Forensic Evidence

• Author: Brian H. Greenspan/Vincenzo Rondinelli/Daniel Brown/Jill Witkin
• Pages: 299-323



Forensic

Evidence

10

I. Forensic DNA in Sexual Oence Investigations, Prosecutions,

and Defences............................................. 

II. Understanding the Technology and DNA ProleInterpretation..... 

A. Documentation ..................................... 

B. Body Fluid Testing ................................... 

C. Forensic DNA Analysis ............................... 

III. Understanding Transfer and Persistence of DNA-Containing Cells

in the Context of a Case..................................... 

IV. Paternity Analysis in Sexual Oence Investigations............... 

This chapter was authored by Cecilia Hageman.

© 2024 Emond Montgomery Publications. All Rights Reserved.

 Prosecuting and Defending Sexual Oence Cases

I. Forensic DNA in Sexual Offence Investigations,

Prosecutions, and Defences

Biological evidence plays a crucial role in the investigation, prosecution, and defence of

many sexual oence allegations. Biological evidence is mobile evidence, and physical

exchanges of blood, skin, semen, and epithelial1 and buccal2 cells may lead to meaning-

ful inferences regarding sexual (and other) physical contact and activity between per-

sons or between persons and objects. Cells that contain deoxyribonucleic acid (DNA)

may be recovered from victims or accused persons, or both, from body cavity, skin, and

other intimate samples, bite marks, undergarments, outer clothing, and fingernails.

Biological evidence may be present at the crime scene on condoms, bedding, furniture,

and carpets. The transfer of semen cells via vaginal intercourse may also be evidenced

through a paternity analysis, where the DNA profile of a child or aborted product of

conception arising from an alleged sexual oence is analyzed to determine the contri-

bution from the semen donor, allowing for a comparison to potential male suspects.

Forensic scientists tasked with examining sexual oence case evidence first review

case circumstances and available evidence items to identify the questions of inter-

est to investigators. They then determine the relevant hypotheses in order to decide

which scientific tests to carry out. A hypothesis is simply a theory or a potential expla-

nation for results; to a scientist, it is considered an allegation to be tested using sci-

entific methods. It is therefore important to begin this discussion with general types

of hypotheses that a scientist can address in sexual oence cases. Scientists aliated

with the (former) Forensic Science Service in the United Kingdom provided a very

useful “hierarchy of propositions”3 to help define and set apart the dierent catego-

ries of questions that investigators and legal counsel address, and how scientists can

define the proper hypotheses and testing conditions for these questions.

The highest of four proposition levels speaks to the probability that the suspect

committed a criminal oence. This consideration of the ultimate issue, the competing

hypotheses of guilt and innocence, is entirely beyond the capability and realm of the

scientist. Indeed, it is incumbent upon the scientist to clearly state conclusions in a

way that does not intrude on this proposition level. Moving downward and in order,

the remaining three proposition levels (listed in Table 10.1) deal with questions of

scientific scope: activity (i.e., “What happened?”), body fluid deposition (i.e., “What

and whose body fluid was deposited?”), and DNA source (i.e., “Who is the source

of DNA from some indeterminate cell deposition?”). In a typical sexual oence case

where DNA profiles are obtained from an evidence item and from a reference sus-

pect, for example, competing hypotheses and typical forensic DNA evidence may

include those outlined in Table 10.1.

1 These are cells that line orifices such as the vagina and the rectum.

2 Buccal cells line the inside of the mouth and can be found in saliva.

3 R Cook et al, “A Model for Case Assessment and Interpretation” (1998) 38:3 Science &

Justice151; R Cook et al, “A Hierarchy of Propositions: Deciding Which Level to Address in

Casework” (1998) 38:4 Science & Justice 231.

© 2024 Emond Montgomery Publications. All Rights Reserved.