How Much Active Management Does There Have to Be for Price Discovery to Take Place?

How much active management does there have to be for price discovery to really take place? I've asked people like Andrew Lowe and MIT who said you can have 90 percent passive the remaining 10% is where all your price discovery will take place. All you need is the marginal investor who's rational and always ready to take advantage of opportunities yes but it's not very clear who the marginal investor is or if they exist if they exist. The markets have become a little bit less efficient over time in the sense that they have become longer deviations from fundamentals eventually they do correct, he says.