Franklin's 200-Year Compound Interest Experiment

When Benjamin Franklin died in 1790, his will contained one of the most audacious financial experiments in history. He left £1,000 each to the cities of Boston and Philadelphia — with a catch. They could only use a portion after 100 years. The rest had to keep compounding for 200 years, until 1990.

Franklin had spent decades writing about the power of compound interest. The will was his final proof. He wasn't just theorizing — he was running a live, multi-generational experiment to demonstrate that he was right.

What the Will Actually Said

Franklin's bequest was specific and structured. Each city received £1,000 (roughly $4,400 in 1790 dollars — equivalent to perhaps $150,000 today). The funds were to be loaned to young tradespeople at 5% annual interest. After 100 years (1890), each city could withdraw £100,000 for public works. The remaining funds were to continue compounding until 1990 — a full two centuries after Franklin's death.

Franklin's projection: at 5% compounded annually, £1,000 should grow to roughly £131,000 after 100 years and over £2,000,000 after 200 years. He was explicitly modeling compound interest — and making it a matter of civic public record.

What Actually Happened

Reality, as always, was messier than the model. Lending to young tradespeople turned out to be administratively complicated. Interest rates fluctuated. The funds were managed with varying degrees of skill and attention over two centuries. But the final results in 1990 were still remarkable:

Boston's fund outperformed Philadelphia's significantly — largely because Boston's administrators invested the funds more aggressively in later decades, including in equities. Philadelphia kept the money in more conservative instruments. The difference between the two cities' outcomes is itself a lesson: the rate of return matters enormously over long periods.

Boston used its $5M to establish the Benjamin Franklin Institute of Technology. Philadelphia's funds went to scholarships and grants.

The Math: What Should Have Happened

At Franklin's projected 5% annual return, £1,000 over 200 years produces roughly £17.3 million (before currency conversion and inflation adjustments). Both cities fell significantly short of this — the actual returns were lower than 5% on average due to administrative friction, periods of mismanagement, and conservative investment choices.

That 10% row is not a typo. At stock market historical average returns (~10% nominal), £1,000 in 1790 would grow to nearly £190 billion over 200 years. This is why Warren Buffett has said the best investment he ever made was being born in the right era — compound interest at equity returns over long periods produces numbers that seem impossible.

The Lesson About Time

The single most important variable in Franklin's experiment wasn't the interest rate. It was the time horizon. The difference between £1,000 at 5% for 100 years (£131,501) vs 200 years (£17.3 million) is a factor of 131x — from the same additional 100 years. The second century produces 131 times more wealth than the first century, despite being the same length of time.

This is the exponential nature of compounding in action. The early years produce small absolute gains. The later years produce enormous ones. Which is why starting to invest at 22 vs 32 isn't just "10 years earlier" — it's often the difference between retiring at 50 or retiring at 65.

Franklin as Investment Scientist

What makes the will remarkable isn't just the monetary outcome — it's the intellectual framework. Franklin designed a controlled experiment. Two identical inputs (£1,000 each), same instructions, same interest rate target, same time horizon. He wanted to demonstrate, in a way that could be verified by future generations, that compound interest was not a trick or a theory but a mathematical certainty.

He was right. Despite both cities falling short of maximum theoretical returns due to real-world management issues, both funds still grew by factors of over 1,000x from their starting point. The proof of concept succeeded.

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