Heterosexuality is romantic attraction, sexual attraction or sexual behavior between people of the opposite sex or gender. As a sexual orientation, heterosexuality is "an enduring pattern of emotional, romantic, and/or sexual attractions" to people of the opposite sex; it "also refers to a person's sense of identity based on those attractions, related behaviors, and membership in a community of others who share those attractions." Someone who is heterosexual is commonly referred to as straight.
Sexual orientation is an enduring personal pattern of romantic attraction 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. Patterns are generally categorized under heterosexuality, homosexuality, and bisexuality, while asexuality is sometimes identified as the fourth category.
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 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.
Fraternal birth order, also known as the older brother effect, has been correlated with male sexual orientation, with a significant volume of research finding that the more older brothers a male has from the same mother, the greater the probability he will have a homosexual orientation. Ray Blanchard and Anthony Bogaert first identified the association in the 1990s and named it the fraternal birth order effect. Scientists have attributed the effect to a prenatal biological mechanism, since the association is only present in men with older biological brothers, and not present among men with older step-brothers and adoptive brothers. The mechanism is thought to be a maternal immune response to male fetuses, whereby antibodies neutralize male Y-proteins thought to play a role in sexual differentiation during development. This would leave some regions of the brain associated with sexual orientation in the 'female typical' arrangement – or attracted to men. Biochemical evidence for this hypothesis was identified in 2017, finding mothers with a gay son, particularly those with older brothers, had heightened levels of antibodies to the NLGN4Y Y-protein than mothers with heterosexual sons.
The sexually dimorphic nucleus (SDN) is an ovoid, densely packed cluster of large cells located in the medial preoptic area (POA) of the hypothalamus which is believed to be related to sexual behavior in animals. Thus far, for all species of mammals investigated, the SDN has been repeatedly found to be considerably larger in males than in females. In humans, the volume of the SDN has been found to be 2.2 times as large in males as in females and to contain 2.1 times as many cells. The human SDN is elongated in females and more spherical in males. No sex differences have been observed in the human SDN in either cell density or mean diameter of the cell nuclei. The volume and cell number of the human SDN considerably decreases with age, although the decrease in cell number is both sex and age-specific. In males, a substantial decrease in the cell number of the human SDN was observed between the age of 50–60 years. Cell death was more common in females than males, especially among those older than 70 years of age. The SDN cell number in females can drop to 10-15% of that found in early childhood.
Androphilia and gynephilia are terms used in behavioral science to describe sexual orientation, as an alternative to a gender binary homosexual and heterosexual conceptualization. Androphilia describes sexual attraction to men and/or masculinity; gynephilia describes the sexual attraction to women and/or femininity. Ambiphilia describes the combination of both androphilia and gynephilia in a given individual, or bisexuality.
INAH-3 is the short form for the third interstitial nucleus of the anterior hypothalamus, and is the sexually dimorphic nucleus of humans. INAH-3 is significantly larger in males than in females regardless of age. One study has shown that INAH-3 is larger in heterosexual males than in homosexual males and heterosexual females, although this data has not been successfully reproduced.
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.
A relationship between handedness and sexual orientation has been suggested by a number of researchers, who report that heterosexual individuals are somewhat more likely to be right-handed than are homosexual individuals.
Childhood gender nonconformity (CGN) is a phenomenon in which prepubescent children do not conform to expected gender-related sociological or psychological patterns, or identify with the opposite sex/gender. Typical behavior among those who exhibit the phenomenon includes but is not limited to a propensity to cross-dress, refusal to take part in activities conventionally thought suitable for the gender and the exclusive choice of play-mates of the opposite sex.
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).
Prenatal stress is exposure of an expectant mother to psychosocial or physical stress, which can be caused by daily life events or by environmental hardships. This psychosocial or physical stress that the expectant mother is experiencing has an effect on the fetus. According to the Developmental Origins of Health and Disease (DOHaD), a wide range of environmental factors a woman may experience during the perinatal period can contribute to biological impacts and changes in the fetus that then causes health risks later in the child's life.
The relationship between the environment and sexual orientation is a subject of research. In the study of sexual orientation, some researchers distinguish environmental influences from hormonal influences, while other researchers include biological influences such as prenatal hormones as part of environmental influences.
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.
H-Y antigen is a male tissue specific antigen. Originally thought to trigger the formation of testes it is now known that it does not trigger the formation of testes but may be activated by the formation of testes.
Epigenetic theories of homosexuality concern the studies of changes in gene expression or cellular phenotype caused by mechanisms other than changes in the underlying DNA sequence, and their role in the development of homosexuality. Epigenetics examines the set of chemical reactions that switch parts of the genome on and off at strategic times and locations in the organism's life cycle. However, epigenetic theories tangle a multiplicity of initiating causes and of resulting final effects and will never lead to a single cause or a single result. Hence, any interpretation of such theories may not focus just one isolated reason of a multiplicity of causes or of effects.
The following outline offers an overview and guide to LGBT topics.
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.
Beginning in the 2010s, various media personalities promoted conspiracy theories claiming that exposure to endocrine disrupting chemical pollutants in the water supply are responsible for an alleged increase in the gay or transgender population. These claims are not supported by scientific evidence, and appear to be a conflation with research suggesting endocrine disruptors can have a feminizing effect on the genital development of non-human animals.
