But pessimistic investors hoping to profit from a Bitcoin price fall actually have had the opportunity to make negative investments for a while. Bitcoin exchanges such as Bitfinex allow shorting of Bitcoins, and it is possible to short Bitcoin-linked exchange-traded notes on online brokerages like the Bitcoin Investment Trust, GBTC. Both of these options suffer from lack of liquidity and of trust in these new institutions; GBTC has not tracked Bitcoin prices accurately, for example. Still, if enough people had managed to take a short position, that might have helped to limit the increases in Bitcoin prices that we have seen.

It is possible that the Bitcoin market will change in a meaningful way now, given the decision of Cboe, the Chicago Board Options Exchange, to start a Bitcoin futures market on Dec. 10 and of the CME Group to do so on Dec. 18. The academic literature tells us that volatility of an underlying asset often falls after the establishment of new futures markets for it. But the ability to short an asset more easily won’t necessarily overcome the power of investor excitement.

In 1936, John Maynard Keynes suggested why. He played down the role of quantitative analysis and probability estimates in human thinking of the assessment of ambiguous future events. People in such situations are vulnerable to a play of emotions and at times a “spontaneous urge to action” that he called “animal spirits.” He argued that much of what happens in financial markets has to do with people learning, from price movements, about each other’s animal spirits.

I believe that Mr. Keynes was correct about animal spirits in general and how they affect markets like the one for Bitcoin. George Akerlof and I expanded on his perspective in our 2009 book Animal Spirits, which argued that the driving force behind human enterprise cannot be reduced to the rational optimization emphasized by traditional economics. Darwinian evolution produced a human species whose behavior sometimes seems to be emotionally driven.

Neuroscientists, psychologists and economists are leading us toward new models of human decision-making. They may help to explain phenomena like the Bitcoin price rise.

Scott Huettel, a Duke neurologist, and other researchers showed in 2006, for example, that when making decisions involving ambiguity, people do not use the parts of the brain required for calculations of probabilities and expected values. And the economists Anat Bracha of the Boston Federal Reserve and Donald Brown of Yale have provided an alternative to conventional economic theory of human behavior under uncertainty. They define a different kind of rationality — one based on Mr. Keynes’ views, not on calculations of utility — in ambiguous situations.

Furthermore, a paper by neuroscientists including Benjamin Lu that was presented at the Society for Neuroeconomics annual convention in Toronto in October, showed that psychologically stressful experiences can result in changes in neurological processes when ambiguous situations arise.