Even people who otherwise support transgender equality often draw the line at transgender women competing against cisgender women. We have a couple of new opinions expressed lately. What makes these more unusual than most is that they are grounded in science.

Steven Petrow investigates at Washington Post.

This charged debate is timely because of a landmark rule change instituted by the International Olympic Committee (IOC) this year. In Rio, transgender men (female-to-male athletes) will be allowed to compete without any restrictions (based on the sexist assumption, I suppose, that trans men could never dominate their sports). Trans women, meanwhile, are no longer required to undergo gender-reassignment surgery to compete in female divisions, and the previously mandated two-year wait after transitioning has been jettisoned.

Meanwhile IOC scientist Yannis Pitsalidis discusses the science at New Scientist.

The new guidance requires testosterone levels in trans women’s blood to be below a threshold for a year before competition. This would usually require hormone treatment or even surgery. What impact does this treatment have?

There is some research that helps. A 2004 study measured levels of testosterone and haemoglobin – related to the oxygen-carrying capacity of blood – in trans women within a year of surgery and reported levels of both in the female range, along with a reduction in muscle mass.

The only study to directly assess performance was published last year. It looked at race times in eight non-elite trans women runners who had competed in both men’s and women’s distance races. It found they had no advantage over other women.

--Pitsalidis

One competitive cisgender female runner, who did not want her name used, explained how “incredibly unfair” this is to her, attributing the rule change to the IOC’s “trying to be politically correct.” Another cisgender female athlete, former Olympic judo competitor Ronda Rousey, went further (and got graphic) when she complained to the media about her competitor Fallon Fox, a trans woman, claiming: “She can try hormones, chop her pecker off, but it’s still the same bone structure a man has. It’s an advantage. I don’t think it’s fair.”

But here’s why we all had it wrong: The first-ever study of transgender athletes showed that the hormone therapy that facilitates male-to-female transition does more than just suppress testosterone. Published last year in the Journal of Sporting Cultures and Identities, the study showed that as testosterone levels approach female norms, trans women experience a decrease in muscle mass, bone density and other physical characteristics.

--Petrow

Together these changes lead to a loss of speed, strength and endurance — all key components of athleticism.

It’s not the anatomy that matters, it’s the hormones.

--Joanna Harper, author of the study

Every athlete, whether cisgender or transgender, has advantages and disadvantages.

--Cyd Ziegler, Outsports

People come in all shapes and sizes. We don’t disqualify Michael Phelps for having super long arms; that’s just a competitive advantage he has in his sport. We don’t regulate height in the WNBA or NBA; being tall is just an advantage for a center. For as long as sports have been around, there have been people who have had advantages over others. A universal level playing field does not exist.

--Chris Mosier, dualthlete

Still, the controversy and trans-shaming continue. Although there are well over 40 openly gay and lesbian athletes competing in the Summer Games, neither of the two reported trans athletes have come out publicly. Until anti-trans stigma is fully erased, they are eligible to compete, but, Mosier said, that doesn’t mean “everybody is ready to accept us.” Now I ask you: Is that fair?