DNA Testing - What Clinton Was Up Against

The Starr Report released to the Internet on Sept. 11, 1998 states that: "Physical evidence conclusively establishes that the President and Ms. Lewinsky had a sexual relationship." This "physical evidence" was from DNA testing, a facet of forensic medicine.

Forensic medicine is a branch of medicine that uses medical science in legal proceedings. Powerful methods of studying biologic matter with genetic analysis of the basic molecules of inheritance are at the forefront of forensic researchers.

Using DNA analysis, forensic medical specialists can precisely match biologic material with persons who have left it at the scene of a crime.

Let us assume that a man's semen is alleged to be on a piece of fabric. To learn whether the cloth is stained by the suspect's semen, DNA testing can be done. Here is how it works:

  1. First, it must be shown that the stain contains semen. Semen glows under ultraviolet light and also changes color when exposed to specific chemicals.
  2. If these tests are positive, the lab technicians cut a swatch of the fabric from the stain and dissolve the organic matter in the stain by putting the swatch in a special solution.
  3. The technicians then look for sperm cells in the solution with an ordinary low-power light microscope. The sperm look like little lifeless tadpoles.
  4. If there are visible sperm under the microscope, the techs next extract DNA from the sperm. This is done with a mild detergent that bursts non-sperm cells. A rinse with water removes broken cells. And then a stronger detergent is used to burst the sperm and recover their DNA.
  5. The lab then compares the DNA from the semen with the suspect's DNA. A blood or saliva sample from the suspect will supply enough DNA for the comparison.
  6. If the stain is small and the amount of DNA minute, a method based on the polymerase chain reaction, or PCR, is employed to make millions of copies of selected segments of the DNA. Note that PCR does not change the DNA but merely amplifies the amount.
  7. The DNA from the semen is then compared with the suspect's. This can be done using what are called restriction fragment length polymorphisms, or RFLPs. To examine RFLPs, the DNA from the sample is cut (restricted) by special enzymes. These restriction enzymes cut different DNAs differently. If the restriction enzymes cuts the suspect's sample and the semen sample into the same number of fragments of the same length, then the semen may be the suspect's.
  8. The odds that the DNA in the semen is the suspect's are then calculated. The odds may, for example, be one in 11 million that the DNA in the semen came from another man, not the suspect.

The DNA tests can take several days or several weeks before results are available, depending upon which techniques have been used by the lab (and which lab has done the DNA testing).

DNA tests can show that the semen is not from the suspect. Or these tests can show that the DNA in the sperm is not detectably different from the suspect's. But DNA testing can never prove with complete certainty that the semen is from the suspect.

The Starr Report states that: "Physical evidence conclusively establishes that the President and Ms. Lewinsky had a sexual relationship." Particularly pay attention to the wording "conclusively establishes."

A dress Ms. Lewinsky had worn had stains on it which "she surmised...were the President's semen." Testing revealed that the stains were "in fact semen." A blood sample was then drawn from the President so that his DNA could be compared by the FBI with the DNA in the semen.

The Starr Report maintains that: "By conducting the two standard DNA comparison tests, the FBI Laboratory concluded that the President was the source of the DNA obtained from the dress." Note specifically that the Report uses the language "...the FBI Laboratory concluded that the President was the source of the DNA..." (from the dress).

Due to the limitations of DNA testing, the FBI Laboratory could not possibly have "concluded that the President was the source of the DNA..." No laboratory in the world could have "concluded that the President was the source of the DNA..." It is simply not scientifically possible to come to this conclusion.

The Starr Report has a footnote that reads, "The FBI Laboratory performed polymerase chain reaction analysis (PCR) and restriction fragment length polymorphisim (sic) analysis (RFLP)." And the Report adds that: "According to the more sensitive RFLP test, the genetic markers on the semen, which match the President's DNA, are characteristic of one out of 7.87 trillion Caucasians." The details of how the number of "7.87 trillion Caucasians" was arrived at are not provided in the Starr Report.

Irrespective of what the statistical odds were for that semen sample coming from another man, it was physically impossible from the DNA testing to have "concluded" that the semen came from any given man on the face of this planet.

Last Editorial Review: 8/13/1998