ScienceDaily reports that:
In a population of initially well-functioning older adults, we found a significant correlation between strong adherence to the Mediterranean diet and a slower rate of cognitive decline among African American, but not white, older adults. Our study is the first to show a possible race-specific association between the Mediterranean diet and cognitive decline.
(Score: 2) by lx on Sunday July 20 2014, @09:15AM
Form the article it's not clear if and how they accounted for cultural differences.
If the typical diet of older White American people is more healthy than the typical diet of older African-Americans then a switch to the Mediterrean diet may have less effect in that population.
From the abstract it appears that they only looked at adherence to the Mediterranean diet and didn't look at differences in overall diet in both groups, but without seeing the actual paper it's impossible to tell.