2025-07-17

The Variabilities of Dopamine (₯) - PART VIII: Trust or Distrust? MeSH: D035502

In neurobiology, the study of trust has historically been dominated by oxytocin, the so-called "social hormone." This hormone is known to reduce anxiety and increase empathy, fostering a sense of trust. Conversely, increased levels of steroid hormones like testosterone can inhibit oxytocin secretion, thereby reducing empathy and trust. Amidst the scientific focus on these two key chemicals, dopamine was often relegated to a supporting role, thought to merely assist in regulating oxytocin to enhance the pleasure derived from cooperation and relationship-building. At best, it was considered a secondary player in the mechanism of trust.

However, since the early 21st century, cognitive neuroscience has begun a wider and deeper exploration of the relationship between dopamine and trust. This interdisciplinary field integrates applications from economic exchange games (such as trust games), psychological frameworks (motivation, emotion, and cognition), and neuroscience (brain circuits, hormones, and neurotransmitters). For example, the ventral striatum (vSTR), a key part of the mesolimbic pathway, regulates reward motivation and promotes reinforcement learning. When individuals learn to trust others through repeated interactions, dopamine neurons cause the vSTR to activate. This activation encodes a "reward prediction error" signal, the difference between an expected reward and the actual reward received. This dopaminergic neural mechanism provides the foundation for neuroeconomic models that explain how the human brain establishes and maintains trust (as illustrated in Figure 2).

The knowledge structure of trust can also be understood through its classification in the U.S. National Library of Medicine's Medical Subject Headings (MeSH). Under the ID D035502 (Figure 3), "Trust" is defined as "Confidence in or reliance on a person or thing." The MeSH hierarchy places "Trust" under the parent term "Interpersonal Relations," which itself falls within the "Behavior and Behavior Mechanisms" category. Its sibling terms include "Social Interaction" and "Social Skills." Furthermore, this classification reveals the relationship between trust and other topics previously discussed in the " Variabilities of Dopamine " series. For instance, "Behavior" has a descendant term "Exploratory Behavior" (curiosity); "Emotion" includes "Happiness," "Fear," and "Pleasure"; and "Motivation" includes "Goals." All these concepts fall under the broad umbrella of "Behavior and Behavior Mechanisms."


Strong evidence for the role of dopamine in trust comes from studies on its receptors. A 2023 neuroimaging study in the Nature Communications linked behavioral performance in trust-based tasks to neural activation. The study found that blocking the dopamine D2/D3 receptors in male participants altered activity in brain regions associated with trust, such as the prefrontal cortex and striatum. This research examined the relationship between dopamine and trust using the concept of "belief volatility": the stability of one's trust-related beliefs. This was measured using a Bayesian inference model, where beliefs are updated based on new evidence. The core concepts include:

  • Prior Belief: the initial level of trust before encountering new evidence.
  • New Evidence (Likelihood): new information or actions from the other person. 
  • Updated Belief (Posterior Belief): the modified level of trust after considering the new evidence.
"Belief volatility" is a metric derived from this statistical framework.

The study showed that antagonizing (blocking) D2/D3 receptors increased the volatility of trust-related beliefs. Higher volatility indicates less confidence; it means that an individual's assessment of another's trustworthiness is unstable and changes rapidly in response to new information. Conversely, lower volatility reflects more stable beliefs, and thus higher confidence, that are less swayed by new evidence. The experimental study by Mikus et al. (2023) provides direct behavioral evidence that dopamine regulates the stability of our beliefs about others.

So how does dopamine shape these trust-related beliefs?

A study by Schuster and Lamm (2025) at the University of Vienna highlights how recent experimental and clinical work has advanced our understanding of this process. It demonstrates that dopamine's role can be conceptualized within Bayesian inference frameworks, involving processes like precision-weighted prediction error computation and hierarchical belief updating. Until recently, dopamine was discussed primarily in the context of reinforcement learning. It was known that phasic (short-burst) dopamine firing tracks the magnitude of reward prediction errors, but growing evidence shows that tonic (sustained) dopamine levels signal the precision or certainty associated with those predictions. In essence, dopamine appears to mediate multiple aspects of belief updating on different time scales. The two key types of dopamine signals are:

1. Phasic Dopamine Signals: these are rapid, short-lived bursts of dopamine, often triggered by unexpected or rewarding events. Think of them as brief flashes that signal a surprise.
2. Tonic Dopamine Signals: this refers to the baseline, sustained level of dopamine in the brain. It sets the overall tone for how sensitive we are to new information.

This research clarifies that dopamine helps the brain update beliefs by managing the uncertainty of new information, allowing it to function optimally by minimizing surprise. According to Schuster and Lamm, modern tools like functional magnetic resonance imaging (fMRI) show that shifts in belief following a prediction error (a phenomenon known as Bayesian surprise) are encoded in dopamine-rich areas of the midbrain. Figure 5 illustrates the key substantia nigra/striatum and midbrain-cortical dopamine pathways involved in trust-based learning.

Schuster and Lamm's work further emphasizes that D2/D3 receptors are key to how the dopamine system regulates belief volatility. These receptors are crucial for distinguishing meaningful information from "noise." Imbalances in D2/D3 receptor function can be detrimental. If signaling is too high, we may struggle to distinguish important information from noise. If it is too low, we might fail to recognize new information that should cause us to update our beliefs. This function also regulates the speed at which we update our views of others. These findings provide new insights into the potential mechanism by which antipsychotic drugs targeting D2/D3 receptors can effectively reduce symptoms of severe distrust, such as paranoia.

In the context of "trust or distrust," variable dopamine helps the brain balance the influence of prior beliefs (old information) with incoming evidence (new experiences). By modulating this system, dopamine adjusts how much weight we give to new information. Increased dopamine signaling can make the brain prioritize new data over established beliefs. It is now clear that the decision to trust or distrust is not just a matter of social hormones but is fundamentally modulated by dopamine.

REFERENCE

  1. Zak, P. J. (2008). The neurobiology of trust. Scientific American, 298(6), 88-95; Zak, P. J. (2019). How Our Brains Decide When to Trust. Harvard Business Review
  2. Krueger, F., & Meyer-Lindenberg, A. (2019). Toward a model of interpersonal trust drawn from neuroscience, psychology, and economics. Trends in neurosciences, 42(2), 92-101.
  3. MeSH: D035502 ; MeSH: D005106 ; MeSH: D005239; MeSH: D006240; MeSH: D009042; MeSH: D006040
  4. Mikus, N., Eisenegger, C., Mathys, C., Clark, L., Müller, U., Robbins, T. W., ... & Naef, M. (2023). Blocking D2/D3 dopamine receptors in male participants increases volatility of beliefs when learning to trust others. Nature Communications, 14(1), 4049.
  5. Schuster, B. & Lamm, C. (2025). How dopamine shapes trust beliefs, Progress in Neuro-Psychopharmacology and Biological Psychiatry, Volume 136. Preprint Online Version, Nov.28.2024. 




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