What Is an Index Fund and Should You Invest in One?
What is an index fund and should you invest in one? The plain-English explanation of how index funds work, why they outperform, and how to start.
📚 Part of our Complete Investing Guide
If you have spent any time reading about investing, you have heard that index funds are the best investment for most people. Warren Buffett has recommended them. Jack Bogle built Vanguard around them. Academic research consistently supports them.
But what exactly is an index fund, and should you actually invest in one?
What Is an Index?
Before understanding index funds, you need to understand what an index is.
A market index is simply a list of stocks or other securities chosen according to specific rules. The index tracks how those securities perform collectively.
The most famous example is the S&P 500 — a list of the 500 largest publicly traded companies in the United States, maintained by S&P Global. When people say "the market went up 1% today," they usually mean the S&P 500 increased by 1%.
Other major indexes include:
- Dow Jones Industrial Average — 30 large US companies
- Nasdaq Composite — all companies listed on the Nasdaq exchange, heavily tech-weighted
- Russell 2000 — 2,000 smaller US companies
- MSCI World — large and mid-cap stocks across 23 developed countries
- Bloomberg US Aggregate Bond Index — thousands of US bonds
What Is an Index Fund?
An index fund is an investment that attempts to replicate the performance of a specific index by holding the same securities in the same proportions.
A fund tracking the S&P 500 buys shares of all 500 companies in the index, weighted by their market capitalization. When Apple represents 7% of the S&P 500, the fund holds 7% of its assets in Apple.
Index Funds vs Actively Managed Funds
The alternative to an index fund is an actively managed fund — where a professional fund manager picks stocks they believe will outperform the market.
Active management sounds appealing. A smart professional, armed with research teams and sophisticated tools, should be able to beat a simple list of stocks.
The data consistently shows otherwise.
According to S&P's SPIVA report — the most comprehensive analysis of active vs passive fund performance — over a 15-year period, approximately 88% of actively managed US large-cap funds underperformed their benchmark index.
The reasons are mathematical:
Costs: Active funds charge 0.5-1.5% annually. Index funds charge 0.03-0.20%. Every year, active funds start 0.5-1.3% behind before making a single investment decision.
Trading costs: Active managers buy and sell frequently, generating transaction costs and tax consequences that further drag performance.
Market efficiency: All publicly available information about a company is already reflected in its price. Consistently finding genuinely mispriced stocks before millions of other sophisticated investors is extraordinarily difficult.
Survivorship bias: Fund performance data is skewed because failed funds close and disappear from records. The average active fund that exists today looks better than average because underperformers closed.
The Different Types of Index Funds
By Asset Class
Stock index funds — track equity indexes. The most common type. Examples: S&P 500 funds, total market funds, international funds.
Bond index funds — track bond market indexes. Lower return potential than stocks but lower volatility. Examples: BND (Total Bond Market), AGG (US Aggregate Bond).
The Two Books That Explain Index Funds Better Than Anyone
The Little Book of Common Sense Investing by John C. Bogle — Written by the man who invented the index fund. If you read one book after this article, make it this one.
A Random Walk Down Wall Street by Burton Malkiel — The academic case for why index investing beats active management, explained accessibly. Essential companion to Bogle.
Prefer audiobooks? All of these are available on Audible — try it free for 30 days and get your first audiobook included.
By Structure
ETF (Exchange-Traded Fund) — trades on stock exchanges like individual stocks throughout the day. Can be bought in fractional shares. Highly tax-efficient.
Mutual fund — priced once per day after market close. Purchased directly from the fund company. Some have minimum investment requirements.
Both structure types can track identical indexes. The practical differences are minor for long-term investors.
Should You Invest in an Index Fund?
For the vast majority of individual investors, the answer is yes.
You should invest in index funds if:
- You want to build wealth over 10+ years
- You do not want to spend significant time researching individual stocks
- You want to minimize investment costs
- You want consistent, reliable exposure to market returns
- You understand that short-term volatility is normal and will not panic-sell during downturns
You might consider alternatives if:
- You have significant expertise in a specific industry and want concentrated exposure
- You have complex tax situations requiring customized portfolio construction
- You are an experienced investor with genuine edge in security selection
For the vast majority of people — including most financial professionals for their personal accounts — index funds are the optimal investment.
The Best Index Funds for Beginners
VTI — Vanguard Total Stock Market ETF. Covers the entire US stock market. Expense ratio: 0.03%.
VOO — Vanguard S&P 500 ETF. Tracks the 500 largest US companies. Expense ratio: 0.03%.
VT — Vanguard Total World Stock ETF. Covers the entire global stock market. Expense ratio: 0.07%.
FZROX — Fidelity Zero Total Market Index Fund. Tracks the US total market with a 0.00% expense ratio. Available only at Fidelity.
BND — Vanguard Total Bond Market ETF. For adding stability to a portfolio. Expense ratio: 0.03%.
How to Start
Open a Roth IRA or brokerage account at Fidelity or Schwab. Search for VTI or FZROX. Buy your first shares. Set up automatic monthly contributions.
The process takes 15 minutes. The results compound over decades.
The Bottom Line
Index funds are not exciting. There are no stories of spectacular returns from brilliant stock picks. There is no cocktail party appeal to owning "the entire market."
What there is: decades of evidence showing that index funds beat the majority of actively managed funds over the long term, at a fraction of the cost, with dramatically less effort.
For most people building wealth in 2026, index funds are not just good — they are the optimal choice.
📥 Free download: The 10-Step Financial Independence Checklist
The exact roadmap I followed to build my financial foundation. 11 pages, professionally designed, free with email signup — no credit card, unsubscribe anytime.
Disclosure: This post may contain affiliate links. ZarWealth may earn a commission if you sign up through our links, at no extra cost to you.
