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Beyond XX and XY: The Extraordinary Complexity of Sex Determination

A host of factors figure into whether someone is female, male or somewhere in between

Crop of diagram explaining how the factors determining biological sex can vary in intersex individuals.
Credit:

Pitch Interactive and Amanda Montañez; Source: Research by Amanda Hobbs; Expert review by Amy Wisniewski University of Oklahoma Health Sciences Center


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Humans are socially conditioned to view sex and gender as binary attributes. From the moment we are born—or even before—we are definitively labeled “boy” or “girl.” Yet science points to a much more ambiguous reality. Determination of biological sex is staggeringly complex, involving not only anatomy but an intricate choreography of genetic and chemical factors that unfolds over time. Intersex individuals—those for whom sexual development follows an atypical trajectory—are characterized by a diverse range of conditions, such as 5-alpha reductase deficiency (highlighted in graphic below). A small cross section of these conditions and the pathways they follow is shown here. In an additional layer of complexity, the gender with which a person identifies does not always align with the sex they* are assigned at birth, and they may not be wholly male or female. The more we learn about sex and gender, the more these attributes appear to exist on a spectrum.

*The English language has long struggled with the lack of a widely recognized nongendered third-person singular pronoun. A singular form of “they” has grown in widespread acceptance, and many people who do not identify with a binary gender use it.

Diagram explains how the factors determining biological sex can vary throughout development in intersex individuals.

Credit: Pitch Interactive and Amanda Montañez; Source: Research by Amanda Hobbs; Expert review by Amy Wisniewski University of Oklahoma Health Sciences Center

Amanda Montañez has been a graphics editor at Scientific American since 2015. She produces and art directs information graphics for the Scientific American website and print magazine. Amanda has a bachelor's degree in studio art from Smith College and a master's in biomedical communications from the University of Toronto. Before starting in journalism, she worked as a freelance medical illustrator.

More by Amanda Montañez
Scientific American Magazine Vol 317 Issue 3This article was originally published with the title “Beyond XX and XY” in Scientific American Magazine Vol. 317 No. 3 (), p. 50
doi:10.1038/scientificamerican0917-50