World Rugby has decided to follow the science when it comes to transgender players.

Transgender men – females who identify as male – will not be permitted to play against women once they have begun taking testosterone because of the advantages it provides.

Women who appropriate manhood will have to prove their physical ability and also provide written documentation that they understand the risks to themselves if they play against men.

Either way it will be difficult for females who identify as male to play Rugby at all.

Transgender women – males who identify as female – will not be permitted to play against women, even if they drop their testosterone levels. They can only play against men or in open or mixed touch competitions.

Males who have been through puberty have distinct advantages over females.

The World Rugby policy clearly describes the science that supports their decision.

The evidence cited points to greater speed, strength and stamina for males, putting females at risk in this full impact sport.

Advantages males enjoy due to their biological reality include:

• Significant increases in total body mass
• Significant increases in lean/muscle mass and muscle density
• Reduction in body fat mass, improving strength and power-to-weight ratio
• Increased height, changed  dimensions of important  levers,  greater  bone density
• Increased haemoglobin levels
• Increased heart and lung size
• Significantly greater strength (between 50% and 60% percent by adulthood, with relatively greater upper body strength)
• Significant speed advantages (between 10% and 15% over various durations)
• Greater capacity to produce force/power (advantages   of between 30% and 40% in explosive movement capabilities)
• Strength-to-weight and power-to­ weight advantages (even after adjusting for mass, height and similar level of performance (elite, untrained etc), males have a 30-40% strength advantage)

World Rugby concluded males cannot compete against females because the “risk of injury is too great”. They particularly described the risk to women in a tackle.

Forces and inertia faced by a smaller and slower player during frequent collisions are significantly greater when in contact with a much larger, faster player. Research has found that the discrepancy in mass and speed is a significant determinant of various head injury risk factors, including neck forces, neck moments and linear and angular acceleration of the head. When two opponents in a tackle are significantly different with respect to mass or speed, these risk factors increase significantly. All these factors are 20% and 30% greater when typical male mass is modelled against typical female body mass in the tackle.

The discrepancy in scrum forces was also noted as “twice as high for elite men vs elite women, and 40% higher for community level men compared to elite women”.

Binary spokeswoman, Kirralie Smith, said it was a sensible and fair decision.

“This policy is one of the few that acknowledges the advantages males have by virtue of puberty,” she said.

“It is not simply limited to testosterone, males have the upper hand due to their size, blood volume, lung capacity, muscle mass and many other factors.

“This policy will keep women safe and ensure the sporting field is fair.”