Black men have the lowest life expectancy of any ethnic group in the United States. Much of the gap is explained by greater rates of chronic illnesses such as diabetes and heart disease, which afflict poor and poorly educated black men in particular.

But why is that? Lack of insurance? Lack of access to health care?

Now, a group of researchers in California has demonstrated that another powerful force may be at work: a lack of black physicians.

In the study, black men seeing black male doctors were much more likely to agree to certain preventive measures than were black men seeing doctors who were white or Asian.

Although 13 percent of the population is black in the United States, just 4 percent of doctors are black.