S&P 600 → 400 → 500: how index migrants perform
By EtfWatch editorial · Published · Data updated
S&P Dow Jones Indices runs three US size indexes that together form the S&P Composite 1500: the SmallCap 600, the MidCap 400 and the S&P 500. A company that grows can climb that ladder, leaving one index the same day it joins the next. This article counts how often that shows up in the EtfWatch register and compares the climbers with companies that joined from outside the family.
What a migration is
The three indexes do not overlap: a stock belongs to at most one of them. When a SmallCap 600 member's market value rises past the mid-cap range, the index committee can move it to the MidCap 400; when a mid-cap grows into large-cap territory, it can be promoted to the S&P 500. Moves also run downwards for shrinking companies, but this article looks only at promotions, since those are the moves that create forced buying by larger-index funds.
A migration is different from a fresh addition in one important way. A migrant is already held by index funds — just the smaller ones. Funds tracking its old index must sell as funds tracking its new index buy. The net demand is the difference between the two, which is usually smaller than for a company no index fund owned before. That is the theory; the rest of this page checks it against what our register actually recorded.
How often it happens
The register currently holds 0 SmallCap 600, 0 MidCap 400 and 28 S&P 500 additions. Matching tickers across those lists, 0 MidCap 400 additions had previously been recorded as SmallCap 600 additions (an unmeasurable share of the MidCap 400 entries), and 0 S&P 500 additions had previously been recorded joining the MidCap 400 or SmallCap 600 (0% of S&P 500 entries).
These counts are a floor, not the true migration rate. Our register only goes back so far, so a company that joined the SmallCap 600 before our history began and was later promoted appears here as a fresh addition. S&P's own announcements frequently describe promotions within the 1500 — something our matching cannot see unless both steps fall inside the recorded period.
Recorded migrations
No ticker in the register has yet been recorded joining a smaller S&P index and later a larger one. The comparison below will fill in automatically once one does.
Migrants vs fresh additions
The table splits each destination index into migrants and everyone else. Groups with only a handful of entries are noisy; a single strong or weak stock can move the average by several points.
| Group | Entries | Median run-up | Avg 1M | Median 1M | Avg 3M | Avg 1Y | 1Y vs index |
|---|---|---|---|---|---|---|---|
| Migrants into MidCap 400 | 0 | — | — | — | — | — | — |
| Fresh MidCap 400 additions | 0 | — | — | — | — | — | — |
| Migrants into S&P 500 | 0 | — | — | — | — | — | — |
| Fresh S&P 500 additions | 28 | +16.9% | +0.5% | +2.2% | +7.3% | +27.6% | +11.3% |
Into the MidCap 400, migrants averaged not measurable over their first month (0 measured) against not measurable for fresh additions (0 measured). Into the S&P 500, migrants averaged not measurable (0 measured) against +0.5% for fresh additions (28 measured). Over a year, the corresponding averages were not measurable vs not measurable for the MidCap 400 and not measurable vs +27.6% for the S&P 500, where windows have completed.
Reading the result
If migrants show a smaller pre-inclusion run-up than fresh additions, that is consistent with the netting effect described above: some of the buying by new-index funds is offset by selling from old-index funds. If migrants do better afterwards, one plausible reason is that they are, by definition, companies whose shares have already risen enough to outgrow their old index — momentum that may or may not continue. Spin-offs and IPO-era companies that join directly from outside often have a short trading history and more volatile first months, which pulls the fresh-addition averages around.
Whatever the split shows today, sample size is the limiting factor. Treat a gap of a few points between groups with fewer than ten members as indistinguishable from chance.
Where to follow these indexes
Each index has its own hub with upcoming changes and its own statistics page: S&P 600 hub and insights; S&P 400 hub and insights; S&P 500 hub and insights. The review calendar lists the next quarterly rebalance dates.
Methodology
A migrant is a ticker recorded as a SmallCap 600 addition before a MidCap 400 addition, or recorded in either smaller index before an S&P 500 addition. Matching is by ticker only, so a reused ticker could in principle create a false match, and promotions whose first step predates the register are counted as fresh additions.
Returns are simple price changes measured from the effective-date close (offset 0) to the close nearest each window: 1M = 30, 3M = 90 and 1Y = 365 calendar days. "Vs index" subtracts the benchmark's change over the same window. Run-up is the move from 30 days before the effective close to that close, and only counts entries with a genuinely recorded announcement date. Windows that have not elapsed, or lack a usable close, are left out rather than treated as zero. Full details are on the About & methodology page.
Historical data only — not investment advice.