Robert Karl Stonjek, 55, of Kings Meadows, has pleaded not guilty to possessing 35 child exploitation videos on the basis that it was for a genuine scientific purpose.

The state does not dispute that Mr Stonjek used the clips to write a psychology paper but said it was not scientific and his conduct was not reasonable.

Yesterday Crown prosecutor John Ransom said his inability to have the paper published in a scientific journal was the "proof ... in the pudding".

"In fact, it was rejected," he said in Launceston's Supreme Court.

Mr Stonjek's defence rested on the concept of being an "independent researcher" whose severe social anxieties prevented him from gaining formal qualifications. Instead the autodidact [self-taught] Mr Stonjek enmeshed himself in online psychology forums where he was treated "like a minor god".

Mr Stonjek told the jury that his paper, Childhood Exploitation Material: a comparative study of downloadable paedophilic material, was "groundbreaking" because it argued "scientifically" that children were not sexual beings.

During cross-examination Mr Ransom said the paper was unoriginal and could have been called "child exploitation material I downloaded and what was in it".

In his closing address, he said the lack of Mr Stonjek's formal qualifications and the absence of ethical oversight undermined any claim that the child pornography was for scientific purposes.

"He viewed what he was seeking to establish as so important ... he was prepared to ignore potential illegality and ethical issues," Mr Ransom said.

"`At the end of the day he crossed the line."

Mr Ransom said the Crown did not suggest the videos were for Mr Stonjek's sexual gratification.

Defence counsel Katie Edwards said the law did not require Mr Stonjek to have formal qualifications to have a legitimate reason for possessing the material.

"Yes, he self-publishes, but that doesn't mean he doesn't have a genuine scientific purpose," she said.

Ms Edwards highlighted singer Justin Beiber, who started out by posting his own video clips online to achieve fame.

"He doesn't have a degree in music," she said.

Ms Edwards said child exploitation material needed to be studied if paedophilia was to be treated or ideas about it challenged.

"Why not him? He clearly had an extensive knowledge in this field," she said.

Ms Edwards argued that Mr Stonjek's comments that children couldn't be used in sexual experiments showed he had considered the ethics before deciding to look at what was "already out there" to conduct research.