What Is an Index Fund?

# What Is an Index Fund?

Research. Education. Perspective.

Difficulty: Foundation Reading time: 15 minutes Last reviewed: August 10, 2026

> Educational Resource > > This article explains index funds and passive investing. It does not recommend any index, index fund, ETF, mutual fund, benchmark, asset allocation or investment strategy.

Executive Summary

An index fund is an investment fund designed to track the returns of a selected market index.

Investor.gov defines an index fund as a mutual fund, exchange-traded fund, or unit investment trust following a passive investment strategy designed to achieve approximately the same return as a particular index before fees.[1][2]

That definition contains two distinct things:

1. The index — a rules-based measurement of a selected market or group of securities.
2. The index fund — an actual investment vehicle that attempts to follow that index.

An index is not ordinarily a security an investor purchases directly.

A fund is.

That distinction matters because the performance of an index fund can differ from the published performance of its benchmark.

Differences can arise from:

• Fees
• Trading costs
• Cash holdings
• Sampling
• Taxes
• Rebalancing
• Corporate actions
• Portfolio implementation

Index funds are often associated with broad diversification and low costs.

Many do provide those characteristics.

But neither is guaranteed by the phrase index fund.

An index fund can track:

• Thousands of securities across the global market
• A single country
• One industry
• A narrow investment factor
• A specialized theme
• A non-traditional rules-based strategy

The correct starting point is therefore not:

"Is it an index fund?"

It is:

"Which index does it track, how is that index constructed, and how closely and efficiently does the fund reproduce that exposure?"

Key Takeaways

• An index fund seeks to track a selected market index before fees.[1][2]
• Index funds can be mutual funds, ETFs or certain other fund structures.[2]
• The index is the benchmark; the fund is the investable vehicle.
• Passive investing is rules-based rather than completely decision-free.[6]
• Index methodology determines which securities are included and how much influence each receives.
• Broad-market and narrow thematic index funds can have very different risk.
• Full replication and sampling are two ways funds can implement index exposure.
• Index-fund returns can differ from benchmark returns.
• Fees reduce investor returns.[5]
• Two funds following the same index can still differ in cost, liquidity, tax treatment and tracking.
• Non-traditional indexes can be significantly more complex than broad market-cap-weighted benchmarks.[9]

What Is a Market Index?

A market index is a measurement tool.

It tracks the performance of a defined collection of securities according to a methodology.

Examples can include indexes designed to measure:

• Large U.S. companies
• Small companies
• International stocks
• Government bonds
• Corporate bonds
• Particular industries
• Investment factors
• Specialized strategies

The index provider determines rules such as:

• Which securities can qualify
• When securities are added or removed
• How securities are weighted
• How corporate actions are handled
• When the index is rebalanced
• How returns are calculated

The index therefore does not simply "exist."

It is constructed.

The Index Is Not the Fund

This is the central distinction.

Suppose an index contains 500 securities.

The index provider calculates a benchmark return using its published methodology.

A separate fund company can launch an index fund designed to track that benchmark.

The fund must then:

• Obtain investor capital
• Buy securities
• Manage cash flows
• Rebalance holdings
• Pay expenses
• Handle distributions
• Implement index changes

The benchmark has no investor redemptions or operating expense ratio.

The fund does.

That is why benchmark return and fund return are not automatically identical.

> Rockwell Forbes Definition > > An index fund is an investable portfolio designed to reproduce, as closely as practical, the economic performance of a specified rules-based benchmark before or after applicable costs.

What Does "Passive" Mean?

Index funds are commonly described as passively managed.

Investor.gov says index funds follow a passive investment strategy designed to achieve approximately the same return as a selected index before fees.[1][2]

FINRA contrasts passive investing with active approaches that involve more discretionary decisions intended to outperform a benchmark or respond to market conditions.[6]

Passive does not mean no decisions are made.

Decisions still exist at several levels.

Index provider decisions

Someone determines:

• Eligibility rules
• Weighting
• Rebalancing
• Methodology changes

Fund-manager implementation decisions

The fund determines:

• How to track the index
• How to handle cash
• How to trade index additions and deletions
• Whether to replicate or sample
• How to manage portfolio operations

Investor decisions

The investor still chooses:

• Which index exposure
• Which fund vehicle
• Which account
• When to buy or sell
• How much exposure to hold

Passive investing reduces some forms of discretionary security selection.

It does not remove judgment from the system.

Index Fund vs. Index

| Index | Index fund | |---|---| | Measurement benchmark | Investable fund or trust | | Built from a methodology | Holds assets or exposure | | Does not have an investor expense ratio | Has operating costs | | Does not process investor purchases or redemptions | Must handle investor flows | | Published return is theoretical benchmark performance | Investor experiences actual fund performance | | Cannot ordinarily be purchased directly | Fund shares can be purchased |

This distinction also explains why index performance should not be presented as though it were exactly what every investor would have earned.

Index Mutual Funds

An index fund can be structured as a traditional mutual fund.

Investor.gov explains that traditional mutual funds generally process purchases and redemptions at the next calculated net asset value, or NAV.[3]

An index mutual fund therefore combines:

• Passive benchmark tracking
• Traditional mutual-fund pricing and redemption mechanics

The fund can invest in:

• Stocks
• Bonds
• Multiple asset types

depending on the benchmark.

Index ETFs

An index fund can also be structured as an ETF.

ETF shares generally trade throughout the day on an exchange.[4][8]

An index ETF combines:

• Passive benchmark tracking
• Exchange-traded share mechanics

Its market price can differ temporarily from NAV.

Trading can also involve:

• Bid-ask spreads
• Premiums or discounts
• Brokerage mechanics
• Intraday liquidity

The investment objective can be nearly identical to that of an index mutual fund while the investor transaction mechanics differ.

Index Mutual Fund vs. Index ETF

| Index mutual fund | Index ETF | |---|---| | Tracks a benchmark | Tracks a benchmark | | Traditional purchases/redemptions generally at next NAV | Shares trade intraday | | No ordinary exchange bid-ask spread on fund share transaction | Bid-ask spread can apply | | Fund processes shareholder flows | Retail investor generally trades with market participants | | Can have minimum investment requirements | Often purchased in share or fractional-share quantities where available | | May offer multiple share classes | ETF share structure differs |

Both vehicles can track the same index.

That does not make the investor experience identical.

How Are Indexes Weighted?

An index needs rules for deciding how much each security matters.

Different weighting systems produce different economic exposures.

Market-capitalization weighting

Companies with larger market capitalizations receive larger weights.

If Company A has twice the eligible market capitalization of Company B, it may receive roughly twice the influence under a straightforward market-cap weighting system.

Many broad stock indexes use versions of capitalization weighting.

Price weighting

Securities with higher share prices receive greater weight.

A $200 stock can have more influence than a $50 stock even if the lower-priced company has a larger market capitalization.

Equal weighting

Each security receives approximately the same weight at rebalance.

A 100-stock equal-weighted index might target about 1% in each constituent before market movements cause drift.

Fundamental or factor weighting

Some indexes weight securities using rules related to:

• Earnings
• Revenue
• Dividends
• Value characteristics
• Momentum
• Volatility
• Quality
• Other factors

These are still indexes if they follow defined rules.

But they can behave very differently from a broad capitalization-weighted benchmark.

Weighting Changes Risk

Consider an index containing the same 100 stocks.

Index A

Weights companies by market capitalization.

Index B

Weights each company equally.

The constituent list is identical.

The portfolio exposure is not.

The equal-weight index gives smaller companies more influence relative to their market value.

The market-cap-weighted index gives larger companies more influence.

The two indexes can therefore produce different:

• Sector weights
• Company-size exposure
• Turnover
• Risk
• Return

"Tracks 100 stocks" is not enough information.

Weighting matters.

Broad Indexes vs. Narrow Indexes

A broad index might cover:

• A large portion of the U.S. equity market
• Global equities
• A diversified bond market

A narrow index might cover:

• One sector
• One industry
• One country
• One theme
• One factor

Both can support legitimate index funds.

But their diversification characteristics are very different.

FINRA's 2025 discussion of non-traditional index funds emphasizes that newer index methodologies can be more complex than traditional broad benchmarks and should be evaluated on their own terms.[9]

> Index Fund Does Not Automatically Mean Broad Diversification > > A rules-based portfolio can still be concentrated.

Number of Holdings Is Not Enough

Suppose Fund A owns 500 companies but its ten largest holdings represent a very large percentage of assets.

Fund B owns 200 companies with much more evenly distributed weights.

Which is more diversified?

The answer cannot be determined from the number of holdings alone.

Diversification also depends on:

• Weight concentration
• Sector concentration
• Geographic concentration
• Correlation
• Common economic drivers

This is why index methodology matters more than the headline constituent count.

Full Replication

One way to track an index is full replication.

The fund attempts to own all securities in the benchmark in approximately the benchmark weights.

This can be practical when:

• The index contains readily tradable securities
• Holdings are transparent
• Transaction costs are manageable

Full replication can make the portfolio conceptually easy to compare with the benchmark.

It still does not guarantee identical returns because the fund has real-world costs.

Representative Sampling

A fund can also use sampling.

Instead of owning every index security, it owns a subset designed to reproduce the benchmark's major characteristics.

Sampling can be useful when:

• The index contains thousands of securities
• Some securities are illiquid
• Full replication would be costly
• Operational efficiency matters

Sampling introduces additional implementation risk.

If the selected portfolio does not behave exactly like the full index, tracking can differ.

What Is Tracking Difference?

Tracking difference is the difference between the index fund's return and the benchmark's return over a period.

Suppose:

• Index return: 8.00%
• Fund return: 7.75%

Tracking difference:

7.75% − 8.00% = -0.25 percentage point

The fund trailed the benchmark by 0.25 percentage point.

Potential sources include:

• Expense ratio
• Trading costs
• Sampling
• Cash holdings
• Taxes
• Rebalancing
• Securities lending economics
• Timing
• Portfolio-management implementation

A tracking difference does not automatically indicate poor management.

Some gap is structurally expected because real funds have expenses and operational constraints.

What Is Tracking Error?

Tracking difference and tracking error are related but not identical.

Tracking difference

The return gap between fund and benchmark over a specified period.

Tracking error

The variability of those return differences over time.

A fund can have:

• A small average tracking difference
• But inconsistent day-to-day or month-to-month tracking

or:

• A somewhat larger but highly consistent gap

Tracking error helps describe the consistency of benchmark replication.

Why Fees Matter

The SEC's July 2025 bulletin emphasizes that mutual-fund and ETF fees reduce investment returns.[5]

Index funds are often lower cost than more research-intensive active funds.

But index funds are not automatically inexpensive.

Expenses can include:

• Management fees
• Administrative expenses
• Other operating expenses
• ETF trading spreads
• Brokerage-related costs
• Taxes
• Transaction costs inside the portfolio

A fund's expense ratio is one important measure.

It is not the only possible economic cost.

Two Funds Tracking the Same Index

Suppose two funds both track the same benchmark.

Fund A:

• Expense ratio: 0.05%
• Tight ETF spread
• Consistent tracking

Fund B:

• Expense ratio: 0.50%
• Wider spread
• Larger tracking gap

The benchmark exposure may be similar.

The investor experience can differ.

Comparison should therefore include:

• Expense ratio
• Tracking difference
• Vehicle structure
• Bid-ask spread for ETFs
• Minimums for mutual funds
• Tax characteristics
• Liquidity
• Securities-lending policy
• Fund size
• Operational history

The index name alone is not a complete product analysis.

Lower Cost Is an Advantage, Not a Guarantee

If two investments produce identical gross performance, the lower-cost one will leave more of that gross result with the investor.

That is arithmetic.

But future gross performance is not known.

A lower expense ratio does not guarantee:

• Positive returns
• Better risk-adjusted returns
• Better diversification
• Better tracking
• Better tax outcomes
• Superior performance versus every alternative

Cost is one of the more knowable variables.

It should not be confused with certainty about investment results.

Index Rebalancing

Indexes change over time.

A benchmark can:

• Add securities
• Remove securities
• Change weights
• Reclassify companies
• Update methodology

When the benchmark changes, the index fund generally needs to adjust its portfolio.

This means passive funds do trade.

They are not static portfolios that buy a fixed list of securities forever.

Index rebalancing can create:

• Turnover
• Transaction costs
• Tax effects
• Temporary tracking differences

The amount depends on the index and fund structure.

Index Reconstitution

Some indexes periodically reconsider the entire eligible universe.

This process can be called reconstitution.

A company can move:

• Into an index
• Out of an index
• From one size category to another

Index funds following the benchmark then need to adapt.

This can produce substantial trading around scheduled index events.

Again, passive means following a rulebook.

It does not mean doing nothing.

Concentration Risk in Index Funds

Market-cap-weighted indexes can become concentrated when a small number of companies become very large relative to the rest of the market.

Narrow indexes can be concentrated by design.

Potential concentration can occur by:

• Company
• Sector
• Country
• Factor
• Industry
• Theme

FINRA warns investors to look through fund labels and understand concentration risk rather than assuming a fund automatically creates broad diversification.[9]

A diversified vehicle structure cannot overcome a concentrated benchmark.

Index Funds and Market Risk

An index fund follows its benchmark down as well as up.

If a broad stock index falls 30%, a fund designed to track that index will generally also experience a substantial decline, before considering tracking differences.

The fund manager ordinarily does not abandon the benchmark because markets look unattractive.

That is part of the passive strategy.

Passive investing therefore reduces certain manager decisions.

It does not reduce the underlying market exposure the benchmark contains.

Index Funds and Valuation

Index funds are sometimes described as "buying the market."

That does not mean valuation disappears.

If a market index becomes expensive relative to its fundamentals, an index fund tracking it still owns that exposure.

If one industry becomes a large portion of a capitalization-weighted index, the fund generally increases its exposure mechanically as that industry's market value rises.

Passive investing does not make valuation irrelevant.

It changes where the valuation decision sits.

Instead of asking:

Which individual stock is undervalued?

the investor may be asking:

Which market or index exposure is being purchased, and what valuation characteristics does it contain?

Index Funds and Turnover

Index funds often have lower portfolio turnover than actively managed funds, especially broad-market funds.

But turnover varies by methodology.

A stable broad index can have relatively low turnover.

A strategy that regularly rebalances factors or equal weights may trade much more.

Higher turnover can create:

• Trading costs
• Tax realization
• Market impact

The word "index" does not guarantee low turnover.

The methodology determines how often the portfolio needs to change.

Traditional vs. Non-Traditional Indexes

FINRA distinguishes traditional broad market indexes from newer or non-traditional indexes that may use specialized weighting or selection methodologies.[9]

Examples can include indexes built around:

• Low volatility
• Momentum
• Dividend yield
• Value
• Quality
• Multiple factors
• Themes

These approaches can still be rules-based and passive in implementation.

But they contain active-like design choices in the methodology.

The index creator decides which rules define the desired exposure.

Investors therefore should understand the strategy rather than relying on the word "index."

Factor Indexes

A factor index systematically favors securities with selected characteristics.

Common examples include:

• Value
• Momentum
• Quality
• Size
• Low volatility

A factor index can be transparent and rules-based.

It can also:

• Underperform for long periods
• Become concentrated
• Experience factor crowding
• Produce higher turnover
• Behave differently from a broad market benchmark

Factor exposure is an investment thesis embedded in an index rulebook.

Thematic Indexes

Thematic indexes target an economic or investment theme.

Examples might focus on:

• Artificial intelligence
• Clean energy
• Robotics
• Cybersecurity
• Space-related companies

The challenge is that a compelling theme does not automatically produce attractive investment returns.

Relevant issues include:

• Valuation
• Company selection
• Definition of the theme
• Concentration
• Profitability
• Competition
• Rebalancing methodology

A rules-based theme can still be speculative or highly concentrated.

Index Funds and Securities Lending

Some index funds lend portfolio securities to qualified borrowers under securities-lending arrangements.

Potential revenue from securities lending can partly offset fund expenses.

But securities lending also introduces:

• Counterparty exposure
• Collateral-management considerations
• Operational risk

Fund disclosures explain the policy and economics.

This is another example of real-world portfolio activity that does not exist in a theoretical benchmark in exactly the same way.

Index Funds and Taxes

Tax results depend on:

• Fund structure
• Account type
• Portfolio turnover
• Distributions
• Investor transactions
• Jurisdiction
• Individual circumstances

Index funds can sometimes have tax advantages associated with lower turnover.

ETFs can also have structural tax characteristics related to creation and redemption.

But tax efficiency is not guaranteed.

A specialized index can have significant turnover.

A mutual-fund structure can distribute realized gains.

Tax analysis should therefore be based on the actual fund and account rather than the word "index."

Index Fund vs. Actively Managed Fund

| Index fund | Active fund | |---|---| | Seeks to follow a benchmark | Manager makes discretionary investment decisions | | Rules-based exposure | Judgment-based security selection or allocation | | Generally evaluates success relative to tracking | Often evaluates success relative to benchmark or objective | | Can have lower costs | Can have higher costs, though not always | | Usually does not seek to avoid benchmark declines | Manager may change exposures within mandate | | Methodology risk matters | Manager-selection risk matters |

FINRA's 2025 active-versus-passive guidance emphasizes that both approaches have potential benefits and limitations.[6]

The appropriate comparison is not ideological.

It is economic.

Index Fund vs. Direct Indexing

FINRA describes direct indexing as a strategy seeking to replicate an index while the investor directly owns the underlying securities rather than shares of a pooled fund.[10]

That can allow:

• Customization
• Individual security ownership
• Greater control over tax-lot decisions

It can also create:

• Complexity
• Trading requirements
• Tracking differences
• Additional costs
• Operational burden

Direct indexing therefore differs structurally from owning an index mutual fund or ETF.

It is a portfolio-management approach rather than a pooled fund.

A Basic Index-Fund Research Framework

Before focusing on past returns, useful questions include:

1. Which index does the fund track?
2. Who maintains the index?
3. What securities are eligible?
4. How are securities weighted?
5. How concentrated are the largest holdings?
6. How often is the index rebalanced or reconstituted?
7. Does the fund fully replicate or sample the benchmark?
8. What is the fund's expense ratio?
9. How large has historical tracking difference been?
10. If it is an ETF, what are typical spreads and liquidity conditions?
11. How much portfolio turnover does the methodology create?
12. Are leverage, derivatives or non-traditional weighting rules involved?
13. What economic risk is the index actually concentrating or diversifying?

These questions help describe the exposure without deciding whether it belongs in a particular portfolio.

Common Misconceptions

"Index fund and ETF mean the same thing."

No. An index fund can be a mutual fund, ETF or certain other structures.[2]

"Every index fund tracks the S&P 500."

No. Thousands of indexes cover different markets, sectors, strategies and asset classes.

"Every index fund is diversified."

No. Narrow indexes can be highly concentrated.

"Passive means nobody makes decisions."

No. Index providers design rules, fund managers implement them and investors select among exposures.

"An index fund exactly matches its benchmark."

Not necessarily. Fees, trading costs, sampling, cash and implementation can create tracking differences.

"The index with more holdings is always more diversified."

No. Weight concentration and common economic exposures also matter.

"Low cost means low risk."

No. A low-cost fund can track a highly volatile or concentrated index.

"Two funds tracking the same index are identical."

No. Costs, vehicle structure, liquidity, tax characteristics and tracking can differ.

"I can buy the index itself."

Ordinarily, an index is a benchmark rather than a tradable security. Investors access index exposure through funds, derivatives or other products.

Frequently Asked Questions

What is an index fund in simple terms?

An index fund is a mutual fund, ETF or certain other pooled vehicle designed to track the returns of a selected market index before fees.[1][2]

Is an index fund the same as an ETF?

No. Some ETFs are index funds, but ETFs can also be actively managed. Index funds can also be structured as mutual funds.[3][4]

Is an index fund passive?

Traditional index funds follow passive, rules-based strategies intended to track benchmarks.[1][6]

Can index funds lose money?

Yes. If the benchmark or underlying assets decline, an index fund can lose substantial value.

Are index funds diversified?

Some are broadly diversified. Others track narrow sectors, factors or themes. Diversification depends on index construction.

What is tracking difference?

Tracking difference is the return gap between a fund and the benchmark it seeks to follow over a specified period.

Why can an index fund underperform its index?

Expenses, trading costs, cash holdings, sampling, taxes and implementation can create differences.

Do index funds have fees?

Yes. Mutual funds and ETFs have operating expenses, and ETF investors can also encounter trading costs such as bid-ask spreads.[5]

What is market-cap weighting?

It is an index methodology in which larger companies by eligible market capitalization generally receive larger weights.

What is an equal-weight index?

It is an index that targets approximately equal weights among its constituents at rebalance rather than weighting them according to market capitalization.

What is a non-traditional index fund?

FINRA uses the term for funds tracking indexes that can use specialized methodologies, factors or strategies beyond traditional broad market benchmarks.[9]

Is passive investing risk-free?

No. Passive portfolios remain exposed to the market, credit, concentration, liquidity and other risks embedded in their benchmarks.

The Bottom Line

An index fund is an investment vehicle built to follow a rulebook.

The index defines:

• What qualifies
• What is excluded
• How holdings are weighted
• When the benchmark changes

The fund then attempts to reproduce that exposure in the real world.

That implementation introduces:

• Fees
• Trading
• Cash flows
• Tracking differences
• Taxes
• Operational decisions

The word passive therefore should not be confused with:

• No risk
• No judgment
• No trading
• Guaranteed diversification
• Guaranteed low cost
• Guaranteed market returns

The most useful index-fund question is not:

"Is passive investing better?"

It is:

"What rules define this index, what exposure do those rules create, and how efficiently does the fund deliver that exposure?"

That is the foundation for understanding index funds as investments rather than treating "index" as a quality label.

Continue Your Learning

1. What Is a Mutual Fund? — Understand the traditional fund structure used by many index funds.
2. What Is an ETF? — Learn how index exposure can trade through an exchange-traded vehicle.
3. Asset Classes Explained — Separate benchmark exposure from the fund vehicle.
4. What Is Asset Allocation? — Understand how index funds can represent broad portfolio exposures.
5. Diversification — Learn why constituent count alone does not determine diversification.
6. Risk vs. Return Explained — Understand the market and concentration risks index funds retain.
7. Common Investing Mistakes — Explore the danger of assuming a familiar index label eliminates due diligence.
8. Liquidity — Understand ETF trading liquidity and underlying-market liquidity.

Sources & References

1. [U.S. Securities and Exchange Commission — Investor.gov: Index Funds](https://www.investor.gov/introduction-investing/investing-basics/investment-products/mutual-funds-and-exchange-traded-4)
2. [U.S. Securities and Exchange Commission — Investor.gov: Index Fund](https://www.investor.gov/introduction-investing/investing-basics/glossary/index-fund)
3. [U.S. Securities and Exchange Commission — Investor.gov: Mutual Funds](https://www.investor.gov/introduction-investing/investing-basics/investment-products/mutual-funds-and-exchange-traded-funds-etfs/mutual-funds)
4. [U.S. Securities and Exchange Commission — Investor.gov: Exchange-Traded Funds](https://www.investor.gov/introduction-investing/investing-basics/investment-products/mutual-funds-and-exchange-traded-2)
5. [U.S. Securities and Exchange Commission: Mutual Fund and ETF Fees and Expenses](https://www.sec.gov/resources-for-investors/investor-alerts-bulletins/ib_mutualfundfees)
6. [FINRA: Active vs. Passive Investing](https://www.finra.org/investors/insights/active-passive-investing)
7. [FINRA: Mutual Funds](https://www.finra.org/investors/investing/investment-products/mutual-funds)
8. [FINRA: Exchange-Traded Funds and Products](https://www.finra.org/investors/investing/investment-products/exchange-traded-funds-and-products)
9. [FINRA: A Look at Non-Traditional Indexes and Funds That Track Them](https://www.finra.org/investors/insights/non-traditional-index-funds)
10. [FINRA: The Basics of Direct Indexing](https://www.finra.org/investors/insights/direct-indexing)

Educational Disclaimer

Rockwell Forbes publishes educational content intended to help readers better understand investing, index funds, passive investing and financial markets.

Nothing in this article should be interpreted as personalized investment, legal, tax or financial advice, or as a recommendation to buy, sell or hold any index fund, ETF, mutual fund, benchmark, security, direct-indexing strategy or portfolio allocation.

Readers should evaluate their own circumstances and consult qualified professionals where appropriate.

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