Barclays Global Investors asks that rude question. Let's start with lost money.
Little investors get a list of funds and are told to do it themselves.
We've tried a hundred ways to help people do this for themselves. But people aren't doing it. Even if you could teach this, most people wouldn't go back to rebalance their portfolios. There will be no sense of strategy that has been communicated to the participant. People are binary - they're in one asset class or another.