AlphaTRADER Academy
How Indices Are Built
Most retail traders treat indices as "the market." They aren't. They're curated lists where losers get removed and winners stay. That's why "the index always rises" is half-truth — and once you see the mechanics, you trade them differently.
"In the long run, the market is a weighing machine. In the short run, a voting machine." — Benjamin Graham
Three Ways Indices Are Weighted
Same word "index" — very different mechanics. Pick a tab.
Used by
Strength
Weakness
The S&P 500 Formula
Since 2005, the S&P 500 uses float-adjusted market capitalization. "Float" = shares actually available for public trading — insider holdings, government stakes, and locked shares are excluded. This means a company with mostly insider-owned shares gets a smaller weight than its full market cap would suggest.
Index weight per company
i = each constituent company; denominator = sum across all 500
Price moves up
Company's weight grows automatically. Index inherits the gain proportionally.
New shares issued
Float adjusts at next review. More public shares = bigger weight (if price holds).
Buyback shrinks float
Fewer public shares → smaller float weight, but EPS pump usually lifts price → roughly neutral.
How Companies Get In (and Kicked Out)
The S&P 500 isn't algorithmic — a human committee at S&P Dow Jones Indices decides who's in. Reviews happen quarterly (March, June, September, December). Eligibility criteria are public and have evolved over time.
To Get In
- ▸ US-domiciled corporation
- ▸ Market-cap threshold (raised over time — currently in the tens of billions USD)
- ▸ Public float ≥ 50% of shares
- ▸ Liquidity minimum (avg daily trading volume)
- ▸ Positive earnings: sum of last 4 quarters AND most recent quarter must be positive (GAAP)
- ▸ Committee approval
To Get Kicked Out
- ▸ Merger / acquisition (company no longer independent)
- ▸ Bankruptcy / Chapter 11 filing
- ▸ Market cap drops far below threshold
- ▸ Loss of representative status (sector imbalance)
- ▸ Going private / delisting
Survivor Bias — The Key Insight
If you understand only one thing about indices, make it this.
The S&P 500 chart you see on TradingView doesn't show you 500 companies bought in 1957 and held forever. It shows a rolling roster where companies that fail are continuously removed and replaced with newer, healthier ones. Of the original 1957 S&P 500 list, only a small fraction (roughly 50–60 names) still belongs to the index today.
Trader takeaway: the "S&P 500 always recovers" narrative is partly an artifact of this curation. The index recovers because its losers get removed and replaced with surviving winners. You can replicate this with an ETF (which rebalances for you), but you cannot replicate it by picking individual 1957-era stocks and holding them.
Try It — Survivor Bias Simulator
Compare two strategies side-by-side: (a) Curated Index — kicks out failed companies, replaces with new ones (what S&P actually does). (b) Buy & Hold 1957 names — keeps every original stock, including the ones that go to zero.
Curated Index (S&P style)
Started at $100k
Buy & Hold 1957 names
Failed stocks drag the portfolio
Survivor Bias Gap
Portfolio value over time — both strategies side by side
The model (transparent math, no hand-waving)
Curated value after T years: $100k × (1 + g)T — kicked-out losers are replaced with fresh stocks that compound at the same rate. Buy & Hold value: $100k × ((1 − f) × (1 + g))T — survivors grow, but the fraction f that fail each year drag the portfolio (stock-by-stock, the failed ones become $0; in aggregate the survival factor compounds). Ratio: B&H / Curated = (1 − f)T — at 3% annual failure for 30 years, that's ~0.40 — the "S&P 500 always rises" chart is reading the green line; the original 1957 list is the red line.
The "Diversification" Illusion
"S&P 500 has 500 companies — fully diversified." Not quite. Because the index is market-cap weighted, the few largest companies dominate the return profile. In recent years, the top 10 holdings have accounted for roughly 30%+ of the entire index — and the top 7 mega-caps in tech have at times pushed that share even higher.
~30%+
Top 10 stocks' share of S&P 500 in recent years
11 → fewer
GICS sectors in the index — but a couple now dominate weight
≠ Equal
S&P 500 vs. S&P 500 Equal-Weight (RSP) deliver very different returns
This is why the S&P 500 and its Equal-Weight sibling (where every company gets the same ~0.2% allocation, rebalanced quarterly) can diverge sharply in periods when a few megacaps dominate the move. The cap-weighted version is essentially a momentum strategy in disguise — it overweights what already worked.
Counter-Example: Nikkei 225
"Indices always recover, just wait." The Japanese Nikkei 225 disagrees. It peaked on December 29, 1989 at roughly 38,915. It took until 2024 — about 35 years — to reclaim that level. Investors who bought the top in 1989 spent an entire working career underwater, even with reinvested dividends.
Why it stalled (what to learn)
- ▸ Demographics: aging population, shrinking workforce → falling consumer demand.
- ▸ Deflation trap: prices flat or falling for ~two decades — no nominal tailwind.
- ▸ Banking crisis: zombie banks carrying bad real-estate loans suppressed credit growth.
- ▸ Concentration in legacy industries: Nikkei 225 is price-weighted and was tilted toward stagnant sectors.
Index permanence isn't a law of physics. It depends on the underlying economy, demographics, and monetary regime. The S&P 500's long-term rise reflects US conditions — not all indices everywhere.
So Why Do Healthy Indices Trend Up?
Survivor bias is part of the answer. But genuine long-term drivers also exist — five of them, working together:
Productivity
Output per worker rises over decades — technology, automation, knowledge accumulation.
Inflation
Even with zero real growth, nominal prices climb. Equities denominated in a depreciating currency drift up.
Money Supply
Central banks expand the monetary base over time. More currency chasing assets lifts asset prices.
Population & Consumers
More consumers → more demand → more corporate revenue. Demographic shrinkage reverses this (see Nikkei).
Reinvested Earnings
Profits compound. Buybacks shrink float and lift EPS. Dividends reinvested at lower prices accelerate compounding.
+ Survivor Curation
The five engines above push real growth. The index amplifies it by continuously kicking out the failures.
Key Takeaways
- 1 Indices are curated lists, not buy-and-hold portfolios. Different weighting schemes (price, market-cap, equal) produce very different returns.
- 2 S&P 500 uses float-adjusted market cap since 2005. A committee — not an algorithm — decides additions and removals quarterly.
- 3 Survivor bias is the underrated reason "the index always recovers" — failed companies are removed; you only see the winners in the chart.
- 4 "Diversified S&P 500" is an illusion — top 10 names sit at ~30%+ of the index in recent years. Equal-weight versions tell a different story.
- 5 Indices don't have to recover — the Nikkei spent 35 years underwater after 1989. Demographics, deflation, and concentration matter.
- 6 Real long-term drivers exist: productivity, inflation, money supply, population, reinvested earnings — plus the curation effect on top.
Test Your Understanding
5 questions — instant feedback, no scoring stored.