Mothers are highly responsive to their offspring. In non-human mammals, mothers secrete dopamine in the nucleus accumbens (NAcc) in response to their pups. Yet, it is still unknown which aspect of the offspring-behavior elicits dopaminergic responses in mothers. Here we tested whether infants’ affective signals elicit dopaminergic responses in the NAcc of human mothers. First, we conducted a behavioral analysis on videos of infants’ free-play, and quantified the affective signals infants spontaneously communicated. Then, we presented the same videos to mothers during a magnetic resonance-positron emission tomography (MR-PET) scan. We traced the binding of [11C]raclopride to free D2/3-type receptors to assess maternal dopaminergic responses during the infant-videos. When mothers observed videos with many infant-signals during the scan, they had less [11C]raclopride binding in the right NAcc. Less [11C]raclopride binding indicates that less D2/3 receptors were free, possibly due to increased endogenous dopamine responses to infants’ affective signals. We conclude that NAcc D2/3 receptors are involved in maternal responsiveness to affective signals of human infants. D2/3 receptors have been associated with maternal responsiveness in nonhuman animals. This evidence supports a similar mechanism in humans, and specifies infant-behaviors that activate the maternal dopaminergic system, with implications for social neuroscience, development and psychopathology.

The decision with whom to form a romantic bond is of great importance, yet the biological or behavioral mechanisms underlying this selective process in humans are largely unknown. Classic evolutionary theories of mate selection emphasize immediate and static features such as physical appearance and fertility. However, they do not explain how initial attraction temporally unfolds during an interaction, nor account for mutual physiological or behavioral adaptations that take place when two people become attracted. Instead, recent theories on social bonding emphasize the importance of co-regulation during social interactions (i.e., the social coordination of physiology and behavior between partners), and predict that co-regulation plays a role in bonding with others. In a speed-date experiment of forty-six heterosexual dates, we recorded the naturally occurring patterns of electrodermal activity and behavioral motion in men and women, and calculated their co-regulation during the date. We demonstrate that co-regulation of behavior and physiology is associated with the date outcome: when a man and a woman synchronize their electrodermal activity and dynamically tune their behavior to one another, they are more likely to be romantically and sexually attracted to one another. This study supports the hypothesis that co-regulation of sympathetic and behavioral rhythms between a man and a woman serves as a mechanism that promotes attraction.