Shelley Taylor, the UCLA psychologist who identified the "tend and befriend" response, says the gene that produces the oxytocin receptor is responsible for influencing self-esteem, optimism and a sense of mastery.

This isn't so surprising, because oxytocin seems to produce most of the positive social emotions -- as well as some less positive ones.

According to the UCLA press office:

At a particular location, the oxytocin receptor gene has two versions: an "A" (adenine) variant and a "G" (guanine) variant. Several studies have suggested that people with at least one "A" variant have an increased sensitivity to stress, poorer social skills and worse mental health outcomes.

 

The researchers found that people who have either two "A" nucleotides or one "A" and one "G" at this specific location on the oxytocin receptor gene have substantially lower levels of optimism, self-esteem and mastery and significantly higher levels of depressive symptoms than people with two "G" nucleotides.