About one third of managers beat the market. The problem is that there is no correlation between year-to-year performance of managers: the manager who beat the market this year is no more likely to beat it next year than any other manager.
That is, if you could figure out which listed companies were going to fare the worst and build a partial index fund with the rest, you'd beat the index.
Similar research style to short selling but with less downside.
For example, Twitter's prospects on its own don't look so great right now. But if that induces you to avoid owning Twitter, you miss out on a quick profit (perhaps even a big one!) if some larger company decides to buy it.
Individual investors pretty much all do worse than market.
EDIT: obviously that means there is skill involved. But it seems that professional managers are close enough in skill that luck is the dominant factor.