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As behavioral genetics enters a second century, the field's oldest research method remains both relevant and controversial. "Twins have a special claim upon our attention; it is, that their history affords means of distinguishing between the effects of tendencies received at birth, and those that were imposed by the special circumstances of their after lives." -- Sir Francis Galton, 19th century behavioral genetics pioneer, Inquiries into Human Faculty and its Development,1875 More than a century after Galton's observation, twin studies remain a favorite tool of behavioral geneticists. Researchers have used twin studies to try to disentangle the environmental and genetic backgrounds of a cornucopia of traits, from aggression to intelligence to schizophrenia to alcohol dependence. But despite the popularity of twin studies, some psychologists have long questioned assumptions that underlie them--like the supposition that fraternal and identical twins share equal environments or that people choose mates with traits unlike their own. The equal environments assumption, for example, has been debated for at least 40 years. Many researchers have found evidence that the assumption is valid, but others remain skeptical (see Further Reading below). Overall, twin studies assumptions remain controversial, says psychologist James Jaccard, PhD, a psychologist who studies statistical methods at the University at Albany of the State University of New York. In response, though, researchers are working to expand and develop twin study designs and statistical methods. And while the assumptions question remains a stumbling block for some researchers, many agree twin studies will continue to be an important tool--along with emerging genome and molecular research methods (see article page 42)--in shedding light on human behavioral genetics."

"In the 1980s, Dr. Lars Olov Bygren, a preventive-health specialist who is now at the prestigious Karolinska Institute in Stockholm, began to wonder what long-term effects the feast and famine years might have had on children growing up in Norrbotten in the 19th century - and not just on them but on their kids and grandkids as well. So he drew a random sample of 99 individuals born in the Overkalix parish of Norrbotten in 1905 and used historical records to trace their parents and grandparents back to birth. By analyzing meticulous agricultural records, Bygren and two colleagues determined how much food had been available to the parents and grandparents when they were young. Around the time he started collecting the data, Bygren had become fascinated with research showing that conditions in the womb could affect your health not only when you were a fetus but well into adulthood. In 1986, for example, the Lancet published the first of two groundbreaking papers showing that if a pregnant woman ate poorly, her child would be at significantly higher than average risk for cardiovascular disease as an adult. Bygren wondered whether that effect could start even before pregnancy: Could parents' experiences early in their lives somehow change the traits they passed to their offspring? (See the top 10 medical breakthroughs of 2009.) It was a heretical idea. After all, we have had a long-standing deal with biology: whatever choices we make during our lives might ruin our short-term memory or make us fat or hasten death, but they won't change our genes - our actual DNA. Which meant that when we had kids of our own, the genetic slate would be wiped clean. What's more, any such effects of nurture (environment) on a species' nature (genes) were not supposed to happen so quickly. Charles Darwin, whose On the Origin of Species celebrated its 150th anniversary in November, taught us that evolutionary changes take place over many generations and through millions of years of nat

"MIT researchers have shown, for the first time ever, that memories are stored in specific brain cells. By triggering a small cluster of neurons, the researchers were able to force the subject to recall a specific memory. By removing these neurons, the subject would lose that memory. As you can imagine, the trick here is activating individual neurons, which are incredibly small and not really the kind of thing you can attach electrodes to. To do this, the researchers used optogenetics, a bleeding edge sphere of science that involves the genetic manipulation of cells so that they're sensitive to light. These modified cells are then triggered using lasers; you drill a hole through the subject's skull and point the laser at a small cluster of neurons. Now, just to temper your excitement, we should note that MIT's subjects in this case are mice - but it's very, very likely that the human brain functions in the same way. To perform this experiment, though, MIT had to breed genetically engineered mice with optogenetic neurons - and we're a long, long way off breeding humans with optogenetic brains. In the experiment, MIT gave mice an electric shock to create a fear memory in the hippocampus region of the brain (pictured above) - and then later, using laser light, activated the neurons where the memory was stored. The mice "quickly entered a defensive, immobile crouch," strongly suggesting the fear memory was being recalled."

"Almost 150 years ago, English scientist Francis Galton coined the phrase "nature versus nurture" - and proposed that research on twins could resolve the debate. Genetics have long seemed to weigh heavily in favor of the role of nature in shaping the people we become. But even identical twins are different to varying degrees, and some researchers believe those differences suggest a third influence at work, called epigenetics. Peter Miller of National Geographic Magazine wrote the magazine's January cover story, "A Thing or Two About Twins." Miller explains how scientists are expanding the field of epigenetics with research on twins."

"To these scientists, and to biomedical researchers all over the world, twins offer a precious opportunity to untangle the influence of genes and the environment-of nature and nurture. Because identical twins come from a single fertilized egg that splits in two, they share virtually the same genetic code. Any differences between them-one twin having younger looking skin, for example-must be due to environmental factors such as less time spent in the sun. Alternatively, by comparing the experiences of identical twins with those of fraternal twins, who come from separate eggs and share on average half their DNA, researchers can quantify the extent to which our genes affect our lives. If identical twins are more similar to each other with respect to an ailment than fraternal twins are, then vulnerability to the disease must be rooted at least in part in heredity. These two lines of research-studying the differences between identical twins to pinpoint the influence of environment, and comparing identical twins with fraternal ones to measure the role of inheritance-have been crucial to understanding the interplay of nature and nurture in determining our personalities, behavior, and vulnerability to disease. Lately, however, twin studies have helped lead scientists to a radical, almost heretical new conclusion: that nature and nurture are not the only elemental forces at work. According to a recent field called epigenetics, there is a third factor also in play, one that in some cases serves as a bridge between the environment and our genes, and in others operates on its own to shape who we are."

"Psychopathic killers are the basis for some must-watch TV, but what really makes them tick? Neuroscientist Jim Fallon talks about brain scans and genetic analysis that may uncover the rotten wiring in the nature (and nurture) of murderers. In a too-strange-for-fiction twist, he shares a fascinating family history that makes his work chillingly personal."

"A MOTHER sits in a playroom with her young son. The phone rings. When she picks it up, a researcher watching through a two-way mirror asks her to look into her son's eyes and ''show him, in the way that feels most natural for you, that you love him''. The mother is doing her best to connect, but this little boy won't return her gaze. He looks at her mouth, where the words are coming from, but it's as if he can't understand what she means. Advertisement: Story continues below Robert Thompson and Jon Venables killed UK toddler James Bulger. Robert Thompson and Jon Venables killed UK toddler James Bulger. Mark Dadds says some children literally cannot see the love in their mother's eyes. Professor Dadds, a parenting expert from the University of New South Wales, has just published results of his work in the British Journal of Psychiatry and the Journal of Child Psychiatry and Psychology that suggest the ability to make eye contact is vital in learning how to love other people. For the past five years, he has been working with children referred to his Sydney clinic for sustained rages, continual aggression, calculated violence and, occasionally, cruelty to animals. These are children with some of the worst behavioural problems, who score highly for ''callous, unemotional'' traits. In his studies in both Sydney and London, it was these children who did not meet their mother's gaze, even when told they were loved. People marvel at the resilience of children who overcome appalling family backgrounds to make good lives. We understand when childhood trauma sends a child off the rails. But we also have to accept that even good parents can have mean children - how else to explain families where only one child seems to be callous and unemotional, while the siblings are not? Dadds distinguishes between children who are emotionally ''hot'' - those who lash out at the world - and the much smaller subset of ''cold'' children - the ones who don't react emotionally, don't car

"One day last summer, Anne and her husband, Miguel, took their 9-year-old son, Michael, to a Florida elementary school for the first day of what the family chose to call "summer camp." For years, Anne and Miguel have struggled to understand their eldest son, an elegant boy with high-planed cheeks, wide eyes and curly light brown hair, whose periodic rages alternate with moments of chilly detachment. Michael's eight-week program was, in reality, a highly structured psychological study - less summer camp than camp of last resort. Michael's problems started, according to his mother, around age 3, shortly after his brother Allan was born. At the time, she said, Michael was mostly just acting "like a brat," but his behavior soon escalated to throwing tantrums during which he would scream and shriek inconsolably. These weren't ordinary toddler's fits. "It wasn't, 'I'm tired' or 'I'm frustrated' - the normal things kids do," Anne remembered. "His behavior was really out there. And it would happen for hours and hours each day, no matter what we did." For several years, Michael screamed every time his parents told him to put on his shoes or perform other ordinary tasks, like retrieving one of his toys from the living room. "Going somewhere, staying somewhere - anything would set him off," Miguel said. These furies lasted well beyond toddlerhood. At 8, Michael would still fly into a rage when Anne or Miguel tried to get him ready for school, punching the wall and kicking holes in the door. Left unwatched, he would cut up his trousers with scissors or methodically pull his hair out. He would also vent his anger by slamming the toilet seat down again and again until it broke. When Anne and Miguel first took Michael to see a therapist, he was given a diagnosis of "firstborn syndrome": acting out because he resented his new sibling. While both parents acknowledged that Michael was deeply hostile to the new baby, sibling rivalr

"Recently we heard about this test that could determine if someone was a psychopath. So, naturally, our staff decided to take it. This week we hear the results. Plus Jon Ronson asks the question: is this man a psychopath? "

"BARBARA HERBERT, a former council worker living in southern England, discovered after the death of the woman she had thought was her mother that in fact she had been adopted. Among her assumed mother's papers, she found a name and address in Finland. When that produced no answer, she contacted the local newspaper in Finland. A reporter dug up the story. Her real mother had been sent to England, two months pregnant, in 1939. She had given birth, been sent back to Finland, and committed suicide at the age of 24. Mrs Herbert had a feeling the story was not over. She seemed to recall somebody saying, "There was another one." So she contacted Hammersmith Hospital, where she was born; and, sure enough, there had been twins. The Registrar-General refused to help her contact her twin. She took the Registrar-General to court, and won. That is how she found her sister. They met at King's Cross station in London. "We just said 'Hi' and walked off together, leaving our husbands standing there," says Mrs Herbert. "It seemed so natural." Mrs Herbert is a bit fatter than her sister, but she can think of no other important difference between them. Their intelligence quotients (IQs) were one point apart. They were tested again a year later; they scored ten points higher, but still only one point apart. Mrs Herbert and her sister Daphne are gold dust to geneticists. Unlike fraternal twins, who are the product of separate eggs fertilised by different sperm, identical twins are natural clones, produced when a fertilised egg splits in two shortly after conception. Such twins, when separated after birth, are thus a scientific experiment designed jointly by nature and by society. They have the same genes but have been brought up in different environments. These curiosities are getting rarer. Until the 1960s, twins offered for adoption in the West were often separated at birth, on the argument that two babies would be too much for one mother. That no longer happens

"It is unusual for the subtitle of a book to undersell it, but Steven Pinker's "Better Angels of Our Nature" tells us much more than why violence has declined. Pinker, a professor of psychology at Harvard who first became widely known as the author of "The Language Instinct," addresses some of the biggest questions we can ask: Are human beings essentially good or bad? Has the past century witnessed moral progress or a moral collapse? Do we have grounds for being optimistic about the future?"

"A few months after the World Trade Center attacks, a strange message appeared on a U.S. Army computer: "Your security system is crap," it read. "I am Solo. I will continue to disrupt at the highest levels." Solo scanned thousands of U.S. government machines and discovered glaring security flaws in many of them. Between February 2001 and March 2002, Solo broke into almost a hundred PCs within the Army, Navy, Air Force, NASA, and the Department of Defense. He surfed around for months, copying files and passwords. At one point he brought down the U.S. Army's entire Washington, D.C., network, taking about 2000 computers out of service for three days. U.S. attorney Paul McNulty called his campaign "the biggest military computer hack of all time." But despite his expertise, Solo didn't cover his tracks. He was soon traced to a small apartment in London. In March 2002, the United Kingdom's National Hi-Tech Crime Unit arrested Gary McKinnon, a quiet 36-year-old Scot with elfin features and Spock-like upswept eyebrows. He'd been a systems administrator, but he didn't have a job at the time of his arrest; he spent his days indulging his obsession with UFOs. In fact, McKinnon claimed that UFOs were the reason for his hack. Convinced that the government was hiding alien antigravity devices and advanced energy technologies, he planned to find and release the information for the benefit of humanity. He said his intrusion was detected just as he was downloading a photo from NASA's Johnson Space Center of what he believed to be a UFO. Despite the outlandishness of his claims, McKinnon now faces extradition to the United States under a controversial treaty that could land him in prison for years-and possibly for the rest of his life. The case has transformed McKinnon into a cause célèbre. Supporters have rallied outside Parliament with picket signs. There are "Free Gary" websites, T-shirts, posters. Rock star David Gilmour, the former guitarist for Pink Floyd, even recorded

"Daniel Kahneman spent part of his childhood in Nazi-occupied Paris. Like the other Jews, he had to wear a Star of David on the outside of his clothing. One evening, when he was about 7 years old, he stayed late at a friend's house, past the 6 p.m. curfew. The intellectual, cultural and scientific findings that land on the columnist's desk nearly every day. He turned his sweater inside out to hide the star and tried to sneak home. A German SS trooper approached him on the street, picked him up and gave him a long, emotional hug. The soldier displayed a photo of his own son, spoke passionately about how much he missed him and gave Kahneman some money as a sentimental present. The whole time Kahneman was terrified that the SS trooper might notice the yellow star peeking out from inside his sweater. Kahneman finally made it home, convinced that people are complicated and bizarre. He went on to become one of the world's most influential psychologists and to win the Nobel in economic science. Kahneman doesn't actually tell that childhood story in his forthcoming book. "Thinking, Fast and Slow" is an intellectual memoir, not a personal one. The book is, nonetheless, sure to be a major intellectual event (look for an excerpt in The Times Magazine this Sunday) because it superbly encapsulates Kahneman's research, and the vast tide of work that has been sparked by it. I'd like to use this column not to summarize the book but to describe why I think Kahneman and his research partner, the late Amos Tversky, will be remembered hundreds of years from now, and how their work helped instigate a cultural shift that is already producing astounding results. Before Kahneman and Tversky, people who thought about social problems and human behavior tended to assume that we are mostly rational agents. They assumed that people have control over the most important parts of their own thinking. They assumed that people are basically sensible utility-maximizers and that w