Schwab Asset Management's D.J. Tierney says extreme concentration in the S&P 500 (SPX) and other major indexes makes diversification increasingly important. He discusses reducing reliance on mega-cap tech through international exposure, fixed income, direct indexing and fundamental indexing, while using tax-loss harvesting and rebalancing to manage concentrated positions. Tierney says international markets are seeing renewed interest as investors look beyond U.S. large-cap growth for long-term diversification.