The inclusion of two biologically male fighters into the Olympic female boxing category has sent shockwaves through the world of sports and sparked many questions regarding the athletes’ rare genetic condition, which has caused significant confusion about their sex status.

Algerian boxer Imane Khelif, 25, and Taiwanese boxer Lin Yu-ting, 28, both have a reported diagnosis of differences in sexual development, or DSD, which is an umbrella term for a variety of conditions in which a person can have either XY or XX chromosomes but not the corresponding physical sex characteristics.

Both Khelif and Lin were disqualified from the female International Boxing Association championship in 2023 following genetic testing confirming that they each had XY chromosomes, but they each were allowed to fight as women under the International Olympic Committee’s guidelines of testing testosterone levels.

Khelif beat Italian female boxer Angela Carini on Thursday after only 46 seconds in the ring when Khelif landed a significant blow to Carini’s face. Lin also won 5-0 against Uzbek fighter Sitora Turdibekova on Friday in a three-round match.

The fight with Carini provoked tremendous controversy online, with many members of the Save Women’s Sports movement in the United States saying it was emblematic of the debate on whether to include male-to-female transgender athletes in women’s sports. But the genetic conditions of Khelif and Lin make them different from transgender athletes.

The Olympic controversy has sparked many questions about the medical diagnosis of the two fighters and whether or not they should be deemed as “female” for the purpose of sports competitions.

Differences in sexual development consists of a group of rare genetic conditions that result in discrepancies in hormones and reproductive organs.

In typical biological development, individuals with XX sex chromosomes have female sex characteristics, and individuals with XY sex chromosomes have male sex characteristics. 

For individuals with a DSD diagnosis, their phenotypic, or physical, sex characteristics do not necessarily correspond to their sex chromosomes. This can result in a multitude of variations for individuals diagnosed with a DSD.

Both Khelif and Lin have reported having a DSD diagnosis, but the specific diagnoses are unknown for medical privacy reasons, making it difficult to speculate about their physiological advantages compared to biological women.

The number of DSD diagnoses worldwide is estimated to be approximately 0.018% of the world population, but this figure is controversial.

If other sex chromosome disorders, such as XXY and single X chromosomes, are included in the statistic, then an estimated 1.7% of the world population qualifies under the umbrella term DSD. 

The primary difference between DSD and identifying as transgender boils down to genetics. 

Whereas a DSD diagnosis requires medical testing, such as DNA screening, blood testing, and ultrasounds, identifying as transgender involves a psychological distinction between biological sex and gender identity. 

Even undergoing gender transition medical treatment–including hormone replacement therapy and surgeries to change physical sex characteristics–will not result in changes to sex chromosomes on the cellular level.

Oftentimes, individuals with a DSD diagnosis are raised and socialized based on their physical sex characteristics identified at birth, namely their genitalia. Both Khelif and Lin were raised as girls from infancy.

The main objection to biological males who identify as transwomen competing in women’s sports is that biological men have a competitive advantage, including having higher levels of testosterone than biological women.

Even trans-identified athletes who have gone through normal male puberty but are subsequently on hormone replacement therapy or testosterone suppressors have a competitive advantage over biological women due to differences in bone density and muscle mass.

But, although both Khelif and Lin are biological males based on their XY sex chromosomes, it is difficult to know whether or to what extent they have the same advantages in competition as male-to-female transgender athletes. That is because the details about their testosterone levels and their physiological development have not been made public.

Both have always competed as women and have strong records, but they are not undefeated.

Khelif has a record of 37-9 and has not lost a match since 2022. Lin has a record of 40-14 and is the No. 1 fighter in the female category in Taiwan.

The term “intersex” falls under the LGBT umbrella of terms, but it was used in a medical sense prior to 2006 when DSD became the standard technical language for genetic conditions. Since then, the word intersex has taken on more of a social connotation. 

Individuals who identify as intersex are more likely to interpret biological sex as a spectrum and to blur the line between biological sex and gender identity.

According to limited survey data in the United States and Australia, individuals with a DSD diagnosis largely prefer the term “intersex” rather than “DSD” to describe themselves. 

In 2023, the IBA disqualified both Lin and Khelif on the basis of genetic testing, during which “both athletes did not meet the required necessary eligibility criteria and were found to have competitive advantages over other female competitors.”

“Based on DNA tests, we identified a number of athletes who tried to trick their colleagues into posing as women,” said IBA President Umar Kremlev in a statement at the time of the original disqualification. “According to the results of the tests, it was proved that they have XY chromosomes. Such athletes were excluded from competition.”

But the IOC has maintained its stance on allowing Khelif to compete, saying that both Khelif and Lin were “victims of a sudden and arbitrary decision by the IBA.”

“The current aggression against these two athletes is based entirely on this arbitrary decision, which was taken without any proper procedure — especially considering that these athletes had been competing in top-level competition for many years,” the IOC said in a statement following the fight between Khelif and Carini.

The IOC has not made public the testosterone tests for either Khellif or Lin.

The controversy has also highlighted the yearslong intense battle between the IBA and IOC.