Across two experiments, interoceptive sensitivity predicted monetary generosity. These results provide evidence for a link between interoception and altruistic behaviour. The link with altruistic behaviour is specific to interoceptive sensitivity, as other measures we obtained did not predict monetary generosity. We can also discount the possibility that the critical property linking altruism and interoception is a general sensory sensitivity to body or environmental signals. In a third experiment (smell sensitivity experiment), we tested participants’ performance detecting faint smells, a perceptual signal that is intimately linked to the perception of internal and homeostatic states in the generation of food-related behaviors24. If altruism were higher in individuals with better overall sensitivity to faint sensory impressions, olfactory detection sensitivity is a plausible candidate to display a pattern of association similar to that observed for interoception. We asked whether smell detection sensitivity would be similarly associated with altruistic behavior in the dictator game. The correlation of smell sensitivity to altruism was much weaker than those obtained in experiments 1 and 2 for interoceptive sensitivity, and statistically not significant [r21 = 0.16, p = 0.488, 95% CI [−0.26, 0.54]]. This indicates that altruistic behavior in experiments 1 and 2 is likely to be specifically linked to their interoceptive sensitivity, rather than sensitivity to any faint sensory signal capable of interacting with internal states.

Low materialism shows a trend to predict both forms of altruistic behavior, monetary generosity and helping (see supplementary materials), but not consistently across the experiments. Empathy showed no association with altruistic behaviour, in contrast to some previous studies (e.g.21,22), and this may be due to our particular empathy measure: the empathy quotient questionnaire18 which measures both affective and cognitive empathy. Had we measured cognitive and affective empathy separately, e.g. with the Questionnaire of Cognitive and Affective Empathy19 and the Multifaceted Empathy Test20, a relation might have been found.

The interoceptive sensitivity experiment points to an association between interoceptive sensitivity and altruistic behavior but cannot demonstrate causality. The interoception training experiment tested one possible form of causation. Its result shows that enhancing the ability to detect one’s heartbeat, through short training, does not enhance altruistic behavior in a subsequent game.

If there indeed was a causal relationship between sensitivity to heartbeats and altruistic behavior (which our interoception training experiment fails to demonstrate), what could be its nature? The relationship might be based on short-term signals, reflecting moment-to-moment changes in the body. Such a relationship would be consistent with a value-based model of decision making25, whereby peripheral signals provide value-weights to guide decisions, or with the somatic marker hypothesis of behavior guidance6. A person might encounter an emotionally evocative situation (like a choice to give away money, or not), that causes a heartbeat signal change (e.g. a strengthening of the signal). That bodily signal is interoceptively detected, and biases the choice towards the altruistic option (the direction remains unexplained, but could relate to avoiding the social stress of not sharing26). Peripheral physiological events have been shown to influence complex cognitive processes on a short time scale (e.g. heartbeat to heartbeat27; giving such a scenario some plausibility. But the results of our interoceptive training experiment show that enhancing the body signal detection through training does not bias choice - it does not make people more altruistic in a subsequent game. This result appears inconsistent with a moment-to-moment account that assumes a direct causal link from interoception to altruism (see supplementary materials). What would explain such a result? One possibility is that our procedure is not sufficiently sensitive to the effect.

A possible alternative account focuses on the perception of distress in others. (It supposes that in the course of the dictator game, participants form a representation of distress in the recipient with whom one does not share.) Altruistic people stand out in their ability to detect distress in faces28. Moreover, extreme altruists (non-directed organ donors) have larger amygdalae (nuclei activated by emotional material), which also respond more strongly to faces communicating distress than amygdalae of control participants29. Given that different levels of interoceptive sensitivity are linked to differences in emotional experience in general10, and that body responses specifically contribute to the perception of distress30, heightened interoceptive sensitivity may enhance perception of distress and support altruism, explaining the results of our experiments. At this stage, the idea is a speculation which may be confirmed in studies including measurement of stress and distress, which our studies do not. Additionally, the functional significance of the amygdala for fear and anxiety may connect that structure to the complex of interoception and altruism. This remains to be explored in future studies. Future studies may also focus on the role emotions and their components (e.g. activation) experienced during the task play in altruistic behaviour.

It is worth noting that although we have assumed that our heartbeat discrimination task reflects interoceptive sensitivity specifically, it is in fact measuring the brain’s ability to integrate or compare exteroceptive (auditory) and interoceptive (cardiac) stimuli to make synchronicity judgments, and thus performance on this task is likely to be dependent on a somewhat different network of brain areas than the ‘cleaner’ heartbeat tracking task31, which requires participants to internally count heartbeats. We chose the heartbeat discrimination task, however because it is not affected to the same degree by confounds such as time estimation ability and beliefs about one’s own heart rate32. Both tasks are problematic, also because they often fail to correlate with each other, and with interoceptive awareness33, particularly in relatively small samples of poorly performing participants. Although differences in performance on our heartbeat discrimination task could be due to variations in attention34, our failure to find a significant association between prosocial behavior and smell sensitivity (see supplementary material), which should also be affected by attention, contradicts this argument.

Our results suggest that altruistic acts may be influenced by the representation of one’s body in the brain. Although the mechanisms underpinning this relationship need to be clarified, the present findings indicate that humans in some sense ‘listen to their heart’ to shape their altruistic behaviors.