A company added to a widely followed index often rises on the announcement. The movement comes from forced buying by funds rather than from any change in the business.
Tracking funds have no discretion
A fund committed to replicating an index must hold the constituents in the specified weights. When the composition changes, the fund must change with it.
This is not a judgement about the company. The obligation follows from the fund's mandate, and it applies regardless of valuation.
Because many funds track the same major indices, the aggregate demand created by a single addition can be large. The amount required depends on the weight assigned rather than on anything the company does.
Active managers benchmarked against the same index face a related pressure. Holding none of a new constituent becomes a deliberate position against it, which some are unwilling to take.
Demand is concentrated in time
Index changes take effect on a specified date, so buying is compressed into a short window rather than spread out.
Concentrated demand meeting a fixed supply of shares moves the price, which is a liquidity effect rather than a valuation one.
Removal from an index produces the mirror image, with forced selling pushing the price down over a similar window. Removals are often larger in effect because the company is usually smaller and less liquid by then.
Anticipation shifts the effect earlier
Index rules are generally published, so eligibility can be estimated before an announcement. Participants position ahead of expected changes.
This moves part of the price response forward, sometimes well before the effective date, and reduces what remains for the event itself.
It also means an expected inclusion that does not happen can produce a fall, as anticipatory positions are unwound.
The effect tends to fade
Once tracking funds have completed their purchases, the concentrated demand ends and ordinary trading resumes.
Prices frequently drift back toward where they were relative to comparable companies, since nothing fundamental changed.
The persistence of any lasting effect is debated, and it is smaller than the immediate movement suggests.
Secondary consequences are real
Inclusion raises visibility, broadens the shareholder base and can increase the number of analysts covering the company.
A larger and more diverse holder base can improve liquidity, which reduces trading costs and can modestly affect the cost of raising capital.
These effects are gradual and structural, and they are quite separate from the price movement around the inclusion date itself.