The vast majority of rapes are committed by persons known to the victim, with only two percent of assault being perpetrated by a stranger, according to one survey of women. Therefore, the initiation and process of a rape investigation depends much on the victim's willingness and ability to report and describe a rape.
Biological evidence such as semen, blood, vaginal secretions, saliva, vaginal epithelial cells may be identified and genetically typed by a crime lab. The information derived from the analysis can often help determine whether sexual contact occurred, provide information regarding the circumstances of the incident, and be compared to reference samples collected from patients and suspects. Medical personnel in the United States of America collect evidence for potential rape cases by using rape kits.
A primary method used by crime labs for testing biological evidence is DNA profiling. The most common form of DNA profiling used in crime labs for identification is called polymerase chain reaction (PCR). PCR allows the analysis of evidence samples of limited quality and quantity by making millions of copies of very small amounts of DNA. Using an advanced form of PCR testing called short tandem repeats (STR), the laboratory is able to generate a DNA profile, which can be compared to DNA from a suspect or a crime scene. Blood, buccal (inner cheek) swabbings, or saliva should also be collected from victims for DNA analysis to distinguish their DNA from that of suspects.
The value of DNA evidence has to be seen in light of the potential of criminals to plant fake DNA samples at crime scenes. In one case, a criminal even planted fake DNA evidence in his own body: Dr. John Schneeberger raped one of his sedated patients and left semen on her underwear. Police drew what they believed to be Schneeberger's blood and compared its DNA against the crime scene semen DNA on three occasions, never showing a match. It turned out that he had surgically inserted a Penrose drain into his arm and filled it with foreign blood and anticoagulants.
Circumstances and type of rape
Abrasions, bruises and lacerations on the victim help in elucidating how a rape was done. 8 to 45 percent of victims show evidence of external trauma. The most common sites of extragenital trauma are the mouth, throat, wrists, arms, breasts and thighs. According to an American study, trauma to these sites comprise approximately two thirds of the injuries, while trauma to the vagina and perineum account for approximately 20 percent of the injuries.
A recent coitus can be determined by performing a vaginal wet-mount microscopy examination (or oral/anal if indicated) for detection of motile sperm. Motile sperm are seen on the slide if less than three hours have elapsed since ejaculation. However, absence of sperm does not exclude the possibility of coitus because only one-third of sexual assaults result in ejaculation into a body orifice. Furthermore the alleged assailant may have had a vasectomy or have experienced sexual dysfunction (roughly 50 percent of assailants suffer from impotence or ejaculatory dysfunction). In addition, acid phosphatase levels in high concentrations is a good indicator of recent coitus. Acid phosphatase is found in prostatic secretions, and activity decreases with time and is usually absent after 24 hours. If an undetermined amount of time has elapsed since the rape or if seminal fluid is scarce, prostate-specific antigen (PSA) may still be detected within a 48-hour period. The seminal fluid of vasectomized men also contains a significant PSA level. Nonmotile sperm may be demonstrated even beyond 72 hours after intercourse, depending on staining techniques.
- Abbey, A., BeShears, R., Clinton-Sherrod, A. M., & McAuslan, P. (2004). Psychology of Women Quarterly, 28, 323-332."Similarities and differences in women's sexual assault experiences based on tactics used by the perpetrator". Accessed 10 December 2007.
- A National Protocol for Sexual Assault Medical Forensic Examinations National Criminal Justice Reference Service (NCJRS). September 2004
- Emergency Management of the Adult Female Rape Victim. American Family Physician, June, 1991. by Diane K. Beebe