Reciprocity motivates to reward those who are kind (= positive reciprocity) and to punish those who are unkind (= negative reciprocity). The neurotransmitter serotonin (5-HT) modulates human behavior in numerous social situations. such as retaliation in response to perceived unfairness. In a placebo-controlled study. https://www.maximilianopellegrini.com/product-category/har-balsam/