集中度上升；分散化并未消失

关键论点

• 标普500前十大权重接近40%，为1990年代中期以来最高，但指数仍保留广泛的分散化收益。
• 有效持股从疫情前约120只降至约50只，但仍远高于集中组合的门槛。
• 学术研究表明25至30只股票可消除大部分个股特定风险，约30只可将可分散风险降低最多86%。
• 前十大持仓相关性趋于下降且离散度收窄，因超大型公司业务线与地域分布多元。

对应原文证据

1. More recent academic research indicates that while the the diversification benefits may be somewhat lower than earlier studies suggested, they remain significant, with portfolios of roughly 30 stocks reducing diversifiable risk by as much as 86% (Surz & Price, 2000)2. · 2000
2. This is particularly relevant today, as much of the concentration in the S&P 500 is driven by large technology-oriented companies that may share common earnings drivers, economic sensitivities, and market narratives.
3. For investors with a constructive view on technology, artificial intelligence, and the broader innovation economy, the S&P 500 remains a compelling core allocation and an efficient way to participate in long-term US economic growth and innovation trends. · long-term