Twins, especially identical twins, are a subject of fascination for the public and for medical researchers.

There is something about them that non-twins cannot quite grasp; their connection is closer than any other two humans and everyone else is on the outside. There are mythologies, horror stories and even an astrological sign (Gemini) that revolve around the twin dynamic.

But where does the mythology end the truth begin?

We talked to several twins, a genetic epidemiologist, and a psychologist to debunk the myths and explain the even more fascinating facts about twins and their relationship to one another.

Twins can read each other's minds: FICTION

The idea that twins might be able to read each other's minds has always been an intriguing part of the twin dynamic. Disappointingly, researchers have tried and failed to prove the twin mind-reading myth.

"As researchers we don't understand everything, but there's no evidence so far that there's any extra-sensory perception (ESP) there," said genetic epidemiologist and deputy director for the Australian Twin Registry, Associate Professor Jeff Craig.

Sarah Colegrove, 52, and her identical twin sister Jenny, are always being asked if they can read each other's minds.

"People love to think that there's telepathy and those sorts of abilities involved," Sarah said.

"[But] there's no sudden feeling that my sister's in big trouble or she's about to die, that's just ridiculous."

Clinical psychologist Katie Wood, the younger sister to identical twin sisters, has a different perspective.

She does not believe in ESP. But she says there is a closeness that means identical twins can communicate without language, especially when they are children.

Dr Wood's interest in her siblings' twin dynamic led to a career as a clinical psychologist specialising in twins, and lecturer at Swinburne University.

"A lot of what goes on between the twins is implicit and unspoken; often they do know what the twin is feeling without having to ask or explain," she said.

"It's a knowing without explaining … It's more about the intensity of their bond and closeness rather than being witchcrafty or magical."

Associate Professor Jeff Craig says some twins feel that they can read each other's mind, despite the fact that there is no scientific evidence.

"I accept that. I believe it has a rational explanation," he said. "They have a stronger relationship than anyone else, and it's something we don't understand. If they say that they feel things at the same time, and they have a connection, then they do."

Twins can feel each other's pain: FICTION

Many experiments over the years have failed to prove that twins feel each other's pain. Sarah agrees that she and her sister Jenny have never been able to feel each other's pain.

Moreover, she feels like if it was ever going to happen, it would have happened when she experienced the worst pain of her life — in childbirth.

And she has a story as her proof that shared pain is a twin myth. Sarah's twin Jenny, her partner, and Sarah's partner were all at the hospital when Sarah was in labour with her first baby. Her three companions left her at the hospital to have dinner.

But while they were gone, the situation escalated, and Sarah went through a painful emergency forceps delivery. However the trio remained at a hotel nearby having dinner, without any idea of the situation back at the hospital.

"My sister was right there and was absolutely clueless," Sarah said.

Similar to the mind-reading myth, there are things that can't be explained. There are twins who say they have felt each other's pain.

And their close relationship and nearly-identical physical structure means there could be a sliver of truth in the theory — as pain can be psychological and can be felt empathetically.

"There is so much more empathy in twins, if you have someone really close you can feel their pain because of the closeness," Associate Professor Craig said.

Twins are identical because they share a placenta in the womb: FICTION

It is a common myth, even among doctors, that identical twins are determined by the shared placenta.

Did you know Ashton Kutcher has a fraternal twin called Michael? ( Getty Images: Adam Bettcher )

It is true that if twins come out at birth with their umbilical cord attached to the same placenta, then they are identical.

But if they each come out with their own placentas, they could be either fraternal or identical.

If they are identical, it could be that the embryo split in two before the development of the placenta.

The only way to check is to get some genetic testing done.

If you are a twin and think you might be identical not fraternal, these days it is easy to get this tested.

If you or your parents are twins, you're more likely to have twins: FACT

This is true, but only true of fraternal twins.

Having identical twins is considered more of a freak occurrence and there is no genetic predisposition yet discovered.

Multiple fraternal births are however genetic, as they involve the presence of more than one available egg for conception, which is considered a genetic trait.

Identical twins should have separate schooling so they develop their identities: FICTION

Parents can really struggle to know whether to keep twins together during schooling or allow them to build their own identities separate to each other.

Six-year-old identical twins Benji and Mason Cox are both in separate Year One classes at school. Their mum, Belinda Cox, says the boys are similar in many ways.

"They lost the same teeth at the same time, they have identical birth marks — but they are on opposite sides," she said.

Benji and Mason's mother Belinda Cox says the boys are similar in many ways — but they're also very unique. ( ABC Health &Wellbeing: Tegan Osborne )

Academically Benji and Mason are also very similar — their IQs are just one point apart.

Ms Cox says she hopes separate classrooms will help them develop the sense that they are each unique. And the boys — who say the worst thing about being twins is "fighting lots" — don't seem to mind at all.

Dr Wood says from a psychological perspective there is no right answer to this question — it depends on the twins. She says a more important part of raising twins is to try and avoid constant comparison.

"You'll often hear people say, he's the brighter twin, quieter twin. That can be really limited for the twins themselves … competition can be exacerbated in twins," she said.

"Right from conception they're kind of competing. They never have single access to the parent. You're competing for the mother's attention, the mother's resources and mother's times."

Dr Wood says it is not uncommon for teachers to schedule one parent-teacher interview with a parent, rather than two separate ones, which poses problems for each individual's development.

There are many examples of twins thriving with their twin close by, and hating separation — especially in primary schooling.

But this need for togetherness can change drastically as twins reach teen years.

"Adolescence can be a tricky time, they have to work out how to separate from their twin as well as their parents," Dr Wood said.

"Sometimes they work really hard to be different. I guess their challenge is to get a balance between their 'me-ness' and 'you-ness' — they've got this third component which I call the 'us-ness'."

Identical twins can't be convicted of a crime that relies on DNA: FACT (but not for long)

Around the world there are many examples of twins avoiding criminal charges, because DNA evidence could not differentiate between the twins, nor prove that either one was the perpetrator.

Twins from the Harry Potter movies, Fred and George Weasley, were always getting up to mischief.

In 2016 UK twin Patrick Hennessy was cleared of dangerous driving and possession of a lock knife charges after he claimed it was his twin James.

And in 2015 French twins were both jailed for ten months. DNA evidence implicated one of them as a serial rapist. But the authorities could not prove which one did it. It was only when twin Yoan confessed that his brother Elwin was released.

But this fact is rapidly becoming fiction, as more is discovered about the way genes function.

Researchers have found that even if samples of DNA have identical sequences, there are other structures attached to the sequence that can be tested, to tell the difference between two identical people.

This extra layer to understanding genes is called epigenetics — which is Associate Professor Craig's speciality.

He describes the epigenetic process as "small molecules that jump on top of our genes and control gene activity". Or, more poetically, "the musicians that play the symphony of life on our genes".

All this means that very soon, criminal twins will no longer able to rely on their twin's existence as a defence.