“Eighth wonder of the world”—earning returns on prior returns, creating exponential growth over time. Educational concept used to motivate young investors to start early, even with small amounts.
The Classic Example
Age 25 vs. Age 35 starting:
- Person A invests $5,000/year from 25-35 (10 years, $50K total)
- Person B invests $5,000/year from 35-65 (30 years, $150K total)
- Both earn 8% annually
- Result: Person A ends with $787K, Person B with $611K
Starting 10 years earlier beats contributing 3x as long due to compounding.
Rule of 72
Estimate doubling time: 72 ÷ interest rate = years to double
- 8% return → money doubles every 9 years
- $10K at 25 → $20K at 34 → $40K at 43 → $80K at 52 → $160K at 61
Cultural Usage
Motivational finance content:
- “Your 20s are for compound interest, not compound spending”
- “Invest $6K/year from 20-30, never again → millionaire by 65”
- “Time in market beats timing the market”
Criticism: Examples often use unrealistic 10-12% returns, ignore inflation, and oversimplify to sell investment products.
Sources:
- “The Compound Effect” (Darren Hardy, 2010)
- Historical S&P 500 returns (~10% nominal, ~7% real)
- Compound interest calculators (NerdWallet, Investor.gov)