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To determine whether female nipple erection is perceived as a sign of sexual arousal or interest, male and female participants were asked to rate photos of real women with and without salient nipple erection on a series of 16 emotional and physiological states, including positive, negative, and sexually aroused states. Nipple erection salience was rated by independent raters, and faces in photos were obscured to prevent discerning emotional states from facial cues. Men clearly projected more sexy and positive emotions onto the stimuli when the stimuli displayed erect nipples. Whereas women did project more positive emotions with erect nipples, they did not differ in their expression of sexy. We also observed that men's self-ratings of sexy and positive emotions were the same as their ratings of the stimuli. Women, however, reported significantly less sexy and positive emotions for themselves relative to the stimuli. Public Significance StatementGiven the research that suggests men attend to nipples and that nipple erection is triggered by sexual excitement (among other triggers), we questioned whether men see nipple erection as a sign of sexual interest. Our findings indicate that men (but not women) see women as sexier when they have nipple erection and also see themselves as sexier, supporting the idea that nipple erection is perceived signaling arousal or sexual interest.

Previous research has shown that men perceive nipple erection as signaling more sexually receptive states. This study intended to determine if this perception changed male hypothetical behavior. For example, would men be more willing to assist women with nipple erection as opposed to those without? Participants were asked to rate pictures of women with and without salient nipple erection (faces were obscured to prevent discerning emotional states). Men perceived women with nipple erection as more deserving of altruism, especially if that altruism involved greater interaction with the woman, and they expected these same women to behave more altruistically toward them. They also believed the women with erect nipples should be included in their social groups. Women, on the other hand, did not perceive them as deserving of greater altruistic behaviors, did not expect greater altruistic behaviors from them, and did not want to include them into their social groups.

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