"[Collective investment trusts] look and act a lot like mutual funds, but generally have lower fees and disclose less about their inner workings to 401(k) participants.... [S]uch investments currently account for $2.4 trillion, or 16%, of the $15 trillion in 401(k)-style and pension plans, up from $1.3 trillion (and 12.7% of the total) in 2009 ... The trusts are accounts available only to retirement plans. They are sponsored by banks and trust companies and are primarily overseen by banking regulators, rather than subject to mutual-fund rules enforced by the [SEC]."