RomneyBainCapital

William D. Cohan asks about it:

The most mysterious of the unexplained mysteries about Mitt Romney’s considerable wealth is how he was able to amass between $21 million and $102 million in his individual retirement account during the 15 years he was at Bain Capital LLC. How did he do it, given the relatively small amounts that the law permits to be contributed to such a plan on an annual basis? 

He goes on to propose several theories. Felix Salmon joins in the speculation:

[T]he recent controversy over when exactly Romney left Bain raises another possibility, which is hinted at in a Maremont article from January:

"Several estate-planning experts said they know of others with IRAs of more than $100 million, but they are rare. Typically, they said, that occurs when founders of companies invest in their own shares, which then take off."

We now know that Mitt Romney, individually, was the sole shareholder of Bain Capital when he took leave of all day-to-day responsibilities in 1999 to concentrate on running the Salt Lake City Olympics. And he remained the sole shareholder of Bain Capital through 2002. So here’s the thesis, taken directly from Henry Blodget: that Romney filled up his retirement account with shares of Bain Capital itself, rather than shares in its funds, or in its portfolio companies.

This would also help explain why it took Romney three years to disentangle himself from Bain Capital.

Interesting as a possibility. But there's only one way to clear this up for good:

Follow your father's advice, Mitt, release the returns!