人工智能（AI）的繁荣创造了有史以来对投资者最友好的股票市场之一。
The S&P 500 ( ^GSPC 0.47% ) has returned 24%, 23%, and 16%, respectively, over the past three years.今年它又上涨了 10%，如果能维持下去，将使这个连续四年实现两位数回报的牛市变得稀有。
人工智能集会也带来了一些问题。
Valuation and concentration within the index have become reasonable risks that could pose higher-than-average downside risk should market conditions change.
估值很高……或者也许并不高？
Whether stock prices are viewed as expensive or not really depends on the metric you're using.
The Shiller CAPE ratio , which measures the S&P 500 relative to inflation-adjusted earnings over the past 10 years, is currently 42. That level has been reached only one before in history -- the lead-up to the tech bubble bursting in 2000.
On the other hand, the forward price/earnings (P/E) ratio for the S&P 500 today is right around 21. That's above average by long-term historical standards, but it's well below the 2024 peak of 23.7. Based on current earnings growth expectations for the next several quarters, stocks seem to be pretty reasonably priced.
但我不想忽视希勒 CAPE 比率告诉我们的信息。 Stocks have been in a rarely interrupted three-plus-year bull market, and there's a lot of optimism still priced in. Investors shouldn't be getting ahead of themselves here.
标准普尔 500 指数
Tech concentration has never been higher
目前，科技股约占标准普尔 500 指数的 40%。在科技泡沫的顶峰时期，这一数字甚至没有那么高。此外，仅前 10 名持股就占该指数的近 40%。 Investors may not mind this when stock prices are going up, but we're sitting right in the middle of an unprecedented period of index concentration.
This isn't necessarily a sell signal, but it is a warning that S&P 500 ETF performance is heavily influenced by just a handful of companies. If anything were to happen to any one of those names, it could drag the index down quickly.
降低投资组合中 S&P 500 风险的选项
If you want to address the valuation and concentration risk problems at the same time, consider the Invesco S&P 500 Equal Weight ETF ( RSP +0.71% ) .单个公司的风险几乎不存在，五个不同行业的权重至少为 9%。而该基金的远期市盈率仅为17，是一个更为合理的长期水平。
过去几年，人工智能的崛起为许多投资者带来了丰厚的回报。但现在是时候审视一下反弹带来的一些风险了。