DNA transfer through non-intimate contact

Male and Female symbols

In sexual offence cases where no semen is detected, the presence of female DNA on penile swabs and underwear of the defendant may be put forward to address the issue of whether or not sexual intercourse took place.  However it is known that DNA can be transferred through non-intimate contact so research was commissioned to investigate the frequency and amount of DNA which might be transferred in this way.  The work was recently published in Science and Justice by the UK and Ireland Association of Forensic Science Providers’ Body Fluid Forum which our expert Julie Allard co-chaired at the time of the research.

The method was designed to mimic a ‘worst case scenario’ where there was extensive non-intimate contact during social interaction – a male donor touched the face of a female participant with his hands, massaging her face and neck for 2 minutes, then holding her hands for 3 minutes.  Throughout the 5 minutes of contact, they spoke to each other.  This scenario provided an opportunity for the direct transfer of cells and/or saliva from the female to the male’s hands.  The second stage required that the male to go to the bathroom immediately afterwards and simulate urination for 30 seconds by undoing his trousers and removing his penis from his underwear, then redressing.  After walking around for 5 minutes, the male took swabs from his penis and sealed his underwear in a bag.  This second stage provided an opportunity for an indirect (secondary) transfer of the female’s DNA to his penis and underwear.  Some experiments were also carried out with a 6 hour time delay between the simulated urination and the DNA sampling.

The DNA profiling method used for the experiments was SGMPlus which was in routine use at the time of the research.  The results showed that with this DNA method the high levels of DNA which may be detected following sexual intercourse were not replicated as a result of transfer by non-intimate contact.  The published paper gives some guidance on the amounts which might be expected as a result of non-intimate contact.

What does this mean for cases profiled using the current DNA17 methods?

Since DNA17 is so much more sensitive than the previous SGMPlus method, we would expect to detect more of the DNA transferred indirectly during the non-intimate contact.  As yet there is no published research which directly addresses how much more in this specific situation, however there are various papers which show that indirect transfers of DNA are readily detected with DNA17.

If you have a sexual offence case where the mechanism of transfer for the complainant’s DNA on the defendant’s penis or underwear is in dispute, we can review the DNA results and provide you with an evaluation regarding whether the findings are more likely given sexual intercourse or some other non-intimate contact.