The core of passive investing is replicating an index, and an index is determined by a provider according to its rules. When passive money reaches a substantial proportion of a market, index rules turn from a descriptive tool into a mechanism with real allocative effect. Securities included in an index receive buying, those excluded face selling pressure, and this has nothing to do with corporate fundamentals. Inclusion and exclusion decisions follow published rules, and applying the rules involves judgement. Capitalisation thresholds, liquidity standards, free float ratios and accessibility conditions are clear in most cases and require judgement in borderline ones.
Whether a market or company near a threshold is included may depend on the provider's interpretation of a particular condition. The consultation process is an important part of this mechanism. Major rule changes or market reclassifications usually go through public consultation, with documents published, views sought and observation periods set. This process gives decisions predictability and transparency, and governments and exchanges participate actively, since a change in classification directly affects the scale of capital flowing into the market. Capital movement on the effective date is an observable effect.
When an inclusion decision takes effect, funds tracking the index must complete their buying on that date to maintain their tracking. This produces changes in volume and price around the effective date, and that change has nothing to do with fundamentals, being purely mechanical rebalancing. Research widely documents a price effect in the securities concerned between announcement and implementation. This mechanism also produces front-running behaviour. Because the effect of inclusion is predictable, active participants buy securities expected to be included ahead of the effective date and sell them to passive money on the date.
The existence of this front-running means part of the price effect occurs at announcement rather than on the effective date. Passive money in effect completes its buying at a higher price. The details of index rules therefore carry real importance. Weight caps, rebalancing frequency and the pace of inclusion factor adjustment are seemingly technical rules that determine the scale and timing of capital flows. Two indices tracking nominally the same market can show noticeable differences in composition and capital flows if they use different rules. That difference ultimately shows up in the returns of the funds tracking them. For investors, understanding this mechanism carries two meanings.
One is knowing that certain price movements come from an index's mechanical capital flows rather than from fundamental information. The other is knowing that a passive fund's actual composition and behaviour depend on which provider's index it tracks and what rules that index uses. This information is all in public documents. One point about scale is worth adding. The larger the pool of passive money tracking an index, the greater the mechanical effect of any change to it. The influence of these decisions has grown with the passive share itself. What was once a marginal technical effect has become large enough that index rule changes are studied as market events in their own right. This is quite separate from anything happening in the underlying businesses.