Investors who have shunned diversification face maybe the best buying opportunity for bonds in decades

Get Mike Santoli's Market Memo in your inbox first. SUBSCRIBE HERE Prudence, in standard investing wisdom, means diversification across asset classes while rebalancing along the way to keep exposures in line and risks well-distributed. Holding fixed proportions of equities and bonds has been more a confounding trap than a useful tool lately. The iShares Core U.S. Aggregate Bond ETF (AGG) is down 4.8% in price terms this year and has suffered a negative 2.2% total return even after interest income, dragging on the S&P 500's 14% year-to-date total return in a diversified portfolio.