Sunday, November 18, 2007

Generational Mobility

The distinguished Harvard scholar Henry Louis Gates Jr writes in today's NY Times:
I have been studying the family trees of 20 successful African-Americans, people in fields ranging from entertainment and sports (Oprah Winfrey, the track star Jackie Joyner-Kersee) to space travel and medicine (the astronaut Mae Jemison and Ben Carson, a pediatric neurosurgeon). And I’ve seen an astonishing pattern: 15 of the 20 descend from at least one line of former slaves who managed to obtain property by 1920 — a time when only 25 percent of all African-American families owned property.
If I understand the empirical pattern Professor Gates is describing, then to me it does not seem astonishing at all. The time span is 87 years, which is about three generations. Everyone has 8 great-grandparents. It seems very possible that a vast majority of today's African-Americans descend from at least one of the fortunate 25 percent.

As a general matter, the literature on intergenerational mobility finds a positive correlation between the economic success of parents and children, but the correlation is not sufficiently strong to persist substantially over three generations. I would venture the guess that over such a long time span, the correlation is only 10 to 20 percent. Maybe it is larger if the sample is restricted to the population of African-Americans, but I would bet against it.