In Western jurisprudence, concurrence (also contemporaneity or simultaneity) is the apparent need to prove the simultaneous occurrence of both actus reus ("guilty action") and mens rea ("guilty mind"), to constitute a crime; except in crimes of strict liability. In theory, if the actus reus does not hold concurrence in point of time with the mens rea then no crime has been committed.

Discussion.

Suppose for example that the accused accidentally injures a pedestrian while driving. Aware of the collision, the accused rushes from the car only to find that the victim is a hated enemy. At this point, the accused joyfully proclaims his pleasure at having caused the injury. The conventional rule is that no crime has been committed. The actus reus is complete, and no rule of ratification applies in the criminal law. Whereas in the law of agency, a principal may retrospectively adopt a transaction as if the agent had originally been authorised to conclude an agreement with a third party ("ratification" of the agent's decision), and so acquires liability under that agreement, an alleged criminal cannot retrospectively adopt an actus reus and acquire guilt. To be convicted, the accused must have formed the mens rea either before or during the commission of the actus reus. In the vast majority of cases, this rule works without difficulty.

Two types of concurrence in criminal law

1. Temporal concurrence – the actus reus and mens rea occur at the same time.

2.  Motivational concurrence – the mens rea motivates the actus reus.

The problem.

Not all events are limited to a particular moment in time. The normal physical rules of cause and effect may see a series of interlocking circumstances conspire to cause a particular injury. If the facts of the example above are slightly changed so that the accident occurs at night at a sharp bend on a very quiet country road. When the driver sees the victim lying in the road, he simply leaves the unconscious person where he fell. Some hours later, when a second car innocently comes around the corner and kills the victim, the first driver is happily asleep in his bed. Thus, he argues that, at the time of the death, he had no mens rea and so cannot be guilty of homicide. This argument fails because of the so-called Single Transaction Principle.