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How sexually dimorphic are we? Review and synthesis
Melanie Blackless1, Anthony Charuvastra, Amanda Derryck
1Department of Molecular and Cell Biology and Biochemistry, Brown University, Providence, Rhode Island.
Summary
Human biological sex is not strictly dimorphic. Studies indicate that variations from the typical male or female development occur in up to 2% of live births, challenging the binary sex model.
Area of Science:
- Human Biology
- Genetics
- Developmental Biology
Background:
- The traditional view of human biological sex assumes absolute dimorphism.
- This binary model is based on an idealized concept of male and female development pathways.
- Deviations from these idealized pathways are often overlooked or miscategorized.
Purpose of the Study:
- To investigate the frequency of variations in biological sex development.
- To challenge the notion of absolute sexual dimorphism in humans.
- To quantify the prevalence of intersex traits in live births.
Main Methods:
- Systematic review of medical literature from 1955 to the present.
- Analysis of studies focusing on deviations from typical male or female development.
- Examination of data on sex chromosome composition, gonadal structure, hormone levels, and genital development.
Main Results:
- The frequency of deviations from the ideal male or female developmental pathway may be as high as 2% of live births.
- The incidence of individuals undergoing "corrective" genital surgery is estimated between 0.1% and 0.2% of live births.
- This suggests a significant prevalence of variations in sex development.
Conclusions:
- Human biological sex is not strictly dimorphic.
- Variations in sex development are more common than previously assumed.
- The concept of a single, universally correct developmental pathway for each sex is not supported by empirical data.