Forbes touts its annual list of the 400 richest U.S. billionaires as evidence “that the American dream is still very much alive,” claiming that 70 percent of them “made their fortunes entirely from scratch.” I noted that in a post the other day, and questioned whether it was true.

Its not. The liberal group United for a Fair Economy has done the heavy lifting on Forbes’s bootstrappers, and its report, “Born on Third Base,” shows that the vast majority of the country’s plutocrats either inherited their money or had significant help from family members. What’s surprising is that this is surprising to Forbes.

Just 35 percent of the Forbes 400 last year were raised poor or middle class, compared to 95 percent of the broader public, as (reasonably) defined by UFE. Twenty one percent inherited enough money to join the 400 without lifting a finger, what UFE calls being “born on home plate.” Another 7 percent inherited at least $50 million or a “large and prosperous company,” 12 percent inherited at least a million bucks or a decent-sized business or startup capital from a relative, and 22 percent were “born on first base,” into an upper class family or got a modest inheritance or startup capital (UFE says it was conservative in assigning people to bases, so its report understates their advantages somewhat). So, at least 62 percent did not, in fact, make their fortunes “entirely from scratch.”

In other words, contra Forbes’s assertion, its 400 list is more a picture of class immobility and stratification than a portrait of an American dream of opportunity for all if who are willing to work hard. It has gotten the story exactly backward.

Of course, this isn’t to say that people who built or massively expanded companies they inherited somehow don’t deserve credit for their work. It’s just to acknowledge that even $50,000 in startup capital or inheritance is a huge advantage on someone with just as much capability but no money. It’s fair to assume that, say, the Koch brothers and Rupert Murdoch, as good a businessmen as they are, would be anywhere near as wealthy as they are today if they hadn’t inherited businesses to expand or that Bill Gates would have created Microsoft without attending an elite high school that afforded them computer access in the early 1970s.

And the false impression that most of these billionaires “built that” from scratch has important implications for policy. As a group they pay less than 20 percent of their income in taxes, largely because of capital gains rates that favor wealth over work.

If anything, the UFE’s report probably understates what the effects of stratification look like now. Most billionaires are older than the average American. It takes a while to accumulate that amount of wealth, and even if you inherit it, you typically don’t until an aged relative kicks over. The median age of the 20 richest Americans is 68. The median age of all Americans is 37.

That 68 year old was born in 1944 and would come of age in the Great Compression, when the income and wealth disparities between the rich and everyone else were at historic lows and social mobility wasn’t as calcified as it is now:

In other words, in 20 or 30 years, don’t be surprised if the Forbes 400 has even fewer Horatio Alger stories—and is still spinning the billionaires it fawns over as proof that the American Dream is alive and well for the rest of us.