You've heard the expression, "Boys will be boys." But what happens when a brawny boy wants to be one of the girls – fiercely competing with females in weightlifting, brutally tackling girls on the football field or even dealing powerful knockout punches to ladies in a mixed martial-arts cage?

It's now happening across America and around the world.

Biological males are joining women's teams, smashing records and dominating in sports such as weightlifting, softball, cycling, track, wrestling, football, volleyball, dodgeball, handball, cricket, golf, basketball and mixed martial arts.

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The movement for "equality" has apparently inspired transgender athletes to join teams of their preferred gender. And that often means biological males are competing against biological females on women's teams. But in the world of sports, critics argue, equality between the sexes simply doesn't exist. Physiologically speaking, there's a gender gap between men and women that cannot be erased.

For example, when it comes to running, men are generally faster than women. As the 2015 edition of Runner's World explained, "At every distance up to the marathon, the gap between men's and women's world record times is nine to 10 percent – and it’s a similar or even higher percentage among recreational runners."

What do YOU think? Should transgendered men be allowed to compete in women's sports? Sound off in the WND Poll right now!

So why do men have some athletic advantages over women?

Ohio University biological sciences lecturer Chris Schwirian told Runner's World: "Faster men's times for 100 to 800 meters are mostly due to men, on average, having greater muscle mass – and a larger portion of it is fast-twitch, which allows them to generate greater force, speed and anaerobically produced energy. At all distances beyond 800 meters, the main reason for the gap is men's higher aerobic capacity [VO 2 max], on average, which is due to their typically having less body fat, more hemoglobin and muscle mass, and larger hearts and lungs than women."

On average, men have longer and larger bones, which gives them mechanical advantages over women, since they have greater leverage, increased height and larger frames to support muscle. Their bones are also more dense, and they have tougher ligaments, making them less prone to sports injury.

Since these physical and physiological factors give most men a clear competitive edge in sports, is it fair or even safe for biological males – with larger muscle mass, hearts and lungs and greater strength, acceleration and speed – to compete against girls and women?

"If men can claim to be women and invade a sport that only women are allowed to compete in, then it’s a safe bet men will win," writes the Federalist's Brandon Morse. "All the accolades, rewards, and recognition will be taken from the women who rightfully deserve them and given to a man who essentially cheated by putting on makeup, injecting himself with hormones, and saying he’s a woman."

Why is all this happening? In his widely acclaimed new book, "The Snapping of the American Mind," award-winning journalist David Kupelian stunningly documents – in a chapter titled "Gender Madness" – precisely what the transgender phenomenon is really all about. Prepare to be shocked.

Allowing biological males to compete against women is not only unfair and dangerous to women, it's worse than that, according to David Kupelian, WND managing editor and author of "The Snapping of the American Mind."

"The insanity of this affects not just the transgendered individual and the women athletes thus victimized," he said. "It also contributes to a sort of mass delusion infecting our whole society in which, thanks to the influence of the powerful LGBT movement, everybody now has to either affirm the absurd and crazy – that a man who is essentially a female impersonator can fairly compete against women – or they have to suffer abuse and persecution as bigots just for speaking the truth."

Kupelian said these "grotesquely unfair matchups" between female athletes and powerful men who identify as women is just one more unfortunate aspect of a revolutionary movement sweeping the nation.

"This sexual anarchy movement – which, for example, recently announced dozens of brand-new genders that never existed in the history of humanity until a few years ago – is really part of a revolutionary political movement rooted in a mixture of personal trauma, utopian ideology and demonic forces that is unfortunately ever-expanding in today's America," he said.

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Despite all of these arguments, the National Collegiate Athletic Association and Olympics have instituted policies allowing transgender athletes to compete on teams that correspond with genders with which they identify, provided the athletes undergo a year of hormone replacement therapy.

To document the growing trend, WND has compiled the following list of headline-making transgender athletes crashing the world of women's sports.

Weightlifting: Biological male takes 1st place in women's contest

Transgender weightlifter Laurel Hubbard, who was born a man, won the Australian international women's competition March 19.

Hubbard, 39, lifted 591 pounds, nearly 20 pounds more than the woman who won the silver medal by lifting 572 pounds.

Weightlifter Deborah Acason, from the Australian Weightlifting Federation, told 1 News Now she's concerned about the fairness of Hubbard competing in the women's division.

"We all deserve to be on an even playing field," Acason said. "It's difficult when you believe that you're not. If it's not even, why are we doing the sport?"

The International Olympic Committee recognizes male and female as genders, and transgender women are classified as female.

Sportswriter Phil Gifford argued that Hubbard should be allowed to compete with women.

"[S]he is completely entitled, I believe, to compete, and anybody who says otherwise is either being, I think, very prejudiced, which is the main thing I would imagine, or just jealous of the fact that maybe this woman has come along and she's better than the female competitors."

Cycling: Biological male dominates women's competition

Transgender cyclist Jillian Bearden, a 36-year-old biological male and Colorado Springs native, won the women's division of the El Tour de Tucson in four hours and 26 minutes in November 2016.

Bearden founded the world's first transgender cycling group, Transnational Women's Cycling Team.

"It's absolutely huge," Bearden told the Daily Star of his El Tour de Tucson victory. "We're at a moment of time – especially now – where not only do we have to come out but we have to be positive. We have to come together in solidarity and move this country in a direction that is accepting of all."

Third-place finisher Suzanne Sonya told Cycling Tips: "I'll take her on any day, but that's just me. I'll take on men, too. I feel bad about saying it, but, no, I do not think it's fair play, and I question her integrity knowing that she's going into these events knowing that she is going to be stronger.

"I'm sure [Bearden] had a rough go at it. It's very difficult to be transgender. But [when it comes to racing], it's problematic to me that she [transitioned] only a couple years ago, and has lived 30 years as a man. Regardless of testosterone levels, she's got muscle memory and a lung capacity that I could never build up. She was a Cat 1 as a male. I could never match a pro man. How fair is that to her female competitors?"

Softball: Male transgender joins girls team

California allowed Pat Cordova-Goff, 17, born a male, to join the girls' softball team at Azusa High School in Azusa in 2014. Goff was already a cheerleader at the school.

State law requires that "a pupil be permitted to participate in sex-segregated school programs and activities, including athletic teams and competitions, and use facilities consistent with his or her gender identity, irrespective of the gender listed on the pupil's records."

"We feel really confident about her ability," Azusa Unified Superintendent Linda Kaminski told local newspaper San Gabriel Valley Tribune. "No. 1 as a district, we want to ensure access to everyone, but we’re also committed to placing students on the team on their merits. ... Based on her skills, Pat did make the team."

"Parents had questions and we answered them as best we could," Azusa High School principal Ramiro Rubalcaba told Fox News. "My experience is that the parents have been pleased.

"Some students and players may feel uncomfortable, but that's only because this is something new to them. But I believe they are all going to be accepting," he said. "And I think the team is going to bring home a championship. That's my prediction."

But Randy Thomasson, president of SaveCalifornia.com, told Fox the state's decision to allow biological males to compete with females is unfair.

"It's intolerable of this young man to not accept an equal standing of girls playing girls," Thomasson said. "It is categorically unfair to biological girls to have to compete with a sexually confused young man with stronger upper body strength, who makes the game board decidedly 'unequal.'"

Track and field: Transgender high-schooler places 3rd at state

High-school runner Nattaphon Wangyot, 18, born a male, participated in the Class 3A girls' sprints at the Alaska state meet in June 2016 and placed 3rd in the 200-meter dash (27.3 seconds) and 5th in the 100-meter dash (13.36 seconds). Wangyot also competed on girls' volleyball and basketball teams.

The Alaska Schools Activities Association has a policy that allows each school district to determine whether transgender athletes may compete in a sport as their chosen gender.

"We didn't want to necessarily create a situation where we were going to bring in a committee and those types of things just because it's just not practical here," ASAA Executive Director Billy Strickland told KTVA-TV 11.

Sixteen runners qualified for state, according to the report.

Female high-school runner Saskia Harrison, who barely missed the cut, told KTVA: "I'm glad that this person is comfortable with who they are and they're able to be happy in who they are, but I don't think it's competitively completely 100 percent fair."

Eagle River junior Peyton Young, also a runner, said: "I don't know what's politically correct to say, but in my opinion, your gender is what you're born with. It's the DNA. Genetically, a guy has more muscle mass than a girl, and if he's racing against a girl, he may have an advantage."

Alaska Family Action President Jim Minnery held a protest against rules that allow male athletes to compete against females.

"We are here today as a voice from the community to ensure that female athletes are not denied the playing opportunities and scholarships otherwise available to them and to make the playing field even again," he said. "Allowing students to play on teams of the opposite sex disproportionately impacts female students, who will lose spots on a track, soccer and volleyball teams to male students who identify as female."

Wrestling: Transgender wins high-school girls tournament

Mack Beggs, 17, was born a girl and reportedly began identifying as a boy at the age of 3. Though Beggs underwent testosterone treatments for more than a year and had the muscle mass of a teenage boy, Beggs competed and took first place in the University Interscholastic League state girls' championship on Feb. 25, 2016.

"If he has been taking hormones, or steroids, he should be wrestling boys," youth counselor Melissa Roush told CNN.

Dr. Brandon Mines, assistant professor at Emory University's Department of Orthopedics, agreed with Roush's statement.

"Testosterone and anabolic steroids are in the same family and have the effect of increasing muscle mass and strength gains," Mines told CNN.

The league allows wrestlers to compete only against their own gender, based on sex listed on birth certificates.

Beggs had a 56-0 season and 110-pound weight class title.

Football: Transgender sues women's team, plays on another

Transgender football player Christina Ginther, who was born a male and is six feet tall, has sued a semi-pro women's football team for discrimination and is now playing for another women's football team.

Ginther, 44, tried out for the Minnesota Vixen women's football team in October after transitioning from male to female. But he was told the league doesn't allow biological males to play against females due to safety concerns.

"I hung up the phone and just felt violated," Ginther said, according to Minnesota Public Radio. "I mean, just the sense of, 'I'm a freak. I'm defective. I am not worthy to be with this team."

So Ginther filed a discrimination lawsuit against the Minnesota Vixen and the Independent Women's Football League, or IWFL.

Minnesota Vixen attorney Greg Van Gompel explained the league policy: "It says, 'A player may not play in the IWFL, unless they are now, and always have been, legally and medically a female, as determined by their birth certificate and driver's license."

Ginther claims his testosterone levels have dropped to near zero after taking estrogen for two years.

Now Ginther plays for the Minnesota Machine, a team in another women's football league. He said he hopes his lawsuit will inspire other transgender athletes.

"A lot of times, we're afraid our voice won't be heard or maybe the consequences of speaking up will just make things worse for us," Ginther said. "But if anything, trans people should feel empowered. Yes, we do have a voice."

Volleyball: Transgender player eyes 2020 Olympics

Transgender volleyball player Tia Thompson, born a male, played with the men's division of USA Volleyball, or USAV, until Thompson was given permission in January to play in the women's division, NBC News reported.

"It took me three years to finally get approved with all the transitioning and hormone therapy and submitting all my paperwork to the Gender Committee," Thompson said.

Under USAV rules, transgender women must undergo hormone replacement therapy for at least a year and change their gender to female on their I.D. documentation.

Thompson is scheduled to compete in the Haili Volleyball Tournament on Hawaii's Big Island this month.

Thompson also has big plans for 2020, when he would like to compete in the Summer Olympics in Tokyo.

"By me coming out and opening the doors, it will get more accepted," Thompson said.

Mixed martial arts: Transgender fighter competes in women's division

Transgender MMA fighter Fallon Fox, born a man, gave his female opponent a concussion and broke her eye socket in 2015.

The woman, Tamikka Brents, suffered a serious eye injury from a damaged orbital bone and needed seven staples after she fought Fox.

The TKO came at 2:17 during the first round of their match.

Brents told WHOA-TV, "I've never felt so overpowered in my life."

"I've fought a lot of women and have never felt the strength that I felt in a fight as I did that night," she said. "I can't answer whether it's because she was born a man or not, because I'm not a doctor. I can only say I've never felt so overpowered in my life, and I am an abnormally strong female in my own right."

Brents said Fox's "grip was different."

"I could usually move around in the clinch against ... females but couldn't move at all in Fox's clinch."

Watch a spectator's video of the brutal Fallon Fox vs. Tamikka Brents fight. (Warning: The following video contains foul language that may offend some readers):

In 2013, UFC commentator Joe Rogan was outraged that Fallon fought as a female athlete.

"She calls herself a woman, but I tend to disagree," he said. "She used to be a man, but now ... she's a transgender, which is (the) official term that means you've gone through it, right? And she wants to be able to fight women in MMA? I say no f---ing way. I say if you had a d--k at one point in time, you also have all the bone structure that comes with having a d--k. You have bigger hands. You have bigger shoulder joints. You're a f---ing man. That's a man, OK?"

Rogan called Fallon a "huge" fighter with a "man's face."

"You want to be a woman and you want to take female hormones? You want to get a boob job? That's all fine. I support your right to live as a woman," he said.

But, Rogan argued, Fallon should fight only men.

"She has to fight guys," he said. "First of all, she's not really a she. She's a transgender, post-op person. The operation doesn't shave down your bone density. It doesn't change. You look at a man's hands, and you look at a woman's hands, and they're built different. They're just thicker. They're stronger. Your wrists are thicker. Your elbows are thicker. Your joints are thicker. Just the mechanical function of punching, a man can do it much harder than a woman can, period."

Basketball: Transgender, 50, joins women's college team

Gabrielle Ludwig, 50, who was born Robert, joined the women's basketball team at Mission College in Santa Clara, California, in 2012.

Ludwig is 6 feet, 6 inches tall and weighs 220 pounds.

On Nov. 30, 2012, a judge awarded Ludwig a new birth certificate designating him a "female."

Ludwig played on the women's team even though he was a male college basketball player in 1980 for Nassau Community College. According to Community College Athletic Association bylaws, a student may only play for two years. But the athletic association made an exception for Ludwig, so he could play as a woman.

The women on the team nicknamed Ludwig "Big Sexy" and "Princess."

Volleyball: Transgender makes women's team

Transgender volleyball player Chloe Psyche Anderson, 24, born a male, joined the U.C. Santa Cruz women's volleyball team in 2016. Anderson is one of the first transgender athletes to play volleyball at the NCAA Division III level.

"Once I decided to come out as transgender, it was the first time I really started to feel good about myself," Anderson told the U.C. Santa Cruz newspaper. "[Before that] I had never really experienced being happy. I really was just totally lost."

Anderson said hormone therapy has caused muscle weakness and coordination problems.

Does he have an advantage because he was born male? Anderson doesn't think so.

"I probably have the least testosterone of anyone on the team," Anderson said.

Anderson lives in a new housing community for transgender students at U.C. Santa Cruz.

Dodgeball: Transgender player competes against women

Transgender Savannah Burton, born a male, used to play on a male dodgeball team. But Burton, a Canadian, transitioned and now plays on a professional women's dodgeball team. In 2015, Burton competed in the world championship.

"I was still playing dodgeball when I started taking the testosterone blockers and estrogen," Burton told Shape.com. "I could definitely see that my throw wasn't as hard as it was. I couldn't play the same way. I couldn't compete at the same level that I had."

Shape noted that Burton, as a male, threw balls so hard that they would bounce off opponents' chests with a loud noise.

Now, he claims he throws more like a girl.

"Now, a lot of people are catching those balls," Burton said. "So it's kind of frustrating that way."

Burton has also competed in a female rowing competition.

Roller derby: Transgender skates on women's team

Skater Vanessa Sites, 31, is the first transgender to compete on Team USA in the World Cup of Roller Derby.

Though Sites has skated in roller derby for at least eight years, he didn't reveal his transgender identity to the media until recently.

"I want people to know me for my skating and not what's between my legs," Sites told the Wilkes-Barre, Pennsylvania, online newspaper the Times Leader.

Sites is a member of the Gotham Girls Roller Derby based in New York City.

"I'm so good at it, so I wanted to be the best," Sites told the paper.

Even though Sites was born a biological male, he doesn't believe he has an unfair advantage when he competes against women.

"I don't think I have an advantage," Sites said. "I get knocked down the same as everyone else."

Sites said other transgender players skate in roller derby, "but I'm one of the few that ... was gifted with skills and really love it. I just worked really hard. I want to be the best."

Cricket: Transgender player joins women's team

Transgender Catherine McGregor, born a male, is a cricket player on a Canberra, Australia, women's team. In late 2016, McGregor said he wanted to play in the Women's Big Bash League.

"I have been pleasantly surprised because there was always the fear that the haters out there would say I don’t deserve to play in the women’s competition because I was born male," McGregor, 60, said. "It hasn't happened though, and I have had all sorts of messages.

"And the women on my team didn't give a stuff. They have welcomed me with open arms, and I can see a real future there," McGregor said. "In fact there is already a trans woman on our team, so it's a complete non-event in terms of controversy for them."

McGregor said he no longer produces testosterone and has high levels of estrogen.

"I know there might be some out there who would say, 'That's just an old bloke in a dress,' but I would argue that any physical advantages I may have had as a born male are offset by my age and the changes to my body," McGregor said.

Handball: Transgender star joins women's team

Transgender Hannah Mouncey, 26, a biological male, was previously captain of a men's handball team in Canberra, Australia. He played 22 games for the men's team, including the 2013 World Championships and qualifications for the 2016 Olympic Games.

But then he began taking hormone treatments and transitioned into a woman in May 2016.

In August 2016, Mouncey announced he would be joining the Australian women's handball team.

"Sport is very physical in nature, and it can be perceived to be a very masculine, macho environment," Mouncey told the Daily Mail last year. "But in general sport is probably one of the more accepting places you could find."

Mouncey said he hopes his story can inspire others.

"The more people that come out as trans from different walks of life hopefully it will make people see … it's something probably more common than they think, and it's OK," he said.

But in October 2016, the Daily Telegraph reported that Mouncey had been sidelined over height fears.

Handball Australia ruled that he couldn't join the women's team in a tournament. The official reason was that he hadn't completed a full year of hormone treatment. But sources told the paper that Mouncey's six-foot-two-inch stature posed a big problem.

"She’s still a 6-foot, 2-inch female and is still 100kg [220 pounds] or so and larger than everyone else with a physical advantage and a certain level of muscle mass … we have to protect the other players," the source told the Telegraph.

"It would not be an issue six months down the line as she will have met IOC guidelines, but the fact Hannah was three weeks out of the 12-month IOC guideline, and no one had dealt with a transgender player in a contact sport before, meant no precedent was set. No one was prepared to break the rules."

Board members reportedly feared they would not be insured if Mouncey injured a female player.

"Being able to play as a woman for the first time after transitioning would have meant feeling normal again," Mouncey told the Telegraph. "In six months, I'll still be 6 foot 2 inches and big, and nothing will have changed. Anyone who thinks I have an unfair advantage is wrong. Being tall means I have extra weight to carry and can't run as fast as the girls."

Golf: Transgender pro-golfer plays in women's tournament

In 2004, pro-golfer Mianne Bagger, born a male, became the first transgender woman to play in a professional golf tournament.

"That was huge," Bagger told ABC News correspondent Jay Schadler of her chance to play in the 2004 Women's Australian Open. "It's like I get there and I'm on the course side of the ropes. I'm here. I'm playing. It was like, wow!"

Bagger claims his physical advantages from being born male are now gone, since he took female hormones.

"The physical changes are, you lose muscle mass and testosterone," Bagger said.

"You lose overall strength. … I certainly wouldn't be out there playing if I felt I had an unfair advantage."