What index rebalancing does to quiet stocks

Every June and December, the major broad indices publish their new constituent lists, and for a short window the normal relationship between price and opinion breaks. Funds tracking the index must buy the additions and sell the deletions in size, on a schedule that is public in advance. The flows are not information. They are plumbing.

The pattern is consistent enough to be dull: additions tend to run up between announcement and effective date, deletions tend to sag, and both partially retrace once the mechanical demand is gone. The stocks most affected are the quiet ones — mid-caps with thin daily volume, where a few days of index buying can equal weeks of normal turnover.

Two practical lessons. First, if you already hold a stock that is being deleted, the rebalance window is close to the worst moment to sell it; the price you see includes a forced seller. Second, buying an addition purely because it was added is buying the flow, not the company, and the flow leaves.

None of this is an argument against index investing. It is an argument for reading the calendar before reading the price.