This Precis provides an overview of the book "The Scent of Eros: Mysteries of Odor in Human Sexuality," which details for a general audience a five-step biological pathway that allows the social environment to influence the genetic nature of mammalian behavior. This pathway is: gene-cell-tissue-organ-organ system. Moreover, though there are many environmental influences on genes, mammalian pheromones are the only known social-environmental stimuli that appear to activate gene expression in neurosecretory cells of tissue in the brain, an organ that is essential to any organ system involved in behavior. Human pheromones appear both to elicit a homologous "neuroendocrine" response and to influence behavior. Thus, human pheromones may fulfill the biological criteria required to link at least one aspect of a sensory-based, nurturing, social environment: olfaction, to the genetic nature of human behavior through a five-step pathway common to all terrestrial mammals and to many other vertebrates.
2. After a Foreward by William E. Hartman and Marilyn A. Fithian and an Introduction by the co-author, Chapter 1 begins with commentary on previously published works by various scientific authorities who have offered their insights into the importance of human chemical communication. Among these authorities are Havelock Ellis, Irving Bieber, and Lewis Thomas, who offered the following statement: "I should think we might fairly gauge the future of biological science, centuries ahead, by estimating the time it will take to reach a complete, comprehensive understanding of odor. It may not seem a profound enough problem to dominate all the life sciences, but it contains, piece by piece all the mysteries." (Thomas, 1980)
3. In Chapter 1, there are fourteen examples of the many questions that may be answered when one considers the likelihood of odorous human communication. Most of these questions concern different aspects of human sexuality. Briefly deliberated are concerns about an ineffective "language of olfaction" and errors in the logic that has been used in the past to deny the importance of odor in human sexuality. The introductory focus then turns to biological consistency among species; the common basis for scientific advancements; and the development of the working hypothesis that odors are a primary influence on human sexuality.
4. Chemical communication and its importance in other species from insects to mammals is more fully detailed in Chapter 2. The term pheromone is defined, with added emphasis of one basic causal relationship, namely, that mammalian pheromones appear to influence the secretion of gonadotropin releasing hormone (GnRH), a hormone with both short-term and long-term effects on neurotransmission. Distinguishing characteristics of pheromones like species-specificity, and the differences between signalling and releasing pheromones are added to the definition. After a brief discussion of mammalian pheromones, the natural production of human odors is discussed and anecdotal evidence of some of their effects are offered as support for the concept of human pheromones.
5. Chapter 3 alludes to Greek mythology; the story of Ariadne's thread, which metaphorically addresses the issue of biological consistency among species. The development of the mammalian sense of smell is detailed from its beginnings in single-celled organisms. Olfactory transduction is briefly discussed. Four crucial turning points in the development of mammalian chemical communication systems, which contribute to species survival, are: (1) the release of pheromones to attract another organism, which occurs in single-celled non-motile organisms, (2) the ability to detect and respond to chemical messengers with movement, which occurs in motile single-celled organisms (3) the development of neural networks devoted to processing chemical signals, which occurs in brainless invertebrates, and (4) phylogenetic advances in the development of these neural networks to include development of the vertebrate brain. Species-specific comparisons and contrasts in structure and function are provided.
6. Chapter 4 offers an ontogenetic perspective on development, both of the mammalian olfactory systems and of the GnRH neuronal system. The ontogenetic connection between the structure and function of olfactory sensory systems and brain development ascends in its significance because it allows the odorous social environment to directly and indirectly influence brain function by acting on GnRH, which in turn has short-term effects on neurotransmission and long-term effects on the hypothalamic-pituitary-gonadal (HPG) axis.
7. Kallmann's syndrome represents a failure of GnRH neuronal migration. Correlates with anosmia and the inability to fall in love are noted, as are correlates with the GnRH neuronal system and brain development in other species. Additional aspects of olfactory transduction and signal processing are discussed. There are differences between the main olfactory system and the accessory olfactory system. The importance of the vomeronasal organ (VNO) for pheromone detection in other species and renewed interest in the recently confirmed presence of the human VNO add to the argument for the influence of odors on human sexuality.
8. Chapter 5 begins with anthropological folklore associated with odor and human behavior and progresses to a discussion of empirical evidence for this link. The metabolism of hormones into pheromones is noted. Specifically addressed are experiments with putative human pheromones and the likelihood of causal physiological and behavioral relationships. Comparisons and contrasts among species again are offered in this regard. The use of mammalian pheromones in fragrances designed to enhance the sexual appeal of humans is examined. The naturally occurring fragrance of musk, present in the secretions of many species, is held in high regard for its universal sex-attractant properties.
9. Chapter 6 reports on experiments with consciously processed human odors, beginning with the classically-conditioned response of infants to their mothers' naturally-scented or artificially-scented breasts. Olfactory imprinting and the importance of the mother-infant bond are linked through non-human animal models to the development of neural templates and the human "love map". Aspects of odor hedonics are detailed.
10. Children can determine the genetic sex of adults, and adults can distinguish between different people using their sense of smell. The importance of mammalian odors in aggression and in other contexts besides the mother-infant bond suggests human correlates. Similarly, clinical and anecdotal evidence that humans are culturally aware of odor-associated customs enhances a more scientific approach to the link between sex and the human sense of smell. Odors and fetishism are linked. The natural superiority of women's olfactory acuity and specificity is linked to estrogen levels and to an important role in female choice: What human beings lack in acuity they make up for in powers of discrimination, which rival those of any other mammal.
11. Chapter 7 is a simplistic overview of prenatal GnRH neuronal system development. Included are genetic predisposition and the importance of GnRH pulsatility in the regulation of the HPG axis. This chapter begins, however, with the importance of chemical communication between ovum and spermatozoa and progresses through basic genetics, neuroanatomy, endocrinology, and endocrine aspects of neurotransmission. Postnatally, odor input is linked to human HPG axis function. Pheromone input appears to be indirectly measurable in assays of luteinizing hormone.
12. Beginning with the "nature" versus "nurture" controversy, Chapter 8 proceeds to the important issue of finding a link between the social environment and genes. The likelihood that genes are involved both in physiological and in behavioral cause and effect relationships is detailed. The influence of pheromones on genes and on concurrent neuroendocrine, reproductive system, and central nervous system development is proposed.
13. Twin studies are discussed, as is recent evidence of master genes that may allow chemical communication at the cellular level to play a primary role in behavioral development and in sexual orientation. Genetic conservation among species, specifically with regard to chemical communication, is addressed. Enzymes and chemical responses are linked with human behavior, as are genes and G protein-coupled receptors through an example of familial precocious puberty. Correlates between adrenal androgen metabolism, pheromone production, sexual dimorphism in the human hypothalamus, and human sexual orientation are offered.
14. Chapter 9 details aspects of human consciousness and of limbic learning and memory. Olfaction plays a key role by providing input to the medial preoptic area of the hypothalamus. Comparisons and contrasts among species and among theories of consciousness are offered.
15. The importance of linking specialized research in diverse disciplines is made known, namely, how a "gay gene" might influence both human neuroanatomy and human sexuality. Dean Hamer has proposed the following: "The most simple hypothesis would be that the Xq28 makes a protein that is directly involved in the growth or death of neurons in the INAH-3. Alternatively, the gene could encode a protein that influences the regulation of this region by hormones." (Hamer & Copeland 1994).
16. Effects of pheromones on other species are favorably compared to the effects of putative human pheromones. A consciously-processed odor stimulus has been used as an adjunct to classically condition the human immune response, thereby adding clinical significance to the effects of odors.
17. Chapter 10 reveals evidence of odor-driven hormonal effects on human behavior and on sexuality, again using cross-species comparisons that link information provided in earlier chapters. Examples supporting a link between pheromones and human sexuality are discussed. The "Law of Propinquity" appears to be invalidated by experience with pheromones that create more of a friendship or kinship bond, perhaps also creating an antibond effect on love. There is evidence that humans mate for genetic diversity on the basis of unconscious odor associations, and that odors may be involved in the Coolidge effect.
18. The sources of human pheromones are detailed in Chapter 11, with a discussion of the role of glandular secretions, fatty acids, bacteria, skin cells, and the relationship between levels or ratios of sex hormones and pheromone production. Important aspects of pheromone distribution are then linked to intimate behavioral associations. Androgenization appears to stimulate secretion of a more masculine pheromone signature. Odors can be used in clinical diagnostics. The inherent difficulties of human pheromone research are briefly discussed.
19. Chapter 12 links human pheromones to various courtship behaviors (e.g., dancing, kissing, et al.,) that appear to become progressively more intimate with increasing exposure to pheromones. Stereotypic attractive qualities (e.g., large breasts or hair color and distribution) with anecdotal evidence of causal relationships between pheromones, attraction, and intimacy are represented both positively and negatively. Culturally, negative representations often appear to correlate well with sexual repression. Racial differences in odor production that may contribute to racial prejudice are briefly addressed.
20. Included in Chapter 13 is a discussion of results from the National Geographic Smell Survey. Data was collected from approximately one and a half million people worldwide. Causes of anosmia and its link both to genetics and to the GnRH neuronal systems are detailed, as are links between damage to the VNO, age-related disorders, olfactory deficits, and behavior.
21. Chapter 14 provides both a historical and a modern-day overview of aromatherapy. Cultural differences in odor hedonics are explained by odor-associated classical conditioning. The roles of chemicals now known to function as human pheromones and of putative human pheromones in fragrances for commercial use is discussed. A brief summation of current research supporting the hypothesis that human pheromones are a primary influence on human sexuality is provided.
22. I believe that "the pheromones of other mammals are the only social-environmental stimuli to influence genes [in GnRH neurons]." Accordingly, human pheromones are the most likely link between the "nature" and the "nurture" of human sexuality. However, a typographical error: insertion of "not" on page 189, in paragraph 2, line 4 (intended to read as above) detracts from the concluding paragraphs.
Thomas, L. (1980) Notes of a biology-watcher: on smell. New England Journal of Medicine 302: 731-733.
Hamer, D. & Copeland, P. (1994) The Science of Desire, Simon & Schuster: 163.