#diversification

Left alone, your portfolio quietly drifts toward whatever has run up the most, right before it usually runs down. Rebalancing drags it back to your target and forces you to sell high and buy low without thinking about it.

Diversification lowers the risk of your portfolio without lowering your expected return. That combination is rare enough that economists call it the closest thing to a free lunch in finance. Here is how it actually works, and where it stops working.