In numerous papers, Breedlove has demonstrated that steroid hormones and sexual behavior affect the developing and adult spinal cord and brain. He also reported that the average digit ratio of lesbians is more masculine than that of straight women,[9] a finding that has been replicated in his[10] and many other labs[11][12][13][14][15][16][17][18][19][20][21] and which indicates that lesbians, on average, are exposed to more prenatal testosterone than are straight women. This finding joins many others that biological influences, such as prenatal testosterone and fraternal birth order,[22] act before birth to affect the later unfolding of human sexual orientation, which is the theme for the documentary project Whom You Love.
He is sole author of two textbooks Principles of Psychology and Foundations of Neural Development and has co-authored textbooks in Biological Psychology and Behavioral Neuroendocrinology.[23][24][25][26]
Breedlove, along with other neuroscientists, researched PTSD being connected to erectile dysfunction. This erectile dysfunction is usually treated by psychotherapy. Breedlove helped to find the receptor that is affected by the PTSD. This was found through a series of stress tests on rats. The receptor is called the gastrin-releasing peptide (GRP) receptor. This receptor is stress-vulnerable and should be targeted in treatment.[27]
He also researched how GRP in the lumbar spinal cord could be stimulated for the purpose of curing erectile dysfunction. It was found that the female rats and the male rats with erectile dysfunction had the same amount of GRP. Once the GRP was stimulated in the male rats, androgen receptors worked, and erectile dysfunction was cured. During the experiment, simple erections, dorsal flips of the penis and cup-like flaring erections of the distal glans were measured in the rats before and after probe stimulation.[28]
Breedlove examined the sex differences in animals to gain an understanding of the sex differences in humans. It was found in rats that the males had more cell numbers in the spinal nucleus of the bulbocavernosus (SNB) than the female rats. These motor neurons appear in both male and female rats, but fade with age in the female rats. Testosterone was also found as the key hormone that is responsible for the differences between males and females.[29]
Breedlove researched the sexual preferences of homosexual men. Using homosexual and heterosexual male participants, it was found that the two groups did not vary according to mating desires. Even though homosexual males cannot reproduce, they, like the heterosexual males, prefer to be with a younger partner. This concludes that both groups' partner references were independent of the evolutionary need for reproduction.[30]
^Tortorice JL (2002). Written on the body: butch/femme lesbian gender identity and biological correlates (Ph.D. Dissertation). Rutgers University. OCLC80234273.
^McFadden D, Shubel E (December 2002). "Relative lengths of fingers and toes in human males and females". Hormones and Behavior. 42 (4): 492–500. doi:10.1006/hbeh.2002.1833. PMID12488115. S2CID1360679.
^Hall LS, Love CT (February 2003). "Finger-length ratios in female monozygotic twins discordant for sexual orientation". Archives of Sexual Behavior. 32 (1): 23–8. doi:10.1023/A:1021837211630. PMID12597269. S2CID1743441.
^Rahman Q, Wilson GD (April 2003). "Sexual orientation and the 2nd to 4th finger length ratio: evidence for organising effects of sex hormones or developmental instability?". Psychoneuroendocrinology. 28 (3): 288–303. doi:10.1016/S0306-4530(02)00022-7. PMID12573297. S2CID21071741.
^Csathó A, Osváth A, Bicsák E, Karádi K, Manning J, Kállai J (February 2003). "Sex role identity related to the ratio of second to fourth digit length in women". Biological Psychology. 62 (2): 147–56. doi:10.1016/S0301-0511(02)00127-8. PMID12581689. S2CID38339150.
^Kraemer B, Noll T, Delsignore A, Milos G, Schnyder U, Hepp U (2006). "Finger length ratio (2D:4D) and dimensions of sexual orientation". Neuropsychobiology. 53 (4): 210–4. doi:10.1159/000094730. PMID16874008. S2CID201838.
^Wallien MS, Zucker KJ, Steensma TD, Cohen-Kettenis PT (August 2008). "2D:4D finger-length ratios in children and adults with gender identity disorder". Hormones and Behavior. 54 (3): 450–4. doi:10.1016/j.yhbeh.2008.05.002. PMID18585715. S2CID6324765.
^Grimbos T, Dawood K, Burriss RP, Zucker KJ, Puts DA (2010). "Sexual orientation and the second to fourth finger length ratio: a meta-analysis in men and women". Behav Neurosci. 124 (2): 278–287. doi:10.1037/a0018764. PMID20364887.
^S. Marc Breedlove (2015). Principles of Psychology. Oxford University Press. ISBN978-0199329366
^S. Marc Breedlove, and Neil V. Watson (2013). Biological Psychology: An Introduction to Behavioral and Cognitive Neuroscience (7th Ed). Sinauer Associates. ISBN978-0-87893-927-5
^Neil V. Watson and S. Marc Breedlove(2016). The Mind's Machine: Foundations of Brain and Behavior (2nd Ed). Sinauer Associates. ISBN978-1-60535-276-3
^Jill B. Becker, S. Marc Breedlove, David Crews and Margaret M. McCarthy (2002). Behavioral Endocrinology, (2nd Ed). MIT Press. ISBN978-0-262-52321-9
^Sakamoto, H., Matsuda, K., Zuloaga, D. G., Nishiura, N., Takanami, K., Jordan, C. L., & ... Kawata, M. (2009). Stress Affects a Gastrin-Releasing Peptide System in the Spinal Cord That Mediates Sexual Function: Implications for Psychogenic Erectile Dysfunction. PLoS One, 4(1), 1-7. doi:10.1371/journal.pone.0004276
^Sakamoto, H., Matsuda, K., Zuloaga, D. G., Hongu, H., Wada, E., Wada, K., & ... Kawata, M. (2008). Sexually dimorphic gastrin releasing peptide system in the spinal cord controls male reproductive functions. Nature Neuroscience, 11(6), 634-636. doi:10.1038/nn.2126
^Morris, J. A., Jordan, C. L., & Breedlove, S. (2004). Sexual differentiation of the vertebrate nervous system. Nature Neuroscience, 7(10), 1034-1039. doi:10.1038/nn1325
^Gobrogge, K. L., Perkins, P. S., Baker, J. H., Balcer, K. D., Marc Breedlove, S. S., & Klump, K. L. (2007). Homosexual Mating Preferences from an Evolutionary Perspective: Sexual Selection Theory Revisited. Archives Of Sexual Behavior, 36(5), 717-723. doi:10.1007/s10508-007-9216-x
Further reading
Breedlove, Marc; et al. (2007), "The mammalian spinal cord", in Einstein, Gillian (ed.), Sex and the brain, Cambridge, Massachusetts: MIT Press, pp. 261–278, ISBN9780262050876.