More than a decade after the Supreme Court allowed teenagers to record their own sexual activity for private use, it has moved to ensure that adults cannot exploit the apparent consent of young people in order to make child pornography for themselves.

The court said on Friday that an Alberta trial judge was wrong to have acquitted two men who made sex videos with two 14-year-old girls in 2008, at a time when 14 was the legal age of consent in Canada. The Conservative government changed the age of consent to 16 that year.

The girls, both drug addicts, had run away from a treatment centre. One girl had a history of prostitution. They landed at the home of Donald Barabash, 60, a friend of one girl's father. He, and sometimes the girls themselves, recorded videos of sexual activity, some of which included a 41-year-old friend of Mr. Barabash's. There was evidence that at least one of the girls believed she would be given drugs in return for making the videos.

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"Fundamentally, the trial judge failed to consider the extent to which the ... two older men may have exercised control over two vulnerable, deeply troubled and runaway girls," Justice Andromache Karakatsanis wrote for the Supreme Court.

The men argued that the girls gave their consent, and that both the sex and the videotapes were therefore legal. In a controversial 2001 decision involving child pornographer Robin Sharpe of British Columbia, the court had struck down two elements of the federal child porn law: Stories, diaries and drawings made solely for personal use were part of Canadians' right to free expression; and sexual recordings made for personal use where the sex was lawful were deemed part of teenagers' right to free speech.

Justice D.R.G. Thomas of the Alberta Court of Queen's Bench, who presided over the trial of Mr. Barabash and his friend, Shane Rollison, said the girls had willingly consented and participated, and even initiated many of the activities. He therefore acquitted the men of child porn charges. The province's appeal court threw out the acquittals, said the court had failed to consider exploitation as a separate issue, and convicted the men, saying in a 2-1 ruling that their guilt was so obvious no new trial was necessary.

On Friday, the Supreme Court said a new trial would be needed, partly because the Alberta prosecutor had wrongly conceded at the first trial that the sex acts were lawful.

Justice Karakatsanis said that under the Criminal Code, sex with a minor who has reached the age of consent cannot be lawful if it involves a relationship of trust, authority or dependency. Judges, she wrote, need to look at the age difference, the evolution of the relationship and the older person's degree of control or influence over the younger one. (This applies today to 16- and 17-year-olds.)

When the court ruled in 2001 that teens had the right to record their sexual activity, it did so, she said, because "two adolescents might arguably deepen a loving and respectful relationship through erotic pictures of themselves engaged in sexual activity."

That had been controversial within the court, with three judges saying the court should defer to Parliament's wishes. There is no guarantee, they said, that "even when a teenager is in possession of a pornographic picture or videotape depicting himself or herself, that it was created in a consensual environment."

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David Matas, a Winnipeg lawyer who represented Beyond Borders and Canadian Centre for Child Protection Inc., intervenors in the Barabash case, also took part in the Sharpe case in 2001. "If you look at the actual facts of the [Barabash] case, they're really horrible." The trial judge's acquittal of Mr. Barabash and Mr. Rollison shows the problem with the exceptions to the child pornography law created by the court in 2001, he said. But Friday's ruling "moves the boundaries in the direction of the best interests of children and cuts down on the license for private use."

Cara Zwibel, director of the fundamental freedoms program at the Canadian Civil Liberties Association, which also intervened in the case, said the ruling was a positive one. "It achieves the right balance between personal autonomy, freedom of expression and the need to protect children."