Rapists, Thieves and Logical Fallacies

A recent study of 40 convicted rapists in the Virginia Prison system is yielding calls to expand the collection of DNA samples from convicted criminals to include those convicted of property crimes such as burglary. Unfortunately, the call for increased DNA collections is based largely on a logical fallacy.

The study examined rapists in Virginia who were convicted based in part on samples of DNA that had been taken from previous crimes. In 60% of the cases, the DNA sample had been collected following a previous conviction for a sexual assault, but in 40% of the cases the DNA had been collected following a conviction for a property crime such as burglary. Although the study has a relatively small sample with only 40 men, it agrees with other studies that find a large percentage of rapists tend to commit other sorts of crimes before committing rape. A study of British rapists, for example, found that more than 75% of them had committed property crimes before committing rape, with the obvious implication being that a significant number of rapes are committed by opportunistic burglars.

On the one hand these studies and others effectively debunk the radical feminist claim that rape is simply an extreme expression of normal male sexuality, and that all men, therefore, are potential rapists capable of sexual violence. In fact, as the Virginia and the UK studies demonstrate, rapists tend to come from a hard core group of career criminals who likely commit numerous acts of crime before moving on to rape. The typical rapist is very different from the average man on the street, and the claim that all men are potential rapists is nothing but a myth.

On the other hand, does this study really mean that it makes sense to take DNA samples from burglars and others convicted of property crimes. No, not unless the rules of logic have suddenly been overturned. Those arguing in favor of widespread DNA collection from burglars are guilty of a logical fallacy known as the undistributed middle. Just because all geese are birds, it doesn’t logically follow that all birds are geese. Similarly, just because a large percentage of rapists are also thieves, it does not follow that a large percentage of thieves are also rapists.

In fact a cursory glance at recent crime statistics shows the inherent problems with trying to catch rapists by DNA testing of thieves. For 1998, the last year for which statistics are available, the Bureau of Justice Statistics’ surveys estimates there were 4.1 million attempted or completed burglaries and 51,000 attempted or completed rapes and sexual assaults. These figures include rapes and sexual assaults not reported to police, but lets assume that half of rape victims not only don’t report rapes to police but also are unwilling to tell an anonymous survey of their rape. If there are 100,000 rapes each year, then there are about 41 times as many burglaries as rapes. Finally, even figuring in the issue of the same person committing multiple burglaries and/or multiple rapes, the population of thieves who are not rapists is still 8 or 9 times larger than the population of thieves who are also rapists, even if we assume that all rapists are thieves.

From a purely financial perspective, taking DNA samples, processing them at a laboratory, and then maintaining them in a computer database is a very expensive proposition to collect the minority of burglars who go on to commit rape; money that could probably be better used on more traditional methods of crime prevention.

Moreover, there’s an additional problem that relates to issues of statistical probability. Everyone’s familiar with the claims that if two random DNA samples are tested and they appear to be identical, the odds that they are not is astronomically high. The problem is that this is only accurate so long as investigators are comparing two random pieces of DNA. If police compare a DNA sample of semen following a rape with the DNA of the chief suspect who lives in the neighborhood and can’t account for his actions at the time of the rape, that is a statistically sound use of DNA. But when police start taking that DNA sample and comparing it to a DNA database of millions of individuals (and if DNA is taken from all people convicted of felony property crimes, that will quickly become a very large database), the probability that a match is a false positive starts to increase relatively rapidly.

A couple of features of DNA collection and crime patterns make such a false positive even more likely. In order to save money on DNA samples, different states actually do very different DNA tests. Rather than analyze the whole string of DNA, such tests look at a number of well-known markers, and to save money further, many states only look at five or six of these markers rather than seven or eight. Of course, the fewer number of marks examined, the higher the risk of a false positive.

Similarly, the astronomical odds assume an even distribution of genes, but this is unlikely to be true. Crime patterns in the United States tend to be disproportionately skewed against African Americans — i.e. a burglar is far more likely to be black than white in proportion to the overall racial makeup of the United States. There are numerous genetic differences that occur in African Americans that don’t occur in Caucasians. We know, for example, that the genes for sickle cell anemia occur almost exclusively in African Americans. Unfortunately this further raises the risk among African Americans that there might be false positives, since the astronomical odds assume that the genes of whites and blacks are evenly distributed in the database, even though they we know this is not the case.

These sorts of problems recently culminated in a false positive DNA match in Great Britain which has already gone a long way to creating a huge national database of DNA on most individuals arrested and convicted of crimes (fortunately for him, the man in question had an airtight alibi — he was in jail at the time on another charge!)

Collecting DNA from everyone convicted of property crimes is likely to be an extremely expensive proposition that will only marginally increase the ability of police to catch rapists, while at the same time dramatically increasing the risk of a false allegation and conviction for rape. Moreover, once it becomes general knowledge that DNA databases don’t really do much to improve the arrest rate for rape, as in Europe the push will occur to expand such testing to everyone ever arrested (a proposition for which police in New York and elsewhere are already clamoring for) or for DNA testing of the general population, which European nations have also started to adopt although it generally costs a lot of money and rarely results in an any arrests much less convictions.

Rather than take DNA from burglars, a better bet might be to make sure people convicted of serious sexual assault spend more time in jail. Much more frightening than the fact that 40% of rapists in Virginia had previously been convicted of property crimes is that 60% of them had been convicted of a previous sexual assault. Given limited funds, reducing recidivism among those convicted of rape or sexual assault would seem a better avenue to reduce rape incidence than randomly testing the DNA of millions of petty criminals.

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