Geminis are always curious. Every Gemini knows well the common questions people ask:
"Do you have telepathic powers?"
"Can you feel the pain of the other twin?"
"Have you ever introduced yourself as the other twin?"
The curiosity aroused by the fact that there are two people who are so similar has inspired ancient myths, as well as music, films, and literature.
However, the scientific community is also interested in twins.
They provide a unique opportunity to study the influence of our genetics and environment on our traits and behavior.
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Are we determined by genetics or upbringing?
There are two types of twins.
Fraternal twins are formed from two separate eggs that are fertilized at the same time, but by different sperm.
They share on average half of their genes, just like other siblings.
On the other hand, identical twins are formed from a single sperm and a fertilized egg that divides into two embryos.
This means that identical twins share almost identical genetic material and are most often physically very similar.
Identical twins are extremely rare - about three in 1.000 births.
Professor Nancy Segal, who is a fraternal twin herself, has dedicated her entire career to this research.
She is a behavioral geneticist and psychologist at California State University, Fullerton, USA, where she directs the Center for Twin Studies.
"Twins allow us to examine the influence of genetics and environment on every trait - from intelligence, running speed, personality, to height and weight," says Professor Segal.
In typical research, identical twins are compared to fraternal twins.
If identical twins are more similar in a certain trait, this indicates that genetics play a role in the development of that character trait.
That genes influence height, weight, and even intelligence may not be a big surprise.
But twin studies have also revealed that genetics have an influence on some of our most personal traits and behaviors.
"Twins have been used to study many different human behaviors - from religiosity and social attitudes, to attitudes towards the death penalty, to substance use and attitudes towards investing and finance," says Professor Segal.
For example, studies in America, the Netherlands, and Australia have found that identical twins are more likely to share similar views on religion than fraternal twins, especially in adulthood.
This indicates that genetics influence religious beliefs.
This does not mean that genetics predetermine us to believe in God, but rather that it influences a complex set of traits, such as intelligence or sensitivity, that may make us more prone to that type of belief, explains Professor Segal.
Twins who grew up apart
Professor Segal made her most interesting discoveries by studying rare cases of identical twins who did not grow up together.
"One is about personality.
"What's fascinating about it is the aggressiveness and how traditional you are... identical twins who grew up apart are just as similar as those who grew up together."
"This suggests that for people who live together and are related, genetic influences are responsible for the similarities, not shared environments."
One of Professor Segal's most famous studies is the research on twins Anne Hunt and Elizabeth Hamel, who were separated immediately after birth.
They entered the Guinness Book of Records as the twins who were separated for the longest time - 78 years.
They were reunited in America, where Elizabeth lived, and where she was found by Anne, who had grown up and lived in England.
The results of this study showed that Anne and Elizabeth had many common character traits, and both even married men named Jim.
Some of the similarities noted in other studies are even stranger.
"Even unusual habits and behaviors are the same."
"For example, a pair of identical twins used a special Swedish brand of toothpaste.
"Another set of identical twins, who grew up apart, met at an airport in Minnesota, and they were both wearing seven rings, three bracelets and a watch," says Professor Segal.
However, there are many more examples of the same habits and behaviors.
"Another set of identical twins had the habit of wearing rubber bands around their wrists and washing their hands before and after going to the toilet."
"They may have been particularly sensitive to bacteria and very dedicated to hygiene," she adds.
A pair of Scottish identical twins, who grew up separated, had the same habit of cutting toast into four pieces and eating only three squares.
This indicates a desire to control their appetite, that is, not to eat everything on their plate, Segal adds.
"All of this indicates that this is not a coincidence."
"We all have unusual and strange habits, and they don't just appear out of nowhere, but in some way reflect our personality," concludes Professor Segal.
Twins named Jim
Both named Jim, they were separated at birth and met again at the age of 39.
Then they discovered that their lives were strikingly similar.
Both men first married women named Linda, and later remarried, this time to women named Betty.
They both had a dog, Toy, and a son they named James Alan.
They bit their nails and spent the summer on the same beach.
These twins participated in research by Dr. Thomas Bouchard from the University of Minnesota.
The study found that they scored remarkably similar on personality tests, even though they had never had contact before.
These unusual examples raise the question: do we really have as much influence over our choices and behavior as we would like?
"Just because something has a genetic basis doesn't mean you don't have the ability to make decisions," says Professor Segal.
She cites divorce as an example of a major life event that genetics can influence, but not completely determine.
So, what role do genes play exactly?
"Probably traits like a difficult temper, stubbornness, those kinds of things."
"But your genes don't tell you, 'Get a divorce!'"
"You make the decision to divorce."
"So I don't think free will is constrained in any way by that," he says.
It is crucial that we do not think that something is absolutely so, emphasizes Professor Segal.
When it comes to our personalities and traits, there is no unequivocal cause that shaped us into the people we are.
"People often think that we are completely shaped by our environment."
"But I think that's a misconception," says Professor Segal.
Based on an episode of the BBC World Service radio show CrowdScience
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