Social chemosignals from breastfeeding women increase sexual motivation Natasha A. Spencer a , Martha K. McClintock a, * , Sarah A. Sellergren a , Susan Bullivant a , Suma Jacob a , Julie A. Mennella b a The Institute for Mind and Biology, The University of Chicago, Chicago, IL 60637, USA b Monell Chemical Senses Center, Philadelphia, PA 19104, USA Received 4 December 2003; revised 3 June 2004; accepted 4 June 2004 Abstract Human pheromones, a type of social chemosignal, modulate endocrine function by regulating the timing of ovulation. In animals, pheromones not only regulate ovulation but also female reproductive motivation and behavior. There is no extant evidence that humans produce social chemosignals that affect human sexual motivation or reproductive behavior as occurs in other mammals. Here, we demonstrate that natural compounds collected from lactating women and their breastfeeding infants increased the sexual motivation of other women, measured as sexual desire and fantasies. Moreover, the manifestation of increased sexual motivation was different in women with a regular sexual partner. Those with a partner experienced enhanced sexual desire, whereas those without one had more sexual fantasies. These results are consistent with previous pheromonal effects on endocrine function, and warrant further study of these social chemosignals as candidates for pheromonal processes, including their effects on other aspects of motivation and behavior. D 2004 Elsevier Inc. All rights reserved. Keywords: Breastfeeding; Chemosignals; Lactation; Sexual motivation; Pheromone; Reproduction Pheromones are defined as those natural compounds produced by one member of a social group that can regulate the neuroendocrine mechanisms underlying behavior, fertil- ity, or development of another group member. Effective in minute quantities by definition, they are specialized types of social chemosignals used for communication within a spe- cies often functioning to indicate reliably the reproductive status of other social group members (Beauchamp, 2000; Karlson and Lu ¨scher, 1959; definitions reviewed by McClintock (2002)). Utilized by many species, there has been great interest in discerning whether humans also utilize social chemosignals as pheromones (Jacob et al., 2001b; McClintock, 2000; Wysocki and Preti, 1998). To date, several studies have shown that natural compounds found in the axillary region of men and women can trigger in women neuroendocrine responses such as altered timing of the preovulatory surge of luteinizing hormone (LH) (McClintock, 2000; Stern and McClintock, 1998) and altered LH pulse frequency (Preti et al., 2003; Shinohara et al., 2001; Wysocki et al., 2001). Thus, such natural compounds are candidates for pheromonal systems that affect behavior. Mammalian model studies have revealed that phero- mones from lactating rats and their pups increase the variability of ovarian cycles in recipient females by length- ening the cycle (McClintock, 1984; Mennella, 1988; Men- nella and Moltz, 1989). They also induce reproductive behaviors—specifically, maternal behaviors—in adult con- specifics (Mennella, 1988; Mennella and Moltz, 1989). Nonetheless, there has been little research on effects of lactating women and their infants on ovarian cycles and reproductive behavior of other women with whom they interact. We have reported that natural compounds collected from lactating women and breastfeeding infants tripled the variance in menstrual cycle length for an overlapping sample of women by acting on the timing of ovulation and the life span of the corpus luteum (Jacob et al., 2004). In addition to investigating effects on menstrual cycle length, another research focus was the psychosexual impact of 0018-506X/$ - see front matter D 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.yhbeh.2004.06.002 * Corresponding author. Department of Psychology, The University of Chicago, 5730 S. Woodlawn Avenue, Chicago, IL 60637. Fax: +1-773-702- 0320. E-mail address: mkm1@uchicago.edu (M.K. McClintock). www.elsevier.com/locate/yhbeh Hormones and Behavior 46 (2004) 362 – 370