Cryptocurrencies have long been heralded as the future of finance, but it wasn’t until 2020 that it finally caught on to an old idea: making money with money. In the crypto world, decentralized finance (or DeFi) encompasses a wide array of blockchain-based applications intended to enhance cryptocurrency holders’ returns without relying on intermediaries — to earn the kind of passive returns an investor might get from a savings account, a Treasury bill, or an Apple Inc. bond.

The idea seems to be catching fire: Deposits in DeFi applications grew from about $1 billion in June to just under $40 billion by late January 2021, suggesting that DeFi could be a major element of crypto from here on out. In the tradition of disruptive innovations — as Clayton Christensen envisioned them — DeFi can be the evolution of blockchain technology that might launch it into mainstream.

The premise of DeFi is simple: Fix the longstanding inefficiency in crypto finance of capital being kept idle at a nonzero opportunity cost. Now, most investors buy crypto with the hope that the value of the currency itself will rise, as Bitcoin has. In general, that strategy has worked just fine. The value of cryptocurrencies has appreciated so rapidly that there just wasn’t much incentive to worry about gains of a few percent here and there.

But the recent rise of stablecoins, which are designed keep their value constant, has changed that calculation. The combined market cap of stablecoins such
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