DNA Tests and the Police

DNA鑑定と警察

The Ashikaga murder case and DNA testing

The Ashikaga case became well-known in Japan as a case where an innocent man was imprisoned because of error in the police investigation.

It was reported that there was a problem with the DNA test that was done at the time of the investigation, and people who heard this began to question the reliability of DNA tests. However, there are a few points about the Ashikaga case that many people misunderstood.

Problems of police investigation in the Ashikaga case

足利事件における警察の捜査の問題点

To begin with, there is the legal issue that the police obtained garbage bags thrown out by the suspect without a warrant and then used samples from the garbage for DNA testing.

Furthermore, there was no way of confirming who the samples used from the garbage belonged to, and the condition of the samples was poor and not ideal for testing.

This created a high possibility for test error, and the sample collection method was incorrect.

On top of this, there were technical errors in the testing method as well, and the investigative work done by the police was not thorough.

Let`s explore one of the problems with the DNA test that was done at that time.

Was there a problem with the accuracy of the DNA test?

足利事件における警察の捜査の問題点

It is said that the DNA test carried out in the Ashikaga case was judged the suspect to be “the criminal with a probability of 1.244 to 1 thousand people.”

So, can a probability of “1.244 for 1 thousand people” be considered reliable?

This is a matter of probability, and the accuracy of DNA testing existing at that time was capable of producing accurate results that could have been used as conclusive evidence.

The test that was actually done at the time of the investigation could have provided conclusive evidence if it had been done correctly. If the number of suspects in the case had been narrowed down beforehand and the DNA matched, the probability that that person was the criminal would have been extremely high.

However, at the time of the incident there were 150,000 people living in Ashikaga-shi, and it was not possible to know who was guilty even if there was a DNA match, as the probability that anyone in the city was guilty was 0.66%. This means that the probability stated by the police was in fact a low, unreliable number.

Likewise, if the DNA did not match, it would also not be possible to exclude the person from the list of suspects.

Nevertheless, the police used the unreliable test results as evidence to identify someone as the criminal, regardless of the real possibility the criminal could be someone else.

If the police grasped the limited accuracy of the DNA test and used the method of DNA testing correctly in the investigation, it would not have created so many problems.

Improvements in DNA test accuracy

現在のDNA鑑定の精度は飛躍的に向上

At the time of the Ashikaga case, the accuracy of DNA testing was very low, being able to differentiate between 2000 individuals. Current methods of DNA testing technology are so sensitive that it is now possible to differentiate between 21,000 trillion (2,100,000,000,000,000 or 2.1 quadrillion) individuals.


With such a dramatic rise in accuracy, we can now conduct DNA tests that yield positive results with a probability of 99.9999%.

There may be people who mistrust DNA tests after hearing about the Ashikaga case, however in the case the error was due more to problems with the investigative methods of the police than with a problem with the DNA test itself.

DNA testing technology is continuously developing, and test results can be used for personal use as well as for a variety of legal purposes.

Click here for more information about Forensic DNA tests >>

True meaning of a POP of 99.99%

父権肯定確率99.99%の本当の意味

99.9999% POP(Probability of Paternity): does not mean `99.9999% chance of a biological relationship.` It essentially represents a 100% probability, and signifies that the alleged father is the biological father of the child.


Theoretically, it is not possible to get a result of 100% POP, and a positive result can be read as “the probability that it is not a parent-child relationship is infinitely close to 0%.“ Therefore, it is impossible to deny a test result of 99.99% or higher POP.

There are some companies out there that advertise DNA testing with a 99.99% probability of a parent-child relationship. However, this would mean that there is a 0.01% chance that there is no biological relationship (a test of low accuracy), and shows that the company lacks a basic understanding of genetic terms.

Click here to read more about the accuracy of DNA tests: Accuracy 35 times higher than the International Standard


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