Understanding Necessary and Sufficient Causes in Science and Medicine

What does it mean to say that "A causes B?" If you think about it, it's not so simple. When non-scientists talk about causality, they generally mean that the first event preceded the second in time and seemed to be related to its occurrence.

Scientists, however, need to be a little clearer. They need to know if exposure to a toxin always makes people sick or only sometimes. They need to understand if a nasty symptom can be caused by one virus or several. It's not enough to simply say that one thing causes another. Scientists have to be able to describe the nature of that association. In order to do so, they have developed terminology to describe the causal relationship between two events. They say that causes are necessary, sufficient, neither, or both.

If someone says that A causes B... 

• If A is necessary for B (necessary cause) that means you will never have B if you don't have A. In other words, of one thing is a necessary cause of another, then that means that the outcome can never happen without the cause. However, sometimes the cause occurs without the outcome. 
• If A is sufficient for B (sufficient cause), that means that if you have A, you will ALWAYS have B. In other words, if something is a sufficient cause, then every time it happens the outcome will follow. The outcome always follows the cause. However, the outcome may occur without the cause.
• If A is neither necessary nor sufficient for B then sometimes when A happens B will happen. B can also happen without A. The cause sometimes leads to the outcome, and sometimes the outcome can happen without the cause
• If A is both sufficient and necessary for B, B will never happen without A. Furthermore, B will ALWAYS happen after A. The cause always leads to the outcome, and the outcome never happens without the cause. 

When you say that one event causes another you may be saying that the first event is:

• Both necessary and sufficient
• Necessary but not sufficient
• Sufficient but not necessary
• Neither necessary nor sufficient

All four circumstances are types of causality that occur in the real world. Some examples are:

• Necessary but Not Sufficient
  • A person must be infected with HIV before they can develop AIDS.  HIV infection is, therefore, a necessary cause of AIDS. However, since every person with HIV does not develop AIDS, it is not sufficient to cause AIDS. You may need more than just HIV infection for AIDS to occur. 
• Sufficient but Not Necessary
  • Decapitation is sufficient to cause death; however, people can die in many other ways.Therefore, decapitation is not necessary to cause death. 
• Neither Necessary nor Sufficient
  • Gonorrhea is neither necessary nor sufficient to cause pelvic inflammatory disease. A person can have gonorrhea without ever developing PID. They can also have PID without ever having been infected with gonorrhea.
• Both Necessary and Sufficient
  • A gene mutation associated with Tay-Sachs is both necessary and sufficient for the development of the disease. Everyone with the mutation will eventually develop Tay-Sachs. No one without the mutation will ever have it.