Many scientist once believed that physical similarities between identical twins are genetic（基因的），while their personalities，intelligence and other differences between them are an effect of their environment.Now scientists are discovering that the boundaries（界限）between genes and environment are not so clear.
Twins Jim Springer and Jim Lewis were adopted（领养）as babies and raised by different couples.When the Jims finally met at age 39，they discovered they had plenty in common.Both were six feet tall，180 pounds.They had the same smile and the same voice.When scientist Thomas Bouchard Jr.invited the Jim twins to his lab，people there found it very hard to tell them apart.
But the similarities didn't stop at the physical.They'd both had dogs named Toy.They had both married women named Linda and then their marriage broke up.They'd both been policemen，enjoyed music，and left love notes around the house for their wives.They had so much in common，it seemed unlikely these were just coincidences （巧合）.
The Jim twins were just one of 137 sets of separated twins.Bouchard tested.When they compared the twins' IQ scores，Bouchard and his team reached a surprising conclusion（结论）.They concluded that intelligence was mostly connected to genes rather than to training or education.It seemed the differences in family and environment had little effect.
However，genes can't control everything，argues geneticist Danielle Reed，who also studies twins.Reed's research shows that though nothing can truly change our DNA，environmental differences that a child experiences before birth and in their first year can sometimes affect the way the DNA behaves，making even identical twins into very different people. "What I like to say is that Mother Nature writes some things in pencil and some things in pen， "She explains. "Things written in pen you can't change.That's DNA.But things written in pencil you can. "
