and level of occurrence was much lower than if sexual intercourse had actually occurred.

The concept of secondary DNA transfer is not novel, and it has been known for some time that it is possible for DNA to be transferred through everyday contact. In fact the very idea of secondary DNA transfer was first described in literature almost two decades ago (Oorschot & Jones, 1997). The aforementioned research follows previous studies conducted investigating similar scenarios but reaching somewhat different conclusions.

Research published last year by the University of Indianapolis conducted their own DNA transfer study in which participants were asked to shake hands for two minutes before one of the participants handled a knife. The study aimed to determine whether this social interaction and handling of the object was sufficient to allow DNA from one individual to be passed to the knife via secondary transfer, without that person coming into any direct contact with the knife itself. Subsequent analysis of the knives showed that in 85% of cases DNA detected on the knife belonged to the participant who had not handled the object, and in one-fifth of the samples they were even the main or only contributor of DNA found on the weapon. This study essentially implies it is possible for someone to be linked to a crime scene via secondary transfer of their DNA to a murder weapon or victim, for instance.

Conversely, research published back in 1997 also conducted similar research, but this time not supporting the idea that secondary DNA transfer can provide misleading results (Ladd et al, 1997). Participants were instructed to shake hands for varying lengths of time before handling an everyday object, such as a coffee mug. The research concluded that a complete DNA profile of the secondary participant (who had not directly handled the object) was never detected. So although various studies have been carried out, although using different experimental conditions, results are to an extent contradictory.

These studies discussed have obvious limitations. The scenarios staged are far from realistic – the average person does not shake someone’s hand for two minutes before handling an incriminating object, which is then immediately swabbed for DNA by investigators. Nor does the research take into account factors that might affect DNA transfer and persistence.

It is worth noting that these concerns are not confined to the research lab. In 2010, former cab driver David Butler found himself imprisoned, accused of murdering 46-year-old Anne Marie Foy. The evidence against him? His DNA allegedly found under the fingernails of the victim. Butler had previously offered up a DNA sample years before during the investigation of a burglary and, although the DNA profile obtained from the victim’s body was merely a poor quality partial match, this was seemingly sufficient to land Butler in prison on remand for nearly eight months. However the DNA evidence was later called into question when it was suggested that Butler, who had a skin condition causing him to shed more skin cells than the average person, could easily have transferred his own DNA to a person or money which was then transferred to the victim via secondary transfer.

Cases such as this highlight the need for further investigation. Although recent research has provided a good starting point for investigating secondary DNA transfer through non-intimate contact, as DNA analysis techniques improve and achieve greater sensitivity, there will be an increased need to extend research. Further studies examining new DNA profiling techniques, different scenarios and the effects of possible affecting factors will be necessary in ensuring secondary DNA transfer in situations of everyday social contact will not be mistakenly interpreted in a criminal investigation.

References

Van Oorschot, R. A. Jones, M. K. DNA Fingerprints from Fingerprints. Nature. 387(1997), 767.

BBC News. DNA test jailed innocent man for murder. [online] Available: http://www.bbc.co.uk/news/science-environment-19412819

Cale, C. M. et al. Could Secondary DNA Transfer Falsely Place Someone at the Scene of a Crime? J Forensic Sci. 61(2016) pp. 196-203.

Jones, S. et al. DNA transfer through nonintimate social contact. Sci Justice. 56(2016), pp. 90-95.