John Michael Bailey, born in 1957, is an American psychologist and professor at Northwestern University. His academic career has largely focused on the biological underpinnings of sexual orientation and paraphilias. Bailey's research has explored the potential genetic and prenatal influences on male sexual orientation, including studies on twins and a notable examination of cases where individuals assigned female at birth were later attracted to women. He has also investigated sexual arousal patterns in relation to sexual orientation, with some of his findings on bisexuality in men generating criticism and debate. Bailey's work has frequently intersected with controversial topics, leading to significant public and academic discussion.

Michael David Bailey earned his Ph.D. in clinical psychology from the University of Texas at Austin in 1989, after completing his undergraduate studies in mathematics. He joined Northwestern University as a professor in the same year. His early research suggested a heritable component to sexual orientation, building on his dissertation work that investigated prenatal stress as a factor in homosexuality. Bailey has conducted significant research into the nature versus nurture debate regarding sexual orientation, notably examining cases of individuals surgically assigned female at birth who later identified as attracted to women, which he described as a "near perfect quasi experiment." His work has also delved into the physiological aspects of sexual arousal, comparing genital and self-reported responses in relation to sexual orientation.

Bailey's 2003 book, "The Man Who Would Be Queen: The Science of Gender Bending and Transsexualism," explored his theories on male sexual orientation, suggesting biological influences, and reviewed Ray Blanchard's typology of transsexualism, including the concept of autogynephilia. The book sparked considerable controversy and faced accusations of scientific misconduct from some individuals who believed they were included as research subjects without adequate consent. Northwestern University investigated these claims, concluding that while research protocols were to be enhanced, the allegations did not meet the federal definition of scientific misconduct. Bailey contended that the attacks were motivated by a desire to suppress his research and theories.

In recent years, Bailey has been involved in research and discussions surrounding rapid-onset gender dysphoria (ROGD), a controversial theory suggesting that social factors can influence adolescents to identify as transgender. A paper co-authored by Bailey on this topic was retracted by Springer in 2023 due to concerns about informed consent, following criticism from researchers and LGBTQ organizations. Bailey also participated as a witness in a 2024 legal challenge regarding gender-affirming care for minors in Missouri, where his research methodology was questioned. His academic work and public statements have continued to be subjects of debate within scientific and activist communities.