When identical twins commit a crime, can DNA really tell who the real culprit is?

The forensic problem

It has been reported that a recent criminal case in France is stalled because DNA recovered from a gun matched a pair of identical twins, and routine forensic tests could not say which brother it came from. The reason is simple and stubborn: monozygotic twins originate from a single fertilized egg and start life with the same genome. Standard police work uses STR (short tandem repeat) analysis — a reliable “barcode” for most people — but those barcodes are effectively identical in monozygotic twins. So where does that leave an investigator? Stuck between a match and reasonable doubt.

New tools beneath the surface

Science has pushed beyond STRs. Whole-genome sequencing can pick up rare somatic mutations that occur after the zygote splits; a 2014 study found only a handful of such differences in adult twins, but those few variants can act as unique identifiers. It has been reported that a U.S. court in 2025 accepted whole-genome evidence distinguishing twins in a murder trial, opening a legal precedent. Mitochondrial DNA, with its faster mutation rate, offers another route and has been admissible in U.S. courts since the mid-1990s, though it hasn’t yet been widely used to separate twins. Finally, epigenetic markers — methylation patterns shaped by life, environment and behavior — can diverge substantially even between twins raised together; a 2025 Korean study reportedly distinguished most twin pairs using only five methylation sites.

Practical and legal hurdles

If the science exists, why are cases like the French one still unresolved? Three realities intervene. First, crime‑scene samples are often minute, degraded or contaminated, and advanced tests need more and better DNA than a dried droplet or a smudge can provide. Second, whole-genome and methylation assays are expensive and slow, and defence and prosecution must be convinced by robust validation before courts will rely on them. Third, and most important, DNA is never meant to be the sole pillar of a conviction: presence at a scene does not equal guilt, and experts warn against consigning a verdict to one molecular readout.

What this means

Laboratory methods can, in principle, point a finger at one twin rather than the other. But can lab precision survive the messy realities of criminal evidence, courtroom rules and the ethical weight of proving guilt beyond a reasonable doubt? Not yet — and perhaps that’s as it should be. As Zhao Bin (赵斌) of Fudan University (复旦大学) argues in an essay republished on Huxiu’s WeChat channel "生态学时空", DNA can sharpen investigations, but human stories and legal safeguards remain essential to deciding who is truly responsible. (See also a recent Nature commentary: DOI 10.1038/d41586-026-00521-z.)