The relationship between biology and sexual orientation is a subject of on-going research. While scientists do not know the exact cause of sexual orientation, they theorize that it is caused by a complex interplay of genetic, hormonal, and environmental influences. However, evidence is weak for hypotheses that the post-natal social environment impacts sexual orientation, especially for males.
The hypothalamus is a small part of the brain that contains a number of nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamus is located below the thalamus and is part of the limbic system. It forms the ventral part of the diencephalon. All vertebrate brains contain a hypothalamus. In humans, it is the size of an almond.
An androgen is any natural or synthetic steroid hormone that regulates the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This includes the embryological development of the primary male sex organs, and the development of male secondary sex characteristics at puberty. Androgens are synthesized in the testes, the ovaries, and the adrenal glands.
Sexual differentiation is the process of development of the sex differences between males and females from an undifferentiated zygote. Sex determination is often distinct from sex differentiation; sex determination is the designation for the development stage towards either male or female, while sex differentiation is the pathway towards the development of the phenotype.
Lordosis behavior, also known as mammalian lordosis or presenting, is the naturally occurring body posture for sexual receptivity to copulation present in females of most mammals including rodents, elephants, and cats. The primary characteristics of the behavior are a lowering of the forelimbs but with the rear limbs extended and hips raised, ventral arching of the spine and a raising, or sideward displacement, of the tail. During lordosis, the spine curves dorsoventrally so that its apex points towards the abdomen.
A gynandromorph is an organism that contains both male and female characteristics. The term comes from the Greek γυνή (gynē) 'female', ἀνήρ (anēr) 'male', and μορφή (morphē) 'form', and is used mainly in the field of entomology. Gynandromorphism is most frequently recognized in organisms that have strong sexual dimorphism such as certain butterflies, spiders, and birds, but has been recognized in numerous other types of organisms.
The hypothalamic–pituitary–gonadal axis refers to the hypothalamus, pituitary gland, and gonadal glands as if these individual endocrine glands were a single entity. Because these glands often act in concert, physiologists and endocrinologists find it convenient and descriptive to speak of them as a single system.
Gonadal dysgenesis is classified as any congenital developmental disorder of the reproductive system in humans. It is atypical development of gonads in an embryo. One type of gonadal dysgenesis is the development of functionless, fibrous tissue, termed streak gonads, instead of reproductive tissue. Streak gonads are a form of aplasia, resulting in hormonal failure that manifests as sexual infantism and infertility, with no initiation of puberty and secondary sex characteristics.
Gender incongruence is the state of having a gender identity that does not correspond to one's sex assigned at birth. This is experienced by people who identify as transgender or transsexual, and often results in gender dysphoria. The causes of gender incongruence have been studied for decades.
Sexual differentiation in humans is the process of development of sex differences in humans. It is defined as the development of phenotypic structures consequent to the action of hormones produced following gonadal determination. Sexual differentiation includes development of different genitalia and the internal genital tracts and body hair plays a role in sex identification.
Fernando Nottebohm is a neuroscientist. He serves as the Dorothea L. Leonhardt Professor at Rockefeller University, as well as being head of the Laboratory of Animal Behavior and director of the Field Research Center for Ecology and Ethology.
Stephen Marc Breedlove is the Barnett Rosenberg professor of Neuroscience at Michigan State University in East Lansing, Michigan. He was born and raised in the Ozarks of southwestern Missouri. After graduating from Central High School in 1972, he earned a bachelor's degree in Psychology from Yale University in 1976, and a Ph.D. in psychology from UCLA in 1982. He was a professor of psychology at the University of California, Berkeley from 1982 to 2003, moving to Michigan State in 2001. He works in the fields of Behavioral Neuroscience and Neuroendocrinology. He is a member of the Society for Neuroscience and the Society for Behavioral Neuroendocrinology, and a fellow of the Association for Psychological Science (APS) and the Biological Sciences section of the American Association for the Advancement of Science (AAAS).
Sexual orientation is an enduring pattern of romantic or sexual attraction to persons of the opposite sex or gender, the same sex or gender, or to both sexes or more than one gender, or none of the aforementioned at all. The ultimate causes and mechanisms of sexual orientation development in humans remain unclear and many theories are speculative and controversial. However, advances in neuroscience explain and illustrate characteristics linked to sexual orientation. Studies have explored structural neural-correlates, functional and/or cognitive relationships, and developmental theories relating to sexual orientation in humans.
The hormonal theory of sexuality holds that, just as exposure to certain hormones plays a role in fetal sex differentiation, such exposure also influences the sexual orientation that emerges later in the individual. Prenatal hormones may be seen as the primary determinant of adult sexual orientation, or a co-factor with genes, biological factors and/or environmental and social conditions.
The neuroscience of sex differences is the study of characteristics that separate brains of different sexes. Psychological sex differences are thought by some to reflect the interaction of genes, hormones, and social learning on brain development throughout the lifespan.
Jeffrey Blaustein is a retired professor and the former head of the Behavioral Neuroscience Division at the University of Massachusetts Amherst, known for his research in behavioral neuroendocrinology. Before the Behavioral Neuroscience Division, he had served at UMass Amherst as the founding director of the Center for Neuroendocrine Studies and as the head of the Biopsychology department. He was named a National Institute of Mental Health Senior Scientist in 1997 and an American Association for the Advancement of Science fellow in 2014. He has served as the president of the Society for Behavioral Neuroendocrinology and as the editor-in-chief of the journal Endocrinology.
The Organizational-Activational Hypothesis states that steroid hormones permanently organize the nervous system during early development, which is reflected in adult male or female typical behaviors. In adulthood, the same steroid hormones activate, modulate, and inhibit these behaviors. This idea was revolutionary when first published in 1959 because no other previous experiment had demonstrated that adult behaviors could be determined hormonally during early development.
David Pafford Crews is the Ashbel Smith Professor of Zoology and Psychology at the University of Texas at Austin. He has been a pioneer in several areas of reproductive biology, including evolution of sexual behavior and differentiation, neural and phenotypic plasticity, and the role of endocrine disruptors on brain and behavior.
Eric Vilain is a physician-scientist and professor in the fields of Differences of Sex Development (DSDs) and precision medicine. He is the Associate Vice Chancellor for Scientific Affairs at the University of California, Irvine Health Affairs and also the director of the UCI Institute for Clinical and Translational Science. He previously was the director of the Center for Genetic Medicine Research at Children's National Medical Center and the chair of the Department of Genomics and Precision Medicine at the George Washington University School of Medicine & Health Sciences in Washington, D.C. Vilain is a fellow of the American College of Medical Genetics, serves on the International Olympic Committee's Medical Commission, and sits on the Board of Scientific Counselors for the National Institute of Child Health and Human Development (NICHD).
Four Core Genotypes (FCG) mice are laboratory mice produced by genetic engineering that allow biomedical researchers to determine if a sex difference in phenotype is caused by effects of gonadal hormones or sex chromosome genes. The four genotypes include XX and XY mice with ovaries, and XX and XY mice with testes. The comparison of XX and XY mice with the same type of gonad reveals sex differences in phenotypes that are caused by sex chromosome genes. The comparison of mice with different gonads but the same sex chromosomes reveals sex differences in phenotypes that are caused by gonadal hormones.
