Extracting DNA from the air – DNA evidence of human occupancy in indoor premises

I have always wondered why DNA is an accepted evidence. It's so easy to contaminate a crime scene or bring someone else hair, skin cells, etc by mistake.

In theory, you could do a "perfect crime" by going to a seedy part of town, picking up a dropped cigarette butt, and leaving it at the crime scene, framing someone else.

In reality, criminals are angry, frightened, in a rush, high or stupid, and they make the most elementary mistakes, so DNA and fingerprints work just fine almost all the time. In like 99% of cases there's not much doubt about who did it, the main thing is to have a watertight case against them when they deny it.

Because people believe it, and courts have accepted as fact that DNA evidence is infallible. Certainly there have been cases where DNA has been successfully challenged, but those are very rare. In the overwhelming number of cases where DNA evidence is present, it is a self-fulfilling prophecy for juries and judges.

If there is DNA evidence that is almost a guilty verdict. It should be more closely scrutinized but not everyone is rich enough to afford a real defense.

>so DNA and fingerprints work just fine almost all the time.

except for all of those innocent folks that have had their lives ruined by that 'almost all the time' caveat, it's great!

here's a report[0] that says something like 80% + of criminal forensic work has major mistakes within it.

[0]: https://www.criminallegalnews.org/news/2024/may/15/report-fi...

Quoting from that:

> At 100% of the 130 examinations analyzed, seized drug analysis was the examination with the highest percentage of Type 2 errors. It was followed by pediatric physical abuse (83% of cases had forensic errors with 22% having Type 2 errors out of 60 cases analyzed), fire debris not including chemical analysis (78% of cases/38% Type 2 in 45 cases), bitemark comparison (77% of cases/73% Type 2 in 44), pediatric sexual abuse (72% of cases/34% Type 2 in 64), serology (68% of cases/26% Type 2 in 204), shoe/foot impressions (66% of cases/41% Type 2 in 32), DNA (64% of cases/14% Type 2 in 64), hair comparison (59% of cases/20% Type 2 in 143), and blood spatter (58% of cases/27% Type 2 in 33).

Also this sample comes from cases where people were exonerated, which could cluster around poor quality police work in general. And is US-based where policing is all kinds of messed up.

Reminds me of the case of David Butler.

"Mr Butler has a rare skin condition, which means he sheds flakes of skin, leaving behind much larger traces of DNA than the average person. He worked as a taxi driver, and so it was possible for his DNA to be transferred from his taxi via money or another person, onto the murder victim."

https://www.bbc.com/news/science-environment-19412819

When we're talking about quality of evidence, saying "in reality" is skipping to your conclusion. Are smart careful people rare among known criminals? Well, yes. What would you expect to see? https://www.econlib.org/archives/2015/02/the_wittgenstei.htm...

> going to a seedy part of town, picking up a dropped cigarette butt, and leaving it at the crime scene

I've sometimes conjectured collecting hair from barbershops and making a dusting bag that steadily fluffs out hairs to dirty up a crime scene. Maybe get some saliva from grocery store sample spoons.

> you could do a "perfect crime" by going to a seedy part of town, picking up a dropped cigarette butt, and leaving it at the crime scene, framing someone else

Framing someone else is not a perfect crime. A perfect crime finds no criminal, accused or convicted.

Creating reasonable doubt as a backup plan in case your perfect caper isn't as perfect as you believe is just good practice.

Rule 43(a) in the criminal procedures handbook IIRC.

That seems an especially good reason to distrust DNA evidence using the technique from this paper. It relies on such a small amount of DNA, in dust, that if someone opened a window and a stranger walked by, the interior of a room might be contaminated with the strangers DNA. Also people are mobile, their clothing will pick up DNA dust from one location and shed it in another location. There may already be DNA dust from a million people in any one location. Even if you suck the DNA directly out of the air, you don't know where that air has previously been.

A few years ago i picked up the old (but famous) cases of Brad Cooper and Casey Anthony as they have tons of available digital forensics evidence.

I double-checked the forensics work and found several mistakes in processes, assumptions and technical conclusions. I sent off my findings to people associated with Project Innocence - not because I found anything that proved Cooper or Anthony's innocence, quite the opposite. Instead, I wanted to let them know that forensics experts can make mistakes.

It is interesting that scientific work have fault-finding processes like peer-reviews, but forensics investigations in court cases does not.