It was almost always to Amos that the economists wrote. They understood him. Amos’s insistently logical mind was much like their own but somehow better: They could see his genius. To most economists, Danny’s mind was a mystery. Richard Zeckhauser, an economist at Harvard who became friends with Amos, spoke for his entire field when he said, “My impression of the way they worked on a paper is that they walked around and let Danny do a variety of things. ‘Guess what, Amos, I went to buy a car and I offered 38 grand and the salesman said 38.9 and I said yes! Did I do a good job?’ And Amos would say, ‘Let’s write that up.’” The economists’ view of the collaboration was that Amos Tversky had set out, like an anthropologist, to study an alien tribe of beings less rational than himself, and his tribe was Danny. “I share your feeling that such behavior is, in some sense, unwise or erroneous, but this does not mean that it does not occur,” Amos wrote, to an American economist who complained about the description of human nature implied by “Value Theory.” “A theory of vision cannot be faulted for predicting optical illusions. Similarly, a descriptive theory of choice cannot be rejected on the grounds that it predicts ‘irrational behavior’ if the behavior in question is, in fact, observed.”
Danny, for his part, claimed that it wasn’t until 1976 that he woke up to the effects their theory might have on a field he knew nothing about. His awakening came when Amos handed him a paper written by an economist. The paper opened, “The agent of economic theory is rational, selfish, and his tastes do not change.” The economists at Hebrew University were in the building next door, but Danny hadn’t paid any attention to their assumptions about human nature. “To me, the idea that they really believed in it—that this is really their worldview—was incredible,” he said. “It’s the worldview in which if people tip in a restaurant to which they will never return it counts as a puzzle.” It was a worldview that took it as given that the only way to change people’s behavior was to change their financial incentives. The idea of it struck him as so bizarre that he could scarcely bring himself to engage with it directly. To Danny the whole idea of proving that people weren’t rational felt a bit like proving that people didn’t have fur. Obviously people were not rational, in any meaningful sense of that term.
He and Amos wanted to avoid getting into an argument about the rationality of man. That argument would only distract people from the phenomenon they were uncovering. They preferred to reveal man’s nature, and let man decide what he was. Their next task, they saw, was to buff and polish “Value Theory” for publication. They both worried that someone would find an obvious contradiction—some Allais paradox–like observation that would render their theory dead on arrival. They’d spend three years doing very little else but searching the theory for internal contradictions. “In those three years we did not discuss anything of genuine interest,” said Danny. Danny’s interest ended with the psychological insights; Amos was obsessed with the business of using the insights to create a structure. What Amos saw, perhaps more clearly than Danny, was that the only way to force the world to grapple with their insights into human nature was to embed them in a theory. That theory needed to explain and predict behavior better than existing theory, but it also needed to be expressed in symbolic logic. “What made the theory important and what made it viable were completely different,” said Danny, years later. “Science is a conversation and you have to compete for the right to be heard. And the competition has its rules. And the rules, oddly enough, are that you are tested on formal theory.” After they finally sent a draft of their paper to the economics journal Econometrica, Danny was perplexed by the editor’s response. “I was kind of hoping he’d say, ‘Loss aversion is a really cool idea.’ He said, ‘No, I like the math.’ I was sort of shattered.”
By 1976, purely for marketing purposes, they changed their title to “Prospect Theory.” “The idea was to give the theory a completely distinct name that would have no associations whatsoever,” said Danny. “When you say ‘prospect theory,’ no one knows what you’re talking about. We thought: Who knows? It may turn out to be influential. And if it is we don’t want it to be confused with anything else.”
In all of this they were slowed, dramatically, by the turmoil in Danny’s life. By 1974 he’d moved out of his house and was living apart from his wife and children. A year later he left the marriage, and flew to London to meet the psychologist Anne Treisman to formally “declare my love.” She reciprocated. By the fall of 1975 Amos was clearly weary of the inevitable fallout. “It is hard to overestimate the amount of time and the amount of emotional and mental energy that is consumed with such affairs,” he wrote to his friend Paul Slovic.
In October 1975 Danny flew to England again, this time to see Anne in Cambridge and to travel with her to Paris. He was at once in a totally uncharacteristic state of elation and worried about the effect of his new relationship with Anne on his old one with Amos. In Paris he found waiting what appeared to be a letter from Amos—but, opening it, he at first found only a draft of what would become “Prospect Theory.” Danny took the absence of any personal note as a subtle message from Amos. Sitting with his new love in the world’s capital of romance, Danny sat down and wrote what amounted to a love letter: to Amos. “Dear Amos,” it began. “When I came to Paris I found an envelope from you. I pulled out your manuscript but there was no letter with it. And I told myself that Amos is very angry with me, and not without reason. After dinner, I was looking for a used envelope to send this back to you and I found your envelope, and then saw your letter inside. We were late for dinner and I just glanced to see how you finish it. And I saw the words ‘Yours, as ever’ and I had goose bumps from emotion.” He went on to write that he’d explained to Anne that he could never have achieved on his own what he had achieved with Amos, and that the new paper they were working on was yet another step. “This is for me the greatest moment in a relationship which I see as one of the peaks of my life,” he wrote. Then he added: “I was yesterday at Cambridge. And I spoke to them about our work on Value Theory. The enthusiasm is almost embarrassing. I concluded with a discussion of the early stages of the isolation effect. And they responded to that especially. In general, they gave me the feeling that I’m one of the world’s greats. They were trying so hard to impress me that I reached the conclusion that maybe the time has come for me to be free of the need to impress others.”
In some strange way, as they approached their moment of greatest public triumph, their collaboration remained a private affair, a gamble with no context. “As long as we stayed in Israel, the whole idea of what the world thought of us didn’t occur to us,” said Danny. “We benefited from our isolation.” That isolation depended on them being together, in the same room, behind a closed door.