Purpose: Attachment research shows that attachment experiences with parents in childhood influence the characterization of personality traits. Meanwhile, it is known that mu-opioid receptor function is involved in human attachment. Furthermore, a few studies suggest that the A118G polymorphism of the mu-opioid receptor gene ( OPRM1 ) is associated with altered mu-opioid receptor function. Thus, we examined if the OPRM1 polymorphism moderates the sensitivity to parental behaviors and thereby contributes to the characterization of personality traits. Materials and Methods: Participants were 725 healthy Japanese. Parenting practices of their parents were evaluated by the Parental Bonding Instrument (PBI) with the care and protection subscales. Personality was evaluated using the Temperament and Character Inventory (TCI). The OPRM1 A118G polymorphism was detected by a PCR method. Results: Multiple regression analyses revealed significant effects of the interaction between the OPRM1 genotype and maternal protection on scores of the self-directedness and cooperativeness dimensions, while significant main effects of the OPRM1 genotype on scores of the TCI were not found. Further analyses showed that there were significant negative correlations between maternal protection scores and the two dimensional scores in the A/A and A/G genotypes with higher correlation coefficients in the former, but not in the G/G genotype. Conclusion: The present study suggests that the OPRM1 polymorphism contributes to the characterization of personality traits by moderating the sensitivity to parental behaviors, especially maternal protection.
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