One of the central difficulties in examining and researching sexual orientation is that the underlying concepts of “sexual desire,” “sexual attraction,” and “sexual arousal” can be ambiguous, and it is even less clear what it means that a person identifies as having a sexual orientation grounded in some pattern of desires, attractions, or states of arousal.

“Few topics are as complex and controversial as human sexual orientation and gender identity,” says Lawrence S. Mayer and Paul R. McHugh. “These matters touch upon our most intimate thoughts and feelings, and help to define us as both individuals and social beings. Discussions of the ethical questions raised by sexual orientation and gender identity can become heated and personal, and the associated policy issues sometimes provoke intense controversies.”

In an attempt to shed light on these often heated discussion, Mayer and McHugh have produced a massive new 143-page report, published in the last edition of The New Atlantis, which “offers a careful summary and an up-to-date explanation of many of the most rigorous findings produced by the biological, psychological, and social sciences related to sexual orientation and gender identity.”

McHugh, the former chief of psychiatry at Johns Hopkins Hospital and arguably one of the leading psychiatrists in the world, and Mayer, a biostatistician and epidemiologist, reviewed more than 500 studies for their report.

While the report is worth reading in its entirety, it’s length may make is inaccessible to the general public. To encourage further examination of the document, I’ve compiled a list of highlights that will provide a broad and extensive—though nowhere near comprehensive—summary overview of their findings.

Sexual Orientation

Problem of Definition

• The concept of sexual orientation is highly ambiguous, and can refer to a set of behaviors, to feelings of attraction, or to a sense of identity.

• One of the central difficulties in examining and researching sexual orientation is that the underlying concepts of “sexual desire,” “sexual attraction,” and “sexual arousal” can be ambiguous, and it is even less clear what it means that a person identifies as having a sexual orientation grounded in some pattern of desires, attractions, or states of arousal.

• There are currently no agreed-upon definitions of “sexual orientation,” “homosexuality,” or “heterosexuality” for purposes of empirical research. Because of this, the authors note, “We will continue to employ ambiguous terms like ‘sexual orientation’ where they are used by the authors we discuss, but we will try to be attentive to the context of their use and the ambiguities attaching to them.”

• Longitudinal studies of adolescents suggest that sexual orientation may be quite fluid over the life course for some people and that those who report same-sex attraction no longer do so as adults.

Genetic and Innate Factors

• Research suggests that while genetic or innate factors (e.g., genes, hormones) may influence the emergence of same-sex attractions, these biological factors cannot provide a complete explanation, and environmental and experiential factors may also play an important role.

• There is some evidence from the twin studies that certain genetic profiles probably increase the likelihood the person later identifies as homosexual or engages in same-sex sexual behavior.

• The largest attempt to identify genetic variants associated with homosexuality, a study of over 23,000 individuals, found no linkages reaching genome-wide significance for same-sex sexual identity for males or females.

• The weight of evidence to date strongly suggests that the contribution of genetic factors is modest.

• Hormonal conditions that contribute to disorders of sex development may contribute to the development of non-heterosexual orientations in some individuals, but this does not demonstrate that such factors explain the development of sexual attractions, desires, and behaviors in the majority of cases.

• Studies of the brains of homosexuals and heterosexuals have found some differences, but have not demonstrated that these differences are inborn rather than the result of environmental factors that influenced both psychological and neurobiological traits.

• There is virtually no evidence that anyone, homosexual or heterosexual, is “born that way” if that means their sexual orientation was genetically determined.

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