Racially motivated stress as an explanation for infant mortality rates?

OK, you have differences in "poor nutrition, inadequate prenatal care, teen pregnancy, heredity, high blood pressure, stress, obesity, low birth weights and prematurity," but some academics have a different theory:

"The pregnancy scares the life out of me because I am pregnant with a baby boy, and I know how black boys are treated in this society," one study participant told researchers from Spelman College and Emory University in Atlanta. . . .

Here is my question: how has the gap between black and white infant mortality rates changed over time? The 1950s and 1960s should have had really high relative infant mortality rages, but they weren't. Well, I looked up some numbers:

The infant mortality rate for Black Americans in 1999 was 2.5 times the rate of White Americans. In 1950 the mortality rate of black infants was only 1.5 times the rate of white infants. . . .