Readers of detective stories will probably familiar with the phrase motive, means and opportunity, which I will refer to as MMO-theory. The logic behind MMO-theory is simple: people will not commit a crime if they do not have a motive for doing so. Often the motive is greed, in fiction hatred also plays a role. And unfortunately too often thrill is another motive for crime.
But a motive alone is not sufficient for being suspect. One should also be able to commit a crime. For instance if one want to steal the contents of a vault, one either needs the key or some other method the break the vault. Anyone would did not have such means cannot logically be the perpetrator.
Even if one has both motive and means, one need the opportunity to commit a crime. For instance if one desires to poison Obama, one needs access to his kitchen to put the poison in his food.
But even if one has motive, means and opportunity it does not prove that one is guilty of the crime. It often happens that multiple persons have a motive, means and opportunity to commit the crime. Though MMO does not necessarily proves the guilt of a suspect it is useful for both real life detectives and writers.
The first goal of detective work is the identification of possible suspects and to exclude clearly innocent people. Only after a possible suspect has been identified, evidence for the guilt of the suspect needs to be collected. In the first phase MMO-theory is quite useful as a tool for detectives.
In what sequence should MMO be evaluated? It depends on the circumstances. If for instance Obama has been found to have died from poisoned food from the White House kitchen, it would be best to start with opportunity. After a list has been compiled of all people who had access to the kitchen during the relevant time frame, we should narrowing down the list of suspects by examining their motives and means.
Sometimes it would make more sense to start with the means. That is for instance the case if one has been killed in public with a rare type of gun. Since the crime has been committed in public, too many people would have had opportunity. A better strategy would investigate who had access to such rare gun.
MMO-theory would run in difficulty if the crime has been committed by intermediaries. In that case there is a separation between the one who has the motive and those who execute the actual crime. In general criminal intermediaries have no inherent motive to commit the crime they have been asked for, other than getting paid.
Intermediaries are not only a theoretical concern for detective writers, they occur often in real live. Two approaches can be followed by solving this “problem”. First even a criminal intermediary needs the means and opportunity to commit a crime, and secondly a person with a motive would need to be able to find a suitable intermediary.
A good detective story should, in my opinion, have multiple persons with a plausible motive and also multiple people with either means or opportunity.