How do scientists find cancer risk factors?

Most cancer risk factors are first identified in epidemiology studies. In these studies, scientists look at large groups of people and compare those who develop cancer with those who do not. The studies may show that people who develop cancer are more or less likely to behave in certain ways, or to be exposed to certain substances, than people who do not.

On their own, such studies cannot prove that a behavior or substance causes cancer. The finding could be a result of chance, or the true risk factor could be something other than the one suspected. Because of this, findings that get attention in the media can sometimes lead to wrong ideas about how cancer starts and spreads.

Scientists can be more confident about a link when many studies point to a similar connection and there is a possible explanation for how the factor could actually cause cancer.