Special Reports

The predictable death of sperm in condoms laced with spermicide could help police pinpoint the time of a rape, and possibly even corroborate the testimony of the victim or the defendant.

An increasing number of rapists are using condoms, forensic medical examiners report. A 1999 study in Oakland, California, found that 13.5% of assailants used one, probably to protect themselves from identification by DNA profiling. Those condoms are sometimes recovered if they are discarded at the crime scene, or at a suspect's home, and can be useful in a police investigation.

This led Ginger Lucero and Ismail Sebetan at the National University in La Jolla, California, US, to wonder whether the spermicide in many condoms would degrade DNA or interfere with genetic profiling. Their worries were unfounded: they found the commonly used spermicide nonoxyl-9 did not affect sperm DNA. But they also noticed that when sperm were incubated with nonoxyl-9 their numbers seemed to fall at a steady rate.

To investigate further, they collected semen samples from five volunteers and placed half in spermicidal condoms and the other half in regular condoms. The team took samples from the condoms, which were kept at room temperature, after 30 minutes, 1 hour, 2 hours and then at regular intervals up to 72 hours. With the help of fluorescent stains that glow when bound to burst cells, they were able to estimate the number of viable sperm that remained after each time interval.

They found that the proportion of live sperm cells in the control condoms remained constant at around 15% over the 72 hours. The number of viable sperm in nonoxyl-9 began at 40% and dropped off in regular steps to 6% by the end of the three days.

Date-rape cases

Sebetan, who presented the findings at the American Academy of Forensic Sciences meeting in New Orleans last week, was surprised that the control samples began with fewer live cells than the spermicidal condoms.

One possibility is that nonoxyl-9 samples may damage cells beyond recognition, so live cells falsely appear to make up a larger proportion of the total. Other cell detection techniques could help refine the results sufficiently to allow the technique to put a precise timing on when a rape happened.

Robert Blackledge of the Naval Criminal Investigative Service in San Diego, California, says the timeline evidence could help support a victim's testimony, or counter it. This would be especially useful in date-rape cases, when there may have been previous instances of consensual sex, or if several condoms are recovered at a suspect's home.

Knowing the age of a semen sample could be invaluable for determining whether such evidence was planted. Blackledge says he knows of cases in which a rape suspect may have used a condom to smear another man's semen onto his victim's clothing to leave a false trail. "Any test that would clearly show that this was not a recent seminal fluid sample would defeat this ploy," he says.