3 Likelihood ratio calculations
In simple terms, the expert is communicating that:
- DNA that matches the suspect’s (Mike) DNA is on the bat.
- The chances of this DNA profile being obtained from someone other than the suspect are very small.
- On the basis of (1) and (2), it is more likely that Mike’s DNA is on the bat than someone else’s DNA.
However, these interpretations do not mean that Mike actually handled the bat, or, as will be of utmost interest to the court, whether he hit the victim with the bat. So, while the likelihood ratio of the DNA evidence may answer the first and second question (Is there DNA on the bat?), and (who’s DNA is that most likely to be?), it cannot reliably answer the third question (How Mike’s DNA came to be on the bat. How Mike’s DNA came to be on the bat is an activity-level question. The offence-level question is the one the jury will need to decide by weighing the evidence provided and determining whether they think Mike is the person who committed the alleged crime.
During the cross-examination, the defence lawyer may contest the answers to all questions but the answer to the third question should be one of particular interest to both lawyers and the court. Research has shown that it is possible for a person to handle an item and leave very little to no DNA behind, or for a person’s DNA to be transferred to an item or surface without direct contact. This is a process known as secondary or tertiary transfer (DNA Judicial Primer; 2025, van Oorschot et al, 2019).
A good line for or of challenging the activity level question might be where (location) on the bat the DNA was recovered from, and how and when the DNA got there. For example, if Mike used the bat to hit the victim, then it would be expected that the victim’s DNA may be transferred, remain on and be recovered from the blood-stained area, and Mike’s DNA could be transferred and remain on the bat handle to be recovered. The expert, therefore, could consider how likely (probable) it is that Mike’s DNA was transferred to the handle of the bat through direct contact or via a different transfer mechanism and then persisted on the bat handle.
Likelihood in Dave’s case
But remember from Week 2 that Mike stated that he was a member of the gym, frequented the locker room, but had nothing to do with the alleged attack. If he handled the bat at any time, innocently, his DNA could have been left on the handle. It could also be transferred to the bat from the locker room environment, without direct contact (secondary or tertiary transfer), although this may be less likely.
Both of these are equally likely; however, the likelihood of transfer and persistence might be different, and is based on what is known about the persistence of DNA on baseball bat handles and what impacts that (time since handling, pressure, humidity, environmental conditions, etc).
The biggest task for the expert in providing any opinion towards question three will be to consider the likelihood of finding Mike’s DNA as a result of holding the bat to hit the victim, as opposed to the same likelihood of Mike’s DNA on the bat for other reasons that are unrelated to the alleged crime. The expert has to determine these likelihoods (probabilities) before they can offer an opinion regarding the likelihood of obtaining the DNA as a result of a particular activity. However, this has not been possible in our case. Therefore, question three is outside the ability and expertise of the expert, and all that the expert can reliably provide, at this point in the case, is an opinion about the likelihood of finding Mike’s DNA on the bat.
Forensic experts might be able to assess how the DNA evidence was transferred to the bat, how long it has remained on it, and how it was recovered from the bat. However, this is complex and remains an area of debate in forensic science (Hunt, 2025; Stacey et al., 2025). Even when experts can provide this information, they cannot say what actually happened. For example, if Mike’s DNA is found on the bat, they cannot determine whether it got there before, during, or after the alleged incident.
Like all forensic evidence, the expert’s role is to support understanding, not decide the case. The final decision rests with the court, which considers all the evidence together, including the presence of DNA, to determine what is most likely and whether Mike is guilty or not guilty.