Doctor Larry Young explains that social personality traits are influenced by levels of oxytocin and vasopressin in the brain.

The oxytocin and vasopressin are neuropeptides that are produced in the hypothalamus. They are projected to other regions of the brain where they act by binding to receptors. The receptor distribution in the brain determines what sort of effect the peptides are going to have. For example, the monogamous prairie vole, very highly social monogamous prairie vole has loads of oxytocin receptors and vasopressin receptors in these reward centers of the brain. If you compare that to other kinds of voles, for example Montane voles who are asocial and don’t form any bonds, they don’t have the receptors there at all. So it seems that the key difference between these species that are very highly social and can bond and ones that cannot is what parts of the brain respond to oxytocin and vasopressin. You can imagine that maybe even in people if there are different levels of these receptors in different brain areas, it could contribute to differences in social personality traits for example.

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