Common Sense Index Investing Bogle Agent Skill

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

| Workflow | User question pattern | Inputs | Steps | Output | Independent trigger? | Distinct references? | Triage score | Should be subskill? | Reason |

|---|---|---|---|---|---|---:|---:|---|---|

| Fund comparison | "Which fund should I choose?" | Fees, turnover, strategy, benchmark, tax status | Compare cost, diversification, active risk, simplicity | Fund verdict | Yes | Yes | 3 | No | Primary index-investing workflow. |

| Cost drag analysis | "Do fees matter?" | Expense ratio, turnover, horizon | Estimate compounding drag | Cost warning | Yes | Yes | 3 | No | Same decision report. |

| Asset allocation | "How much stocks/bonds?" | Age, horizon, risk capacity, income need | Apply simple stock/bond discipline | Allocation range | Yes | Yes | 3 | No | Fits the same portfolio policy. |

| Product skepticism | "Should I buy ETF/smart beta/adviser pick?" | Product structure, costs, incentives | Check complexity, trading temptation, conflicts | Use/avoid verdict | Yes | Yes | 3 | No | Same Bogle cost/simplicity lens. |

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

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

Query Type 2: Portfolio simplification

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

Query Type 3: Cost or adviser review

• Map all explicit and hidden costs.
• Estimate compounding drag qualitatively or quantitatively if data exists.
• 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

• Investors as a group receive market return minus costs.
• Broad diversification eliminates avoidable selection risks.
• Low cost is a durable edge.
• Past performance is not a reliable selection method.
• 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.