Evaluating the value and risks of DNA evidence in Japanese criminal trials: Changes before and after the lay judge system

• We examined 12,391 criminal cases included in a commercially available database. • The estimated utilization rate of DNA evidence in criminal trials is about 2%. • Except for illegally obtained evidence, DNA evidence is rarely deemed inadmissible. • The conviction rate in DNA evidence-involving trials is nearly equal to that in non-DNA trials. • The value and risks of DNA evidence-involving trials have changed over the past 35 years. DNA evidence can be used to link perpetrators to crime scenes or victims. We examined all Japanese criminal court cases included in a commercially available LEX/DB Internet (LEX database) from 1989, the year DNA analysis was first used as evidence in criminal trials, through 2024. We primarily analyzed data on the validity, conviction, and sentencing rates of trials involving DNA evidence. The conviction rate in these trials was not higher than the generally published conviction rate in all phases. This was either nearly equivalent to or slightly lower than that in trials without DNA evidence. However, after 2009, when trials shifted from judge-only to lay judge trials, remarkable changes in the weighting of the admissibility and probative value of DNA evidence were observed. Specifically, from 1989 to 2009, the rate of admissibility of DNA evidence was high, and the average conviction rate was higher than that of trials without DNA evidence. However, after the transition to lay judge trials, the conviction rate in trials involving DNA evidence fell below the average conviction rate compared to those not involving DNA evidence. It can be presumed that the value of DNA evidence and its probative weight have increased during lay judge trials. Contrarily, sentencing trends in DNA-evidence-involving trials showed some variation in the diverse judgment criteria of lay judges after the transition to lay judge trials; however, no significant changes occurred. These data will stimulate discussions regarding quality control and maximize the value of DNA evidence.