Oxytocin is a hormone that promotes bonding during the early stages of relationship development, positive feelings toward relationship partners1, including feelings of trust.2 In fact, oxytocin has been implicated in a variety of positive relationship behaviors, including attachment, social memory, sexual behavior, and orgasm, as well as maternal caring and bonding behaviors.3 As a result, the media often refers to oxytocin as the “cuddle hormone.” However, recent research suggests that the so-called “cuddle hormone” may have a dark side by increasing relationship violence. How They Did It Researchers randomly assigned 93 undergraduate students to receive a nasal spray containing either (a) oxytocin or (b) a saline solution (i.e., a placebo spray). Importantly, the administration of the spray was double-blind; neither the researcher nor the participant knew which spray the participant was receiving. Following the spray, researchers provoked participants in an attempt to raise stress levels and establish a context for aggression. The provocations involved giving a brief speech to an audience who disagreed with the speech and experiencing a “cold pressor task” in which extreme cold is applied to the participant’s forehead (resulting in moderate physical pain). Participants then completed a measure of trait aggression (i.e., how much the person is naturally inclined toward aggression) as well as a measure of how likely individuals were to be aggressive toward their partners that asked about the likelihood of engaging in several behaviors toward their romantic partners (e.g., throwing things, twisting their arm/hair, shoving).