Last week, University of Pennsylvania student Lia Thomas won the swimming women’s division National Collegiate Athletic Association (NCAA) Division One national championship for 500-yard freestyle. Two years back, Thomas, who had gone by the first name of Will, was finishing up competing with the university’s men’s division swimming team. What changed in between? Thomas’s opinion about wanting to compete as a man or a woman changed. Thomas, also took off a season of competition and received hormone suppression medical intervention. While Thomas’s opinion has been backed up by the people enforcing the rules of swimming competition, many swimming competitors and fans look at the situation differently. To them, Thomas is an unwelcome interloper. Thomas competing in the women’s division, many of them think, strikes at the heart of the structure upon which athletic competition relies for validity and vitality. How is it fair that a competitor’s own opinion about the nature of the competitor must be accepted even if it appears to contradict reality? A boxer is not allowed to compete in a lower weight division because he believes he is lighter. A twenty-year-old baseball player cannot compete in Little League because he thinks he is younger. The maximum weight and age limits are based on facts, not each athlete’s opinion. The boxer is weighed. The prospective Little League participant’s age is verified. Maybe the boxer can lose enough weight to compete in a lower weight class. But, the twenty-year-old baseball player cannot shed some of his years. Thomas and other athletes making similar moves from men’s to women’s divisions in sporting competitions seem more like the too old baseball player than the too heavy boxer. Male attributes do not just fall away like pounds. And even if major medical efforts are pursued, some of the results of having been born and grown as a male remain, including strength effects. Throw out longstanding convention in favor of competitors’ opinions and you are throwing out much of what makes athletic competition work well. A defining aspect of athletic competition is rules creating an objective basis for measuring accomplishment and advancing fair competition. Thomas’s participation in the women’s division may not alone create a major long-term detriment for women’s division collegiate swimming. But, what if there is a flood of other athletes seeking to compete on the same basis? And plenty of people have worried that similar bending of longstanding convention as to who competes in women’s divisions would devastate women’s divisions of many sports. This worry seems founded in any athletic competition, such as swimming, where males tend to have a significant edge over females. Thomas and other individuals seeking to move from men’s divisions to women’s divisions have their opinions backing their efforts. But, what of the contrary opinions of the many individuals who compete in, and are the fans and financial backers of, women’s division sporting competition? When many of these individuals walk away, how will women’s divisions thrive? The likely answer is that the women’s divisions will not.