Oxytocin
Oxytocin (Oxt) is a peptide hormone and neuropeptide. Oxytocin is normally produced by the paraventricular nucleus of the hypothalamus and released by the posterior pituitary. It plays a role in social bonding, sexual reproduction, childbirth, and the period after childbirth. Oxytocin is released into the bloodstream as a hormone in response to stretching of the cervix and uterus during labor and with stimulation of the nipples from breastfeeding. This helps with birth, bonding with the baby, and milk production. Oxytocin was discovered by Henry Dale in 1906. Its molecular structure was determined in 1952. Oxytocin is also used as a medication to facilitate childbirth.
In-group favoritism
Further, oxytocin influences the responses of individuals in a particular group to those of another group. The in-group bias is evident in smaller groups; however, it can also be extended to groups as large as one's entire country leading toward a tendency of strong national zeal. A study done in the Netherlands showed that oxytocin increased the in-group favoritism of their nation while decreasing acceptance of members of other ethnicities and foreigners. People also show more affection for their country's flag while remaining indifferent to other cultural objects when exposed to oxytocin. It has thus been hypothesized that this hormone may be a factor in xenophobic tendencies secondary to this effect. Thus, oxytocin appears to affect individuals at an international level where the in-group becomes a specific "home" country and the out-group grows to include all other countries.