## Facts

- Mr. Lamb and his friend O'Donaghue were together when Mr. Lamb, as a practical joke, pointed a revolver at O'Donaghue and pulled the trigger.
- The revolver had a five-chambered cylinder with two bullets, neither initially aligned with the barrel.
- Mr. Lamb mistakenly believed the revolver would not fire, unaware that pulling the trigger would rotate the cylinder and align a bullet to be discharged.
- Mr. Lamb's act resulted in the death of O'Donaghue.
- Expert evidence supported that such a mistake regarding revolver mechanics was common.
- At trial, the judge did not instruct the jury to consider whether the necessary mens rea for assault existed.
- Mr. Lamb was convicted of constructive (unlawful act) manslaughter.

## Issues

1. Whether a conviction for constructive manslaughter requires proof that the base act was both unlawful (a criminal offence) and accompanied by the corresponding mens rea.
2. Whether Mr. Lamb's genuine mistaken belief about the revolver's mechanism could negate the intent required for assault and thus preclude a manslaughter conviction.
3. Whether the trial judge erred in failing to direct the jury regarding the necessity of finding mens rea for the base unlawful act.

## Decision

- The Court of Appeal held that for constructive manslaughter, the unlawful act forming the basis of liability must itself be a criminal offence with its own mens rea.
- Mr. Lamb's honest mistake concerning the operation of the revolver prevented the formation of the requisite intent for an assault.
- Because the necessary intent for assault was absent, there was no unlawful act to found a conviction for manslaughter.
- The trial judge's failure to direct the jury on the necessity of finding mens rea for assault was a critical misdirection.
- The conviction for manslaughter was quashed.

## Legal Principles

- For constructive manslaughter, the predicate act must be both unlawful (a criminal offence and not merely a tort) and committed with the required mens rea.
- A genuine mistake of fact may negate the mens rea necessary for the base offence, preventing liability for manslaughter.
- The requirement for a clear judicial direction to the jury concerning the mental element of the base unlawful act is essential.
- The presence of a dangerous act alone is insufficient; criminal culpability depends on both actus reus and mens rea.

## Conclusion

The Court of Appeal in R v Lamb [1967] 2 QB 981 clarified that a conviction for constructive manslaughter cannot stand where the defendant’s genuine mistake negates the mens rea for the base unlawful act, here assault, so that no criminal offence forms the basis for liability. The case remains influential for its emphasis on the dual requirements of an unlawful act and mens rea in unlawful act manslaughter.
