John McEnroe says Serena Williams can’t compete with the top men in tennis. Experts say there are reasons the gap between women and men athletes remains.

How would Serena Williams rank if she played in the men’s circuit of tennis?

Some wonder why it matters at all.

Former tennis star John McEnroe courted controversy last month when he stated in an interview with NPR that Williams would “be like 700 in the world” if she competed against men.

He made the claim in response to a question from the interviewer, who asked why McEnroe described Williams as “the best female player” rather than “the best player in the world, period.”

McEnroe went on to compare the level of play between men and women in tennis and other sports.

“Maybe at some point a women’s tennis player can be better than anybody. I just haven’t seen it in any other sport, and I haven’t seen it in tennis. I suppose anything’s possible at some stage,” he said.

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Some researchers writing in the 1990s speculated that women might eventually close the gap in certain athletic competitions, such as running events.

Women have since continued to break records and push standards of performance.

But so have men.

As a result, many performance gaps remain.

“Sex differences between the world’s best athletes in most events have remained relatively stable at approximately 8 to 12 percent,” Øyvind Sandbakk, PhD, managing director of the Centre for Elite Sports Research at the Norwegian University of Science and Technology, told Healthline by email.

“The exceptions are events in which upper-body power is a major contributor, where this difference is more than 12 percent, and ultra-endurance swimming, where the gap is now less than 5 percent,” he added.

In most cases, the gap favors men. But in some events, women have had the edge.

“Although women generally do not outperform men in endurance events, open-water ultra-endurance swimming offers an exception,” Sandbakk said.

“In the 32-kilometer Catalina Channel Swim and 46-kilometer Manhattan Island Marathon Swim, the fastest women ever were faster than the best-performing men. Although these two races are currently the exceptions where women outperform men, the sex differences in open-water swimming are indeed smaller than in other sports.”

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To account for differences in athletic performance between men and women, experts point to biological and social factors.

“The physiological advantages in men include a larger body size with more skeletal muscle mass, a lower percentage of body fat, as well as greater maximal delivery of anaerobic and aerobic energy,” Sandbakk explained.

“While the exercise efficiency of men and women is usually similar, women have a better capacity to metabolize fat and demonstrate better hydrodynamics and more even pacing, which may be advantageous, in particular during long-lasting swimming competitions,” he added.

Sex-related hormones appear to influence those trends.

Some women with intersex conditions and other differences in sex development have unusually high levels of testosterone.

But on average, women tend to have lower levels of circulating testosterone than men.

This likely contributes to lower average muscle mass and cardiovascular capacity among women, which in turn affects their athletic performance.

Many women also face sociopolitical barriers that affect their opportunities to train and participate in elite-level sports.

“In many countries, there are cultural restrictions, religious restrictions, and organizational restrictions,” Laura Capranica, MSc, a professor of sport sciences at the Foro Italico University of Rome, told Healthline.

“So you will not have the same athletic base from which you can have the best, the most talented women proceeding through sports careers,” she said.

While the situation has started to improve in many countries, girls and women continue to receive fewer opportunities and less economic and social support in the world of sports.

Disparities in opportunity and participation can be seen at all levels, right up to the underrepresentation of women leaders in national sports organizations.

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Given the average differences in physiology and opportunities, some experts question the usefulness of comparing the sports performance of men and women.

On the one hand, it’s possible to acknowledge differences in biology without devaluing the efforts and achievements of women athletes.

“Part of the reason that we divide a lot of sports by gender is because the ability level is, just on average, fundamentally different between men and women. It’s not that different in some ways than when we segregate wrestling by weight class,” Alice Dreger, a sex researcher and historian of medicine and science, told Healthline.

“I don’t know why that has to be seen as an antifeminist claim,” she said. “There are lots of ways to support women in sports, while not denying the reality of hormones mattering, and height mattering, and oxygen processing mattering, and all that kind of stuff.”

On the other hand, overly simplistic comparisons may contribute to inequitable attitudes about women and distract from more useful conversations.

“Why are people interested in comparing women and men? It often has a sort of discriminatory philosophy underneath,” Capranica argued.

“I would stop doing that kind of comparison, which is quite useless. Instead, I would focus on social opportunities for sport,” she continued. “I want women to have our own opportunities to play, to compete, and to use sport as a springboard, as a social and personal tool for empowerment. How can we bridge the gaps in opportunity? In my opinion, that should be the question we focus on.”