TORONTO | Tue Feb 7, 2012 12:32pm EST

(Reuters) - Spanking children can cause long-term developmental damage and may even lower a child's IQ, according to a new Canadian analysis that seeks to shift the ethical debate over corporal punishment into the medical sphere.

The study, published this week in the Canadian Medical Association Journal, reached its conclusion after examining 20 years of published research on the issue. The authors say the medical finding have been largely overlooked and overshadowed by concerns that parents should have the right to determine how their children are disciplined.

While spanking is certainly not as widespread as it was 20 years ago, many still cling to the practice and see prohibiting spanking as limiting the rights of parents.

That point of view highlights the difficulty in changing hearts and minds on the issue, despite a mountain of accumulated evidence showing the damage physical punishment can have on a child, says Joan Durant, a professor at University of Manitoba and one of the authors of the study.

"We're really past the point of calling this a controversy. That's a word that's used and I don't know why, because in the research there really is no controversy," she said in an interview.

"If we had this level of consistency in findings in any other area of health, we would be acting on it. We'd be pulling out all the stops to work on the issue."

Durant and co-author Ron Ensom, with the Children's Hospital of Eastern Ontario in Ottawa, cite research showing that physical punishment makes children more aggressive and antisocial, and can cause cognitive impairment and developmental difficulties.

Recent studies suggest it may reduce the brain's grey matter in areas relevant to intelligence testing.

"What people have realized is that physical punishment doesn't only predict aggression consistently, it also predicts internalizing kinds of difficulties, like depression and substance use," said Durant.

"There are no studies that show any long term positive outcomes from physical punishment."

While banned in 32 countries, corporal punishment of children retains at least partial social acceptance in much of the world. Debates on the issue typically revolve around the ethics of using violence to enforce discipline.

With the study, Durant hopes parents will start to look at the issue from a medical perspective.

"What we're hoping is that physicians will take that message and do more to counsel parents around this and to help them understand that physical punishment isn't getting them where they want to go," she said.

She also hopes that countries that allow the practice - including Canada - will take another look at their child protection laws.

Canada is one of more than 190 countries to have ratified the United Nations Convention on the Rights of the Child, a 1989 treaty that sets out protections for children.

The treaty - which has been ratified by all UN member states except for the United States, Somalia and South Sudan - includes a passage stating that countries must protect children from "all forms of physical or mental violence".

"If we had two or three studies that showed that if you took 500 mg of vitamin C a day you could reduce cancer risk, we would all be taking 500 mg of vitamin C a day," Durant said.

"Here, we have more than 80 studies, I would say more than 100, that show the same thing (about corporal punishment), and yet we keep calling it controversial."

(Reporting By Cameron French; editing by Rob Wilson)