Even though opioid receptor families are similar to each other in many ways, their structural differences lead to differences in functionality. Thus, mu-opioid receptors induce relaxation, trust, satisfaction and have a strong analgesic effect. This system is also thought to be important in mediating complex social behaviors involved in the formation of stable, emotionally committed relationships. Social attachment was demonstrated to be mediated by the opioid system through experiments administering morphine and naltrexone, an opioid agonist and antagonist, to juvenile guinea pigs. The agonist decreased the preference of the juvenile to be near the mother and reduced distress vocalization whereas the antagonist had the opposite effects. Experiments were corroborated in dogs, chicks, and rats confirming the evolutionary importance of opioid signaling in these behaviors. Researchers have also found that systemic naltrexone treatment of female prairie voles during initial exposure to a male reduced subsequent mating bouts and nonsexual socialization with this familiar partner, when a choice test including a novel male was performed afterwards. This points to a role for opioid receptors in mating behaviors. However, mu-opioid receptors don't have specificity for regulating social behaviour as they induce a relaxing effect in a wide spectrum of non-social contexts.