The trust game captures an essential dilemma of human sociality.

Betrayal becomes more poignant if it occurs after promises were made.

Trustors do well being mindful of the trustee’s incentives to defect.

The best way to find out if you can trust somebody is to trust them. —Hemingway

Interpersonal trust, along with the disappointments that occasionally occur, lies at the heart of human sociality. To understand its challenges and how ordinary people navigate the landscape of social uncertainty is one of the main tasks of social psychology.

Psychologists and behavioral economists have turned to a stylized experimental game to capture the essential elements of trust, that is, its necessary and sufficient characteristics (Berg et al., 1995). In this game, a trustor receives a small endowment of $10. If they transfer the money to a trustee, its value is multiplied, and the trustee then chooses between keeping it all—and thank you very much indeed!—and sharing the newfound wealth equitably with the trustor. Whereas the trustee merely needs to choose between self-regard and reciprocity, the trustor faces a decision under uncertainty. There are no clear paths leading to an optimal decision, but trustors can use a set of social heuristics to navigate this uncertain space (Krueger et al., 2026).

This trust game is an abstraction, and as such, it reflects a bare-bones sort of social ecology. In its canonical form, the two players do not know each other, they do not know how their roles were assigned, and they have no expectation to play with each other ever again. There is also the interesting convention that the trustor makes the first move by either transferring the money or by walking away with the small gain of the show-up fee of $10. Real-world situations are more diverse than this standard game, and this variability is not random. I here review some ways in which common trust situations go beyond the standard experimental treatment.

Consider two scenarios, which, I believe, are not uncommon in life outside of the laboratory. In the first scenario, a seeker of romance has started dating a promising prospect. The first few dates have gone well; promises are made, and expectations are raised. Trusting the prospect, the seeker has made investments of time and money. Then, seemingly out of the blue, the prospect (the trustee) informs the seeker (the trustor) that this budding relationship is not for them after all. The reasons remain unclear. This may be a case of cold feet or someone more attractive has come along, among other possibilities.

In the second scenario, a buyer (trustor) negotiates with a vendor (trustee) the price of an object, let’s say, a medium-sized hard-shell suitcase. The two parties settle on what seems to be a good price after the vendor agrees to grant a 10 percent discount. As is usually the case in such interactions, the buyer has no clear idea how much profit the vendor has made beyond a general sense that the vendor did not sell at a loss. However, the next day, the buyer sees the same suitcase offered elsewhere for another 10 percent less, which suggests that even this lower price would yield a profit.

In these scenarios, in contrast to the canonical game, the trustee moves first (or at least at the same time) by persuading the trustor not to walk away but to invest. In the first scenario, the prospect expresses interest in a dating relationship and encourages the seeker to invest in it. Being dumped means a loss of that investment. In the second scenario, the vendor advertises their wares and entices the buyer with a discount. Seeing a cheaper suitcase elsewhere does not amount to a loss of investment but to the realization that the investment was larger than it had to be. So, in psychological terms, it can now be construed as an unnecessary loss.

In these scenarios, the trustor experiences a betrayal more acutely than in the standard game because it comes after the trustee had projected good faith. Making assurances of cooperation tends to work and is essential to the smooth functioning of dyads, groups, and whole societies (Dawes, 1980). Broken promises or the discovery of formerly concealed information may not, as in the second scenario, affect the trustor’s material outcomes, but it does degrade social outcomes, and it reduces social capital.

The figure below shows a modified trust game in extensive form. Afonso, the trustor, can move right to exit the game and keep the show-up fee of $10, or he can move down with the presumption that Bernardo, the trustee, will honor the deal so that both gain $20. Bernardo, however, has the option of moving down to attain a comparative advantage. Afonso either loses his expected gain (romance) or not (business), while Bernardo’s gain is greater than $20.

The numbers used in this illustration may change, which might affect choice and sentiment while leaving the basic structure of the game intact. The main departure from the standard game is that promises are made and expectations raised; this makes betrayal especially painful should it come. Trustees are motivated to make promises, not only because these can lead to mutually beneficial cooperation but also because they open the door to relative advantage and exploitation. Trustors do well to anticipate the trustee’s mixed incentives, but research shows them to be egocentrically focused on their own outcomes while neglecting the outcomes of those on whose cooperation they depend (Evans & Krueger, 2011).

The payoff structure of the modified game is recognizably similar to that of the original game. Yet, it is more closely related to the sorts of experiences people have and the stories they tell about trust working well or breaking down. There is no need for a radically new theory, just the need for people to tune their perception to help them recognize when they are in such a game, especially when they are cast in the role of the trustor. The dilemma of trust will not go away, but it becomes more navigable when properly understood.

Given its inevitable material and emotional risks, trust offers one irreducible benefit: It reveals the trustee’s character (see Hemingway’s epigraph). This is one reason why it is so hard to re-establish trust once it has been broken.

Berg, J., Dickhaut, J., & McCabe, K. (1995). Trust, reciprocity, and social-history. Games and Economic Behavior, 10, 122–142.

Dawes, R. M. (1980). Social dilemmas. Annual Review of Psychology, 31, 169-193.

Evans, A. M. & Krueger, J. I. (2011). Elements of trust: Risk and perspective-taking. Journal of Experimental Social Psychology, 47, 171-177.

Krueger, J. I., Evans, A. M., & Aizenberg, B. (2026). Heuristic trust. In J. P. Forgas (ed.), The psychology of trust. The Sydney Symposium of Social Psychology, 27, 79-100.

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