In a meta-analysis and review of the effect of oxytocin on social behavior done by Carsten De Dreu, the research reviewed shows that oxytocin enables the development of trust, specifically towards individuals with similar characteristics - categorised as 'in-group' members - promoting cooperation with and favoritism towards such individuals [REF].
Race can be used as an example of in-group and out-group tendencies because society often categorizes individuals into groups based on race (Caucasian, African American, Latino, etc.).