A large body of evidence links altered opioid signaling with changes in social behavior in animals. However, few studies have attempted to determine whether similar links exist in humans. Here we investigate whether a common polymorphism (A118G) in the mu-opioid receptor gene (OPRM1) is associated with alterations in personality traits linked to affiliative behavior and attachment. In a mixed sample (N = 214) of adult healthy volunteers and psychiatric patients, we analyzed the association between the A118G polymorphism of the OPRM1 and two different psychological constructs reflecting individual differences in the capacity to experience social reward. Compared to individuals expressing only the major allele (A) of the A118G polymorphism, subjects expressing the minor allele (G) had an increased tendency to become engaged in affectionate relationships, as indicated by lower scores on a self-report measure of avoidant attachment, and experienced more pleasure in social situations, as indicated by lower scores on a self-report measure of social anhedonia. The OPRM1 variation accounted for about 3.5% of the variance in the two measures. The significant association between the A118G polymorphism and social hedonic capacity was independent of the participants' mental health status. The results reported here are in agreement with the brain opioid hypothesis of social attachment and the established role of opioid transmission in mediating affiliative behavior.
There is evidence that both early experience and genetic variation play a role in influencing sensitivity to social rejection. In this study, we aimed at ascertaining if the A118G polymorphism of the μ-opioid receptor gene (OPRM1) moderates the impact of early maternal care on fearful attachment, a personality trait strongly related to rejection sensitivity. In 112 psychiatric patients, early maternal care and fearful attachment were measured using the Parental Bonding Inventory and the Relationship Questionnaire (RQ), respectively. The pattern emerging from the RQ data was a crossover interaction between genotype and maternal caregiving. Participants expressing the minor 118 G allele had similar and relatively high scores on fearful attachment regardless of the quality of maternal care. By contrast, early experience made a major difference for participants carrying the A/A genotype. Those who recalled higher levels of maternal care reported the lowest levels of fearful attachment whereas those who recalled lower levels of maternal care scored highest on fearful attachment. Our data fit well with the differential susceptibility model which stipulates that plasticity genes would make some individuals more responsive than others to the negative consequences of adversity and to the benefits of environmental support and enrichment.
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