As my fellow HowStuffWorks blogger Marshall Brain pointed out in an earlier post, the International Amateur Athletics Federation (IAAF) has called into question whether South African runner Caster Semenya is indeed a female. Shortly after the 18-year-old won the women’s world 800 meters, the IAAF announced that Semenya will have to undergo gender testing.

In 2006, another female athlete competing at the Asian Games also underwent gender testing and failed — after passing the same exams a year earlier. Santhi Soundarajan was stripped of her silver medal in the women’s 800 meter after her gender test results showed more Y chromosomes than usually present in female samples. But as Molly and I discussed in the Stuff Mom Never Told You episode, “Why is it so easy to fail a gender test“, no absolute standard exists for determining whether someone’s anatomy, hormones and chromosomes constitute a male or a female.

To test for biological gender, doctors usually start out with a physical exam of secondary sex characteristics, such as body hair and genitalia. Next up, they evaluate the hormone levels in blood samples. Things start getting tricky at this point since there’s no set amount of testosterone or estrogen that males and females must have.

If the testing remains inconclusive, doctors move on to the chromosomes. Even on the chromosomal level, things aren’t as simple as XX and XY. Let’s say someone has a Y chromosome present, which would indicate maleness. Yet a faulty SRY gene on that Y chromosome would stop it from expressing; consequently, the person could end up with female traits. In recent years, doctors have also discovered other genes, such as WNT4 in females, which act similarly to SRY in triggering gender characteristics.

In fact, according to the Intersex Society of North America, 1 in 1,666 babies don’t possess XX or XY chromosome pairings. Moreover, Scientific American reports that one in about 4,500 babies are born with ambiguous genitalia.

Gerald N. Callahan’s 2009 book “Between XX and XY” explores these concepts of gender and intersexuality (the preferred term to describe people with indeterminate genders). In an interview with Salon, he commented:

“…The process [of sex development] is controlled by a series of enzymes and the reaction may be more or less complete. It’s not just two poles where that whole process can end up. In between what we call the ideal biological male or ideal biological female, there’s a whole range of other possibilities that don’t differ from our basic preconceptions to the extent that we have names for them or call them a disorder.”

Because of the inherent flaws of gender testing, the International Olympic Committee banned the practice in 1999. And with research on sex development continually revealing the fluid nature of biological gender, it may be time for the IAAF to call it quits, too.

More from HowStuffWorks:
Why would a female athlete fail a gender test?
How Gender Reassignment Works
How Gender Identity Disorder Works