However, the actual situation is not as clear-cut as the above implies. In reality, not all felonious actions will apply in most jurisdictions. To "qualify" for the felony murder rule, the felony must present some degree of danger. If while passing a forged check, the receiver, who happens to be a hemopheliac, gets a paper cut and bleeds to death, most courts will not hold the defendant guilty of murder.

On the other hand, many activities that are inherently very dangerous cannot apply for the felony murder rule. Aggravated assault, for instance, does not. The reason is that virtually all murders result from an assault! (It's hard to cause the death of someone without causing them bodily harm.) But aggravated assault is a felony. Thus if the felony murder rule were to apply in the case of aggravated assault, it would essenitally reduce the culpability requirements carefully set by the legislature for murder to those requirements of assault. For this reason aggravated assault would be said to "merge" with murder.

To counter the common law style interpretations of what does and does not merge with murder (and thus what does not and does qualify for felony murder), many states explicitly list what offenses qualify. The American Law Institute's Model Penal Code lists robbery, rape or forcible deviate sexual intercourse, arson, burglary, kidnapping, and felonious escape.

Other issues also loom. For instance, whose actions can cause the defendant to be guilty of felony murder? There are two schools of thought. One is the agency theory; the other is the proximate cause theory. The former states that only deaths caused by the agents of the crime can result in a felony murder conviction, while the latter holds that any deaths that result from a crime would qualify. As an example of the distinction, take the following hypothetical.

Say John Doe is robbing a bank. John is a bit careless however, and is not paying attention long enough that one of the tellers has a chance to hit the silent alarm. Police arrive, and corner John. Rather than give up nicely, John decides to try to fight his way out, and begins shooting. Officers return fire, and one of them tragically misaims and the bullet strikes and kills a bystander. In this case, jurisdictions that follow the agency theory would hold that John is not guilty of felony murder in the death of the bystander, as the death was immediately caused by the actions of the police, who are not agents of the crime. Jurisdictions following the proximate cause theory however adopt an opinion much closer to a but for relationship: the death would not have occurred but for the commission of the crime, so John is guilty of felony murder.

Note that if John Doe was not alone and was with, say, his wife Jane Doe, if John kills someone (even accidentally) during the comission of the robbery, both John and Jane are guilty of felony murder since they are both agents of crime, and conspired together. Even Joe Shmoe driving around the block in the getaway car would be guilty of felony murder despite the fact that he likely didn't even know that anyone was killed. This is essentially universally held.