Common Sense Index Investing Bogle

The Little Book of Common Sense Investing — Index Investing Skill

Knowledge source: The Little Book of Common Sense Investing by John C. Bogle.

Overview

Use this skill to evaluate investment fund choices, portfolio simplicity, cost drag, index fund discipline, asset allocation, and buy-hold behavior using Bogle's common-sense investing framework. It is educational decision support, not personalized financial advice.

When to Use This Skill

Use this skill when the user asks:

• "Should I use an index fund or active fund?"
• "Are these fund fees too high?"
• "How should I think about ETFs, smart beta, or advisers?"
• "What stock/bond allocation is sensible?"
• "Am I overcomplicating my portfolio?"

Core Principle

Own the broad market at very low cost and hold it for the long term. Before costs, investors collectively earn the market return; after costs, the more they pay intermediaries, trade, and chase winners, the less they keep.

Workflow Inventory

Architecture Justification

The workflows are askable separately, but Bogle's method is intentionally simple: cost, broad diversification, tax efficiency, and patient holding are inseparable. A single-file skill preserves the "simplicity and parsimony" rule.

DIMENSION 1: Broad Market Ownership

The Rule: Prefer owning the entire market through a low-cost traditional index fund over trying to select winners.

Key questions to ask:

• Is the fund broadly diversified or making a narrow bet?
• Does it eliminate stock-picking, sector, and manager risk?
• Is the investor seeking market return or chasing alpha?

Decision criteria / Checklist:

• Broad-market exposure.
• Low tracking error to the intended market.
• No unnecessary sector, style, or manager concentration.
• Clear role in the portfolio.

Warning signals:

• Fund selection based on recent outperformance.
• Concentrated funds marketed as safer than they are.
• Complexity presented as sophistication.

Agent instruction:

When reviewing a fund, first determine whether it gives broad, low-cost market ownership or introduces avoidable active risk.

DIMENSION 2: Cost Matters

The Rule: Costs are the most reliable predictor of investor outcomes because every dollar paid to intermediaries is a dollar not compounding for the owner.

Key questions to ask:

• What expense ratio, trading cost, tax cost, advisory fee, and spread apply?
• How long will the investor hold it?
• Does higher cost buy a reliably higher net return?

Decision criteria / Checklist:

• Compare total all-in cost.
• Penalize turnover and tax inefficiency.
• Treat low cost as a durable advantage.
• In bonds, apply extra cost sensitivity because expected returns are lower.

Warning signals:

• Ignoring small annual fees over long horizons.
• Adviser recommendations biased toward high-cost products.
• Performance claims shown before costs, taxes, or survivorship bias.

Agent instruction:

Always run cost analysis before accepting a performance story.

DIMENSION 3: Reversion and Performance Chasing

The Rule: Past fund winners usually revert; chasing them turns a winner's game into a loser's game.

Key questions to ask:

• Is the user selecting based on recent performance?
• Are failed funds excluded from the track record?
• Is the fund's alpha statistically and economically durable after costs?

Decision criteria / Checklist:

• Examine long periods, not hot streaks.
• Watch for survivorship bias.
• Prefer repeatable structure over manager story.
• Assume excess return is rare and competed away.

Warning signals:

• Top-quartile fund rankings used as buying triggers.
• High turnover after disappointing performance.
• Smart beta marketed as guaranteed outperformance.

Agent instruction:

When a user cites performance, test whether it is durable, net of costs, and free of survivorship bias.

DIMENSION 4: Simple Allocation and Holding Discipline

The Rule: Decide stock/bond allocation, keep costs low, rebalance, and hold through cycles.

Key questions to ask:

• What horizon, income need, and risk capacity does the investor have?
• Is the portfolio simple enough to maintain?
• Does ETF trading flexibility tempt speculation?

Decision criteria / Checklist:

• Use a simple stock/bond mix appropriate to risk capacity.
• Favor buy-hold implementation.
• Rebalance by policy, not emotion.
• Be cautious with ETFs that invite trading.

Warning signals:

• Trading index products as if they were speculation vehicles.
• Portfolio crowded with overlapping funds.
• Changing allocation after market moves rather than life or risk changes.

Agent instruction:

For allocation questions, give ranges and rebalancing discipline, not market forecasts.

Query Response Framework

Query Type 1: Compare funds

1. List each fund's exposure, cost, turnover, tax efficiency, and complexity.
2. Apply broad-market and cost tests.
3. Identify conflicts or performance-chasing risks.
4. Give Bogle-style verdict: prefer, avoid, simplify, or needs data.

Query Type 2: Portfolio simplification

1. Identify current holdings and overlaps.
2. Reduce to broad low-cost building blocks.
3. Propose stock/bond policy and rebalancing rule.

Query Type 3: Cost or adviser review

1. Map all explicit and hidden costs.
2. Estimate compounding drag qualitatively or quantitatively if data exists.
3. Recommend lower-cost alternatives where appropriate.

Output Format

## Bogle Common-Sense Investing Review
**Decision:** ...
**Verdict:** Prefer index / Avoid active cost / Simplify / Needs data

| Test | Finding | Result |
|---|---|---|

## Portfolio or Fund Recommendation
...

## Cost Discipline
...

## Citations
...

Critical Reminders

1. Investors as a group receive market return minus costs.
2. Broad diversification eliminates avoidable selection risks.
3. Low cost is a durable edge.
4. Past performance is not a reliable selection method.
5. ETFs can be good tools but bad behavior enablers.

CITATION RULES

Every substantive Bogle-method claim must include a citation to the original text.

Quote files:

• cost-matters-quotes.md — costs, taxes, performance chasing, bond costs, speculative return, survivorship bias, ETFs, and Graham margin of safety.
• index-fund-quotes.md — owning the market, common sense, cost hypothesis, alpha, arithmetic, eliminated risks, reversion, simplicity, and investor underperformance.
• investing-wisdom-quotes.md — buy-and-hold, dividends, allocation, timeless advice, Gotrocks, smart beta, adviser conflicts, and simplicity.

Citation format:

"Author's exact words here."

— The Little Book of Common Sense Investing, cited excerpt

Anchor mapping:

• cost-matters-quotes.md: #tyranny-of-costs, #two-percent-costs, #taxes-are-costs, #past-performance-no-guarantee, #bond-costs-even-worse, #speculative-return, #survivorship-bias, #etf-trading-danger, #graham-margin-of-safety
• index-fund-quotes.md: #own-entire-market, #common-sense-investing, #cost-matters-hypothesis, #alpha-negative-sum, #relentless-arithmetic, #we-get-what-we-dont-pay-for, #index-fund-eliminates-risks, #reversion-to-mean, #simplicity-majesty, #investments-perform-better-than-investors
• investing-wisdom-quotes.md: #buy-hold-prosper, #dividends-best-friend, #asset-allocation-critical, #time-tested-advice, #gotrocks-parable, #smart-beta-costs, #advisers-conflicts, #keep-it-simple

Rules:

• Cite cost-related decisions with a cost anchor.
• Cite product or behavior warnings with ETF, adviser, smart-beta, or performance anchors.
• Do not provide personalized regulated advice; frame as educational policy.