The defence has finished laying out its case in the Grace Millane murder trial, putting forward an expert in modern sexual practices who told the court BDSM has become more mainstream and socially acceptable.

The 27-year-old man accused of murdering the British backpacker, whose name is suppressed, has pleaded not guilty. His lawyers say Millane, 21, was strangled to death by accident during consensual sex. The prosecution say she was murdered.

In the second and final day of defence arguments, Professor Clarissa Smith, from the University of Sunderland in the UK, gave evidence to Auckland high court via video link.

She told the court that the BDSM sub-culture was generally not one of aggression and that practices were designed to be mutually pleasurable.

“To people on the outside these things may not look interesting or pleasant, but for those on the inside it certainly is. That’s what they’re looking for.”

Although there was a lack of robust data on what proportion of the population engaged in BDSM it was estimated about 25% of adults expressed some interest in one or more elements of it, and this rate was likely higher among young people, the jury heard.

Smith said although consent and safe practice was paramount in BDSM, inexperienced people might not always know where to access the best information – and alcohol, drugs or passion could get in the way.

“Sometimes the desire for something will override one’s sensible head. And taking a risk feels like there is a reward at the end of it – so why not? Where things like drink or drugs are involved, or just simply being carried away in the moment can mean that safety is not one’s first consideration.”

Smith said the “gameification” of hook-up apps, among other things, had led to more young women “owning” their sexuality and being unafraid of asking for what they wanted – or didn’t want.

The court has previously heard Millane had accounts on websites and apps for those interested in BDSM.

The defence called a British man who matched with Millane on BDSM websites in the past. The man, whose name is suppressed, described her knowledge of BDSM practices as minimal, saying she seemed “naive”.

“I felt like Grace was more naive and trusting in the BDSM area ... The users could be any undesirable person online and Grace seemed to have a naivety.”

The prosecution and defence will make their closing arguments on Thursday. The accused has chosen not to give evidence.