今年距离指数投资首次引入给普通投资者已满 50 年。如今，它正面临来自 SpaceX、Anthropic 和 OpenAI 等公司一波大型首次公开募股的新考验，问题在于指数基金是否会被迫大规模买入它们并承受风险。如今的 IPO 管线包括估值达到一万亿美元的公司，使它们跻身市场上最大的公司之列。

指数化投资在半个世纪前开始时，IPO 规模较小，而且公司较大比例的股份从一开始就上市并交易。对于即将到来的 IPO，上市时可能只会释出较小一部分股份，更多股份会随时间进入市场供公众投资者购买。经自由流通量调整的市值意味著，一家公司在指数中的权重基于投资者实际可以购买的股份，不包括内部人士和其他私人投资者持有、未在交易所上市的股份。

像 SpaceX 这样的公司可能拥有一万亿美元估值，但其股份可能只有一小部分可供交易，即所谓的自由流通股。即使自由流通股比例相对较小，SpaceX 仍可能成为迄今最大的 IPO，因此可交易的美元价值仍将相当可观，支撑深厚的流动性和有效的价格发现。Facebook 2012 年 IPO 后最初几个月下跌近 50 per cent，而这家公司如今名为 Meta，市值超过 $1.5tn。

This year marks 50 years since index investing was first introduced to everyday investors. Today, it faces a new test from a wave of mega initial public offerings from companies such as SpaceX, Anthropic and OpenAI, with questions about whether index funds will be forced to buy them at scale and be exposed to risks. Today’s pipeline of IPOs includes companies with trillion-dollar valuations, placing them among the largest companies in the market.

When indexing began a half-century ago, IPOs were smaller and a larger share of the company was listed and traded from the outset. With upcoming IPOs, a smaller portion will probably be made available at listing, with additional shares entering the market for public consumption over time. Float-adjusted market capitalisation means a company’s weight in the index is based on the shares investors can actually buy, excluding shares held by insiders and other private investors that are not listed on an exchange.

A company like SpaceX may carry a trillion-dollar valuation, but only a fraction of its shares is likely to be available for trading, the so-called free float. Even with a relatively small float, SpaceX will probably be the largest IPO to date, so the dollar value available to trade will still be substantial, supporting deep liquidity and efficient price discovery. Facebook’s 2012 IPO fell nearly 50 per cent in the first few months after its debut, while the firm, now called Meta, has a market capitalisation of more than $1.5tn.