Many are wondering how Mitt Romney swelled his IRA account to (if rumor is to be believed) $100 million.

Bain, like many other private-equity firms, allowed employees to co-invest in its takeover deals. This posed a risk they could lose their whole investment, as they sometimes did. But because of the firm's success during the Romney era, employees ended up able to share in returns for Bain investors that averaged 50% to 80% annually.

Bain added a couple of unusual twists that made co-investing even more rewarding. It allowed employees to co-invest via tax-deferred retirement accounts, and to do so by buying a special share class that cost little but yielded much larger gains than other shares when deals proved successful, according to former employees and internal Bain documents analyzed by The Wall Street Journal.