Why do men perform lewd acts on trains?

Why do men masturbate on public transport?

At least five men have allegedly exposed themselves, or performed lewd acts in front of a woman on a Melbourne train in the last three weeks.

It's a crime that baffles as much as it frightens and appalls people.

On Wednesday, police released images of a man who allegedly exposed himself to two women on separate trains earlier this year.

Police have been told the man boarded a train at Oakleigh station about 4.40pm on March 26 and allegedly masturbated in front of a woman. He allegedly did the same on a train leaving Huntingdale station on the afternoon of May 23.

The number of sexual offences on public transport reported to Victoria Police rose by 40 per cent in 2016-17 to 350 reports, up from 249 the year before.

The offences include sexual assault, obscene exposure and other lewd behaviour - are mostly reported on trains.

A need for power, a desire for attention or a wish to shock fellow passengers may motivate offenders, experts say.

Forensic psychologist Dr Lisa Warren, from Monash University, says it can be a misguided way of trying to attract women.

"The reasons for each are a little bit different but the commonality is that it's sexually gratifying and there is the distortion the person who is the target might be gratified by it as well. Basically a misguided way of trying to pick up," she said.

"As for why they would do it on public transport, there are two reasons: firstly you have a captive audience, you are on something that is moving, people can't get away easily. The other thing is people are much closer, and the whole idea of personal space is a bit different on public transport, particularly if you've got much closer confined spaces."

She said it was important to reinforce the message that commuters should feel comfortable reporting incidents to authorities, even if they think it was minor.

"What I hope is something like the #metoo movement is having impact on people's willingness to report," she said. "We should encourage people to report. This is absolutely unacceptable behaviour. If your instincts tell you it's not right, it's not right."

Clinical psychologist Pan Tsomis also said there was a range of intentions behind the behaviour, including the drive to get "attention and affection".

"Sometimes it's a lack of social skills to form normal relationships, or to attract people in the normal way, so it's that more spontaneous cause and effect notion: if I flash someone they will be impressed and I will gain their attention," he said.

"Obviously that's a distorted way of thinking ... sometimes it is a power thing, not wanting to show off, wanting the other person to looked shocked, intimidated, scared."

Recent incidents:

Crimestoppers and Victoria Police last year launched their "Hands Off" campaign, releasing images to help identify alleged offenders of sexual offences committed on public transport.

Dr Bianca Fileborn, a lecturer in criminology from UNSW, said the campaign had been a good start but police must encourage women to come forward and report incidents even if they aren't on the serious end of the scale.

She said her research into street harassment had found most of the women she spoke to who had been harassed on public transport had not reported it.

"People didn't really feel like they were able to report a lot of the time, there is still the sense of 'will this be taken seriously, was this bad enough to go to police'," she said.

WHAT TO DO IF YOU ARE THE VICTIM OF SEXUAL HARASSMENT ON PUBLIC TRANSPORT:

Call triple zero or press the button in the carriage to speak to the train or tram driver.
Witnesses should also call triple zero.
Take photos of perpetrators, but only if it is safe to do so.
If an individual feels like they are in a vulnerable position, send a text message to a friend or family member where they can advise them they are in that position so that person can ring triple zero and police can attend.

Anyone with information can call Crime Stoppers on 1800 333 000 or submit a confidential report. Centre Against Sexual Assault: 1800 806 292. National sexual assault helpline 1800RESPECT: 1800 737 732.