No children should be playing contact sport before the age of 12, say some of the world's leading experts on head injuries in sport.

The group of Boston scientists told the Herald that the younger you started sports like rugby and American football, the greater your chances of suffering degenerative brain diseases.

Dr Bob Cantu, the "Godfather of Concussion", has also discouraged football clubs from allowing heading the ball in under-12s.

In a series of exclusive interviews, the scientists at Boston University's Alzheimer's Disease Center team showed experiments which found mice subjected to the same forces as tackle football suffered repeated violent wobbles to the head, which were imperceptible to the naked eye.

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Dr Lee Goldstein said these repeated ordinary hits - not the more high-profile but rarer concussions which grabbed the headlines - were the reason for the startling finding of his colleague Dr Ann McKee in 2017 that 110 out of 111 brains of former US National Football League players showed signs of the incurable brain disease chronic traumatic encephalopathy (CTE).

McKee said even allowing for the bias of self selection by families who donated the footballers' brains, the rate of serious brain disease among NFL players and college players was far above the general population.

"You can split hairs, you can use the studies to say we're blowing this out of proportion but the average 7-year-old knows this is real."

Some scientists believe that impacts suffered as children can have an effect later in life.

Cantu said tackling or heading might be too much for the brain development of children under 12.

He said a key factor in developing CTE was not the severity of the trauma but "the total amount of hits you took and when you started taking them".

"If you started taking them before the age of 12 that increases your risk," he said. "What we can best correlate the risk of CTE with, is total number of hits to the head you took over the period of life you were taking hits to the head."

Cantu said impacts received when young came at a greater price than those as an adult, regardless of force, and is adamant there should not be any tackle codes, including rugby, until high school.

"Your neck is weak and your brain isn't myelinated and it's easier to disrupt brain fibres," he said.

His views are echoed by McKee, the neuropathologist who made last year's Time Most Influential People list.

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McKee believes the professionalisation of sport has encouraged parents to allow or even encourage their children into dangerous practices.

"We're experiencing a vortex where all these things are making the disease [CTE] worse.

"We're seeing if you start playing tackle football before 12 as opposed to after 12, and you're an NFL player, you'll have more structural damages in your nervous system – regardless or not as to whether you have CTE... You'll also have greater cognitive difficulties and executive dysfunction later in life."

McKee said there was ample evidence that those that started American football earlier were less resilient to the effects of CTE and she expects the same in rugby, highlighting that one of the "most damaged" brains she had worked with was former Australian player Barry "Tizza" Taylor.

Dr Ann McKee, professor of neurology and pathology. Photo / file
Dr Ann McKee, professor of neurology and pathology. Photo / file

While rugby and American football have important technical distinctions, and most agree that the latter is more dangerous due to the number of impacts on every single play, the fact that collisions are an integral part of the sport connects them.

New Zealand Rugby head of medical Ian Murphy said there was still much to learn about the risk-reward of contact youth sport and said his organisation would continue to offer tackle rugby to children, with a continuing onus on technique.

Murphy said New Zealand Rugby has made huge strides in making its stakeholders increasingly aware of the risks involved with concussion and that has led to, anecdotally at least, a culture shift in the game.

He said that ACC data suggests that there are very few recorded rugby injuries in the 7-12 age bracket. If injuries at a young age are a harbinger for worse to follow, Murphy said, then some comfort should be taken in the low levels.

"You're seeing an emerging discussion around non-contact forms of contact sports at younger ages. That's emerging slowly. It's arguably not a bad thing but I think there'll always be a group that likes to play the contact form of games," Murphy said.

"There are some people who think, 'She'll be right, no worries', and there's a group that believes in [no contact sport] under 12. Our current view, and my current view, is somewhere in there lies the right thing to do.

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"New Zealand Rugby's position is contact is going to take place in the game through the grades if you wish to play that form of the game."

Cantu added that while the numbers of CTE afflictions in NFL players were eye-popping, he didn't believe rugby would hit comparable levels.

"I don't think statistically it will be where boxing or [American] football is, or has been. But unquestionably it's going to be in a high-risk group."

New Zealand under-17 assistant coach Sam Wilkinson, who runs the Melville football academy in Hamilton said no parents had expressed "huge concerns" about their kids heading the ball and that modern-day parents were never shy about expressing opinions.

"In youth football you don't see massive amounts of heading and in training you use only age-specific football sizes and we're mindful of the quantities," Wilkinson said.

* This series was made possible through an nib health scholarship.