Sex verification in sports (also known as gender verification, or loosely as gender determination or a sex test) occurs because eligibility of athletes to compete is restricted whenever sporting events are limited to a single sex, which is generally the case, as well as when events are limited to mixed-sex teams of defined composition (e.g., most pairs events). Practice has varied tremendously over time, across borders and by competitive level. Issues have arisen multiple times in the Olympic games and other high-profile sporting competitions, for example allegations that certain male athletes attempted to compete as women or that certain female athletes had intersex conditions perceived to give unfair advantage. Sex verification is not typically conducted on athletes competing in the male category. The first mandatory sex test issued by the International Association of Athletics Federations (IAAF), the world's track and field governing body, for woman athletes was in July 1950 in the month before the European Championships in Belgium. All athletes were tested in their own countries. Sex testing at the actual games began with the 1966 European Athletics Championships’ response to suspicion that several of the best women athletes from the Soviet Union and Eastern Europe were actually men. At the Olympics, testing was introduced in 1968. In some cases, these policies have led to athletes undergoing unnecessary surgery such as female genital mutilation and sterilization. Subsequent reports have shown that the tests could cause psychological harm. Sex verification—identifying athletes whose hormone levels are abnormal compared to others of their purported sex — can cause sex identity crises, elicit demeaning reactions (publicly and privately), isolate athletes socially, and lead to depression and sometimes suicide. History Sex verification in sports began in the 1940s with "femininity certificates" provided by a physician. It subsequently evolved into visual inspections, physical examinations, chromosome testing, and later testosterone level testing. These tests were all designed to ensure that athletes were only allowed to compete as their sex, but mostly resulted in the exclusion of intersex athletes. It is not always simply checking whether a person's sex chromosome pair is XX vs. XY, or comparing their levels of key sex hormones to distinct reference ranges, to determine an athlete's sex. Variations in pairings of inherited chromosome, other genetic aspects, and pre- and postnatal physical development at subcellular to organ levels mean some people are not unambiguously female or male. Fetuses start out as undifferentiated, then