A couple of years ago, the philanthropy reporter for the Jewish Telegraph Agency tried to divvy up by ancestry the Forbes 400 of America's richest individuals and came up with the headline "At least 139 of Forbes 400 are Jewish."
It was an interesting start, but was also bogged down by obvious errors. (E.g., No, George Lucas isn't Jewish.)
The blogger n/a of "race / history / evolution notes" then took on the task of improving the Jewish Telegraph Agency's published list, which he did in 2009. His latest version is the 2010 Forbes 400 list.
Obviously, this kind of thing isn't easy to do. I'm sure there are mistakes in it, but I haven't noticed any glaring ones. The most likely source of errors is when some obscure rich guy happens to have a name that doesn't align well with his ancestry. But those are hard to catch.
His task is simplified by using a group called "Northwestern Europeans" that makes up 50.75% of the 2010 Forbes list, down from 72% in Nathaniel Weyl's count of the 1987 list. "Northwestern Europeans" lump together English, Scottish, Welsh, Dutch, Irish (Catholic and Protestant), Scandinavians, French, Belgians, Swiss, Germans, and Austrians. It's not much of a bloc.
A more subtle pair of methodological problems are what to do with people of mixed ancestry and with people who were adopted. A dual example is the late Steve Jobs. He's listed as one of the eight (2.0%) who are "Middle Eastern" because his biological father is Syrian.
I think it would be better to divide up ancestry into fractions when possible.
And, though trans-ethnic adoptions are fairly rare, when they are known about, I think it makes sense to treat Nature and Nurture as, more or less, equally important. So, ideally (by my lights), Jobs would be classed as 1/4th Middle Eastern (i.e., half nature, no nurture) and 3/4th NW European (half nature, all nurture).
It's not clear how much these refinements would alter the list. It would depend if interethnic marriages tend to be skewed by sex.
Overall, I doubt if the summary percentages would be changed a whole lot by refinements. For example, the JTA count was pretty laughable in part, but the overall number of Jews the JTA reporter came up with was almost identical to what n/a's much more careful study arrived at. That's because random errors tend to neutralize each other.
Here are n/a's 2010 percentages:
Northwestern European 50.75%
East Asian 2.0%
Middle Eastern 2.0%
Eastern European 1.75%
South Asian 1.0%
Hispanic 0.5% (one of the two is actually a Basque)
Black 0.25% (Oprah)
The low Eastern European percentage is interesting. There are a fair number of Slavs in the U.S., but they tend to maintain a low profile. Moreover, they have a fairly high rate of Anglicizing their names to make them easier to spell. For example, I know two Polish-American identical twin millionaires (not Forbes billionaires) from Chicago who, upon setting out to make their fortune agreed to ditch "Wojtyla" (as in the late Pope) for a much easier to pronounce and spell English name. In contrast, while Italian names are recognizably non-English, they are reasonably easy to spell and pronounce, plus have more glamor associated with them due to all the Italian celebrities since the Renaissance.
As I've mentioned before, Italians make a pretty good apples to apples comparison to Jews: they arrived about the same time in the U.S., they live in about the same parts of the country, they are both non-rural, they are both non-Protestants, and so forth. Italian-Americans strike me as pretty typical white American gentilers in terms of achievements on average. They make up about 8% of the white gentile population and about 6% of the white gentile Forbes 400. That's pretty good considering their late start in America. (For example, there are five Hearsts on the Forbes 400, and their fortune goes back to William Randolph Hearst's dad striking it rich in the Comstock Lode before the Civil War.) But it's also nothing all that special either.
Italians are usually thought to be about three times as numerous as Jews in America (roughly 6% of the total population versus 2%) , but they are almost an order of magnitude less common in absolute terms on the Forbes 400 (17 v. 143), which suggests Jews are about 20 or 25 times more likely to make the Forbes 400 on a per capita basis. That's a big difference.
Weyl's 1987 count found Jews made up about 23 percent, which suggests that the Jewish proportion of the super-rich has grown by 50% over 23 years (keep in mind that there is a lot of turnover in the Forbes 400 list due to different sectors of the economy going in and out of fashion — e.g., oil, tech, finance, real estate, etc.) But, there have also been a sizable number of Jews among the very richest for a long time.