Sexual Health STDs Understanding Necessary and Sufficient Causes in Science and Medicine By Elizabeth Boskey, PhD Elizabeth Boskey, PhD Facebook LinkedIn Twitter Elizabeth Boskey, PhD, MPH, CHES, is a social worker, adjunct lecturer, and expert writer in the field of sexually transmitted diseases. Learn about our editorial process Updated on October 31, 2020 Medically reviewed Verywell Health articles are reviewed by board-certified physicians and healthcare professionals. These medical reviewers confirm the content is thorough and accurate, reflecting the latest evidence-based research. Content is reviewed before publication and upon substantial updates. Learn more. by Matthew Wosnitzer, MD Medically reviewed by Matthew Wosnitzer, MD LinkedIn Twitter Matthew Wosnitzer, MD, is a board-certified urologic surgeon and physician scientist. He specializes in male infertility. Learn about our Medical Expert Board Print 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. Ghislain & Marie David / de Lossy Cultura / Getty Imagse 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. Necessary Causes vs. Sufficient Causes 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 sufficientNecessary but not sufficientSufficient but not necessaryNeither necessary nor sufficient Real-World Examples 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. Was this page helpful? Thanks for your feedback! Sign up for our Health Tip of the Day newsletter, and receive daily tips that will help you live your healthiest life. Sign Up You're in! Thank you, {{form.email}}, for signing up. There was an error. Please try again. What are your concerns? Other Inaccurate Hard to Understand Submit 3 Sources Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. Centers for Disease Control and Prevention. HIV. About HIV/AIDs. MedlinePlus. Pelvic inflammatory disease. NIH U.S. National Library of Medicine. Genetics Home Reference. Tay-Sachs disease.