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.nacrack.com/super-save-2001-2004-LEXUS-LS430-Sedan-Rear-Bumper-Cover-w-o-Park-Assist-Paint-Code-3P2-hot-choice/