Colorectal cancer is the third leading cause of cancer-related deaths for both men and women in the U.S. It used to be that wealthy, white northerners had the highest death rates. But in the past few decades the trend has shifted, and now the people at highest risk are poor, black southerners.

Between 2008 and 2012, the death rates due to colorectal cancer were highest for blacks that held a high school diploma or less, according to a paper published Monday in the Journal of Clinical Oncology using data from the Centers for Disease Control and Prevention.

But this isn't so much about education or race as it is about poverty. "It's not about your skin color. It does matter," Dr. Ahmedin Jemal, vice president of surveillance and health services research at the American Cancer Society and an author of the study, tells Shots. But the effect of race, he says, "is very small compared to the disparities that you see by socioeconomic status."

Indeed, Jemal and his colleagues found that non-Hispanic whites who didn't finish college were two times more likely to die of colorectal cancer than those who had at least a bachelor's degree.

Smoking is more common among low-income groups, Jemal says, and is a risk factor for colorectal cancer along with obesity and physical inactivity.

"I think the reason that you're finding it in blacks and in lower incomes is probably less access to [health] care," says Ann Zauber, an attending biostatistician at Memorial Sloan Kettering Cancer Center. As screening tests including colonoscopies became more widespread in the late 1980s, people with health insurance were more likely to get screened, and to get precancerous polyps removed before they became cancerous.

Geography also plays a role. The authors found even among the most educated whites, the death rate for colorectal cancer in Mississippi was 8.9 per 100,000 people, compared to 3.8 per 100,000 in Connecticut. Low taxes on cigarettes could be one reason why, Jemal says.

And state policies can also really help, Zauber says. She points to Delaware's programs like the Delaware Cancer Consortium and the Delaware Cancer Treatment program, which ensured insurance coverage for screening for all state residents. It's credited with the 41 percent drop in colorectal mortality rates for African Americans between 2002 and 2009.

Jemal and his colleagues think that half of all premature deaths from colorectal cancer of people between the ages of 25 to 64 could be prevented by reducing ethnic, socioeconomic and geographic inequalities.

Says Jemal, "Colorectal cancer is one of the most preventable cancers by reducing risk factors and by improving access to care."

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