Bubbles have been frequent in economic history, and they occur in the laboratories of experimental economics under conditions which -- when first studied in the 1980s -- were considered so transparent that bubbles would not be observed.

We economists were wrong: Even when traders in an asset market know the value of the asset, bubbles form dependably. Bubbles can arise when some agents buy not on fundamental value, but on price trend or momentum. If momentum traders have more liquidity, they can sustain a bubble longer.

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