When markets are rising diversification can help investors from putting too much money in the wrong places. On the flip side, performance will always be just average and those who are able, can outperform.
But, unfortunately when it matters most, as markets decline in earnest, diversification fails. The chart below shows how correlated an in lock step the global markets are when in a steep decline. If you are looking for the solution to manage portfolio risk, diversification does little at the times it is needed most. As such, you better have a plan.