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The Problem of False Convictions

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Characteristics in cases that led to erroneous convictions and cases that led to near misses.

Characteristics in cases that led to erroneous convictions and cases that led to near misses. Source: Jon B. Gould et al., Predicting Erroneous Convictions: A Social Science Approach to Miscarriages of Justice. Final report to the National Institute of Justice (February 2013).

 

The National Institute of Justice funded American University researchers to study factors that lead to wrongful convictions. They examined 460 cases of violent felonies that occurred from 1980 to 2012, and they asked the question: Why are some innocent people convicted while others are released?

This month, the NIJ posted the final report. Gould and his colleagues identified 10 factors that led to a wrongful conviction:

  • A younger defendant
  • A defendant with a criminal history
  • A weak prosecution case
  • Prosecution withheld evidence
  • Lying by a non-eyewitness
  • Unintentional witness misidentification
  • Misinterpreting forensic evidence at trial
  • A weak defense
  • Defendant offered a family witness
  • A “punitive” state culture – defined by the number of executions per population.

In addition to the report, the NIJ offers videos of Jon Gould explaining the study.

Forensic Firearms Identification

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Silver to bullets

Poster printed by Sir Joseph Causton & Sons, Ltd, London, 1915. Source: Library of Congress.

Forensic firearm examiners determine whether a certain weapon fired a bullet or cartridge found at a crime scene. Early efforts linked spent ammunition with a class of weapon. Following the 1862 shooting of Confederate General Stonewall Jackson, for example, investigators concluded that the General had been accidentally shot by his own side. The spherical projectile removed from the General had been fired from a smooth-bore musket, a type of weapon that the Union Army no longer used.

In 1912, Professor Victor Balthazard at the University of Paris formulated the basic principles of firearms examination. Using enlarged photographs, he compared marks created by a firearm on the surface of bullets and cartridge cases found at a crime scene with marks on ammunition that he had fired from a suspect weapon. In this way, he could connect crime scene ammunition to a particular firearm.

During the 1920s in New York, four men rediscovered Balthazard’s principles and initiated modern firearms identification: Charles E. Waite, Calvin Goddard, Philip O. Gravelle, and John E. Fisher. Gravelle had extensive experience with a comparison microscope to study fine details in cloth patterns. He suggested that they might be able to use the instrument to compare fired bullets and cases.

In a signal event of firearms identification, the group bought two comparison microscopes and modified them. They added a comparison bridge, and rotatable mounts for bullets and cartridge cases. Through the eyepiece of the bridge, two pieces of spent ammunition could be examined, one on each stage of the two microscopes.

Police departments and the courts became aware of the value of “fingerprinting” bullets, especially after Goddard testified about his findings in the 1929 St. Valentine’s Day Massacre. Within a decade, firearms identification became an established technique of criminal investigation.

Unraveling DNA Mixtures

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DNA mixture

 

The New York Times’ Liz Robbins recently reported a development in forensic DNA analysis. Over the past several decades, improvements in synthesizing DNA from trace amounts greatly increased the sensitivity of DNA profiling. Yet increasing sensitivity does not provide a solution to cases in which evidence contains trace amounts of DNA from several people.

Theresa A. Caragine and Adele A. Mitchell of the New York City medical examiner office’s forensic biology lab may have a solution. Their Forensic Statistical Tool is an algorithm for a software program that enables analysis of a DNA mixture uncovered from a crime scene and determines the probability that the DNA brew includes a defendant’s DNA profile.

Before judges allow results of the technology in court, the technique must survive Frye hearings. A Frye hearing is a challenge to the general scientific acceptance of new technology. Several Frye hearings are scheduled in New York courts.

Bacterial DNA Profiling

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 Finding DNA

Earlier this month, researchers at Washington University School of Medicine (St. Louis) and at the European Molecular Biology Laboratory (Heidelberg, Germany) reported a new type of DNA profile: DNA profiles from human gut bacteria. The researchers analyzed microbial DNA in 252 stool samples from 207 individuals, focusing on 101 species of microbes commonly found in the intestine. They found many types of DNA differences, the sort of differences that generate unique DNA profiles. For 43 subjects, the researchers collected two stool samples one month to six months apart. The scientists found little variability in the microbial DNA. In short, they discovered variability in DNA profiles between subjects and consistency in DNA profiles in subjects over time.

“The microbial DNA in the intestine is remarkably stable, like a fingerprint,” said George Weinstock, associate director of The Genome Institute at Washington University. “Even after a year, we could still distinguish individuals by the genetic signature of their microbial DNA.”

It’s not immediately obvious how this discovery could be incorporated into a crime story. But, if you can do it, then consider yourself on the cutting edge.

A more apparent application of bacterial DNA profiling to mystery stories is found in the discoveries of Noah Fierer and his colleagues at the University of Colorado in Boulder. They found that a typical human hand shelters about 150 species of bacteria. The types of bacteria living on skin vary greatly. In a 2008 study, they identified more than 4,700 different bacteria species living on the hands of 51 people. Yet only five species lived on the skin of every participant of the study.

In 2010, the researchers reported that computer users leave DNA traces of bacteria on computer mice and keyboards. These DNA traces more closely match the DNA of bacterial colonies that inhabit the hands of the individual who used the computer, compared with bacterial DNA traces of randomly selected people. The researchers obtained useful samples of bacterial DNA two weeks after a person touched computer equipment.