Why compound interest becomes a wealth multiplier over decades
Simple interest pays a fixed percentage of your principal every period. On $10,000 at 7% for 30 years, you earn $700 a year, and the account reaches $31,000. Compound interest reinvests your earnings into the principal, so you earn returns on your returns. The same $10,000 at 7% compounded over 30 years grows to $76,000. The difference, $45,000, comes entirely from reinvesting interest instead of withdrawing it.
The gap between simple and compound widens dramatically with time. In year one, compound beats simple by only $700 (the reinvested interest). In year 10, the gap is roughly $10,000. By year 30, it's $45,000. This exponential curve is the reason Einstein allegedly called compound interest the eighth wonder of the world. Early investors benefit hugely because their money has decades to compound, while late starters struggle to catch up despite higher contributions.
The power of consistent reinvestment
The speed of compounding depends on two factors: the rate of return and the time horizon. High returns (12% instead of 7%) accelerate compounding significantly, but time horizon is equally powerful. A 10% return over 40 years beats a 15% return over 10 years. This explains why starting retirement investing at age 25 instead of 35 is worth hundreds of thousands of dollars by age 65, even with identical contribution amounts.
Compounding assumes you never withdraw earnings, only reinvest. The moment you begin withdrawing dividends or interest, you break the cycle. This is why young investors in growth assets (stocks) outperform income investors: stocks reinvest earnings into capital gains, while bond investors are often tempted to spend interest. Staying invested and not touching capital is the hardest and most valuable discipline in building wealth.