"Diversification is protection against ignorance. It makes little sense if you know what you are doing." - Warren Buffett
Another one of my favorites from the Oracle of Omaha, "Risk comes from not knowing what you're doing."
Most people practice modern portfolio theory, it's what I was taught when I studied Finance in college. There's some data to support it but the benefits are for those who have large portfolios and if you look at how most have created real wealth it's not through diversification, imo. The market is tough for most people, there are rocket scientists that work on the "street" everyday and try to beat the market but don't. If you want the markets historical return take 10% of your annual earnings and buy index funds that track the s&p 500. If the past is any indication you'll get about 10-11% annualized returns over time. It's easy and diversifies your portfolio for you.
Personally, I take the Oracles advice and specialize in real estate. The coc returns from my rentals beat the market before I even consider tax benefits, appreciation, or loan ammortization. There's people around here much smarter than I but that's my experience and makes it an easy decision for me.