There were no women athletes in the first modern Olympic games. The next time around, in the 1900 Paris games, out of 997 athletes there were 22 women, who competed in just five acceptably ladylike sports: tennis, sailing, croquet, equestrianism and golf. Over a century later, the introduction of women’s boxing meant that the 2012 Olympics were the first to feature women competing in all sports. But that moment of parity has been followed almost immediately by a drastic challenge to the very definition of women’s sport, as the International Olympic Committee brought out new rules last November on the inclusion of trans athletes.

The change of rules has been followed by news that two unnamed British athletes who were born male are now in contention to compete as women in the 2016 Games in Rio. However, although 2016 could be the first Olympics with transgender athletes, it’s not the first time they’ve been permitted: the 2003 Stockholm Consensus on Sex Reassignment in Sport confirmed that trans athletes could participate, provided they had undergone sex reassignment surgery, had been receiving hormone therapy for a minimum of two years, and had legal recognition of their new gender.

The 2015 Consensus scotches one of those stipulations, and limits the other two: now, trans competitors no longer need to have genital surgery, and female-to-male transitioners can compete as men without restriction. For male-to-female transitioners, the legal status requirement has been replaced with a declaration of gender that is binding for four years, and the hormone treatment rule is now that the ‘athlete must demonstrate that her total testosterone level in serum has been below 10 nmol/L [nanomoles/litre] for at least 12 months prior to her first competition’.

There’s an obvious reason for this disparity in regulations between transmen and transwomen. Joanna Harper is a medical physicist and a transwoman herself, and she participated in the 2015 Consensus. As she points out, the inclusion of trans athletes is likely to have a greater impact on women’s sports than on men’s: ‘Realistically, if we take away all the societal limitations on transgender people, transgender women would be overall advantaged with respect to cisgender [i.e. non-transgender] women. Whereas I would think overall transmen would be disadvantaged with respect to cisgender men,’ she says. ‘If we're looking ahead 30-40 years from now, I think we will see more transwomen in sport than transmen.’ That means that it will be overwhelmingly female athletes rather than male ones who lose team places and podium positions to trans rivals.

Harper has conducted the only peer-reviewed research into the effects of gender transition on sporting performance. Her study of eight distance runners who transitioned from male to female concluded that, when their performances were benchmarked for age, the runners placed comparably before and after transition, and experienced no advantage from running as women. There are some caveats to this, however. Firstly, eight subjects is a very small sample – especially when one of them was excluded as an outlier after an intensified training schedule post-transition led to drastic improvement in results, while another subject whose training plummeted was left in the analysis. Secondly, even if trans athletes don’t have an edge in women’s distance running, they still might in other sports.

Transwomen have higher muscle mass than women in general, which can be an advantage in sports such as sprinting that rely on explosive power. Height, hand-size, foot-size and skeletal proportions are obviously unaffected by hormone treatment, so transwomen could have the opportunity to dominate in sports where grip, elevation or reach are vital. Only when smallness is a prerequisite for success can female athletes expect to hold out against male-to-female challengers: ‘I doubt,’ says Harper, ‘that there will ever be an elite transwoman gymnast.’ But just how extensive these residual differences are is unknown, because research into the effects of transition on athleticism consists of a tiny handful studies to date.

The problem with measuring performance in sport, of course, is that athletic achievement is inherently a matter of exceptionalism. For transwomen athletes, the quandary is that they are required to be unexceptional in order for their participation to be seen as fair; but by their very nature, all athletes are outliers within their peer group, and that ability will often derive from an arguably unfair physical bequest. For example, left-handed fencers dominate their sport and are consequently overrepresented – and, Harper argues, we accept that, so by implication we should also accept any advantages an athlete accrues by being born male and transitioning to complete in women’s events.

But the analogy is an imperfect one. For one thing, whatever the benefits of handedness to sporting performance, athletes have never been segregated by it – whereas separate events for male and female athletes has long been considered necessary on account of their differing ranges of physical ability. Secondly, handedness is not a controlled substance under IOC regulations. Testosterone, however, is, and athletes are routinely disqualified when found to have unacceptable levels of the hormone in their blood. Indeed, so great is the concern about testosterone advantage, the 2015 Consensus contains a recommendation that women with hyperandrogenism (elevated testosterone levels, which can be related to an intersex condition or to polycystic ovarian syndrome) should be allowed to compete in men’s events if excluded from women’s.

That gives trans athletes a peculiar exemption when it comes to an otherwise highly controlled substance. Transmen are permitted to compete while receiving testosterone injections that would qualify as doping in their male rivals, while transwomen athletes have undergone what is in effect a regimen of testosterone treatment throughout their adolescence. It also implies a level of perhaps unmerited trust that unscrupulous coaches will not exploit this in way that mirrors the Soviet doping scandals (for example, directing a mediocre male athlete to reassign as female in order to excel). Is it possible to reconcile allowances for trans athletes with the principles of fair competition?

Transwoman tennis player Renée Richards played on the women’s circuit in the 1970s. 30 years later, she decided that the tensions were irreconcilable: ‘I know if I’d had surgery at the age of 22, and then at 24 went on the tour, no genetic woman in the world would have been able to come close to me… There is one thing that a transsexual woman unfortunately cannot expect to be allowed to do, and that is to play professional sports in her chosen field.’ For Harper, women’s sports exist ‘to give women an opportunity to shine’. If transwomen begin to outshine their female rivals, a fraught era awaits.

Sarah Ditum is a journalist who writes regularly for the Guardian, New Statesman