There is a mystery in jazz and a mystery in genes and they are both in the air one night at a D.C. club called Twins Lounge. The owners, identical twins from Ethiopia named Kelly and Maze Tesfaye, are an indistinguishable service dynamo shuttling out of the kitchen with plates of fiery lentil stew and foamy sop-bread. The woman onstage leading a quartet through a medley of jazz standards -- Miss Sharon Clark, by name -- is also an identical twin. She's a bounteous woman in a red sequined dress draped with the long coils of her crimped hair. The band is tight. Sharon's rich, sensuous voice coquets above the piano, the drums, the bass. The sounds merge in harmony and scatter in crazy syncopation, like raindrops becoming rainbows becoming raindrops again. Between "I Only Have Eyes for You" and "Girl Talk," Sharon trundles over to fix the sound levels and chats up the audience, the way a hard-working entertainer will do. "How's everybody tonight?" she asks abstractedly. Cough, cough. Tap, tap on the microphone. A buzz of feedback. "Now I want to encourage you all to pick up a copy of my new CD," she says. "Sherry's got 'em in the back." In the back, more or less hidden in shadow, Sharon's identical twin, Sherry, sits with her hands twisted around a big snifter of brandy. Sherry and Sharon are 36 and have always been together. Since childhood they have shared a special language, and they have sung together, and they have counseled each other and argued with each other, and played tricks on their dates and math teachers. But just as no two jazz musicians handle a tune the same way, Sharon and Sherry's riffs on their genetic score have resulted in two distinct ways of life. On this night, Sherry wears a simple dark dress, and her hair is straightened. Unlike Sharon, who never gets sick, Sherry seems to get whacked by whatever comes down the pike, and at the moment she's recovering from a flare-up of diabetes. She lost 57 pounds on a recent diet and doesn't much look like Sharon these days. And the many ways they are not identical go way below the skin; these twins just move through life differently. Things get to Sherry. "We've always been extremely close and had a very strong bond. But I've always been more cautious, always seeking acceptance," she says, gripping the brandy as she watches Sharon flirt with the piano player. "You know, Sharon never gets nervous at her gigs. But I do." Identical twins are the clones of nature, each descended from the symmetrical splitting of a single fertilized egg into cells that contain the identical sequence of billions of even tinier DNA molecules. They occur about once every 250 births, which makes them about a third as common in America as fraternal twins, who descend from two separately fertilized eggs and are no more similar genetically than other siblings. Identical twins are far more familiar than, say, septuplets, but there is still something a little eerie about them, from the double-your-pleasure Doublemint girls to the ghost girls haunting the snowbound lodge in "The Shining." Maybe it's the disorientation induced by a human optical illusion. Maybe it's the fungibility of existence suggested by two lives apparently as interchangeable as bootleg videotapes. If a twin's fate is demonstrably linked to her double's by invisible clumps of nucleic acid, does that mean the rest of us are just as dominated by our DNA? How important are genes, anyway, in determining whether people are beautiful or ugly, stupid or smart, violent or meek, worried or blissful? Are Hell's Angels really born to lose, as it says on their helmets, or were they just brought up wrong? In the case of the Clark girls, Sharon Inez, 6 pounds 15 ounces, was born at 10:49 a.m. on December 21, 1961, at Fort Belvoir Hospital, where her father, Staff Sgt. Curtis Clark, was a water purification specialist. Seven minutes later Sherry Lynn entered the world, half an inch longer but eight ounces lighter. When the twins were a year old, their mother, Mary, noted in her baby book that Sharon "eats very good except for the usual messiness. Likes all foods except spinach." As for Sherry, she "fusses at all mealtimes, but loves spinach." Growing up together in a big household in a semirural corner of Alexandria, the twins were a source of family amusement. At age 3, their older sisters, Sylvia, Tina and Rosalind, would plunk them down in little chairs and dress them as royalty. Sharon, in knee socks and a towel turban, would be the king. Sherry was the queen, festooned with costume jewelry. "Sharon would have this exuberant clowning expression all the time, while Sherry was always more serious-looking, even as a baby," says Tina. "That was one of the ways we could tell them apart." Sharon was headstrong and heedless; Sherry was polite and careful. Sherry was sugar, Sharon spice. At school, the girls excelled in drama and song, learning from their father's jazz collection -- "cocoa music," they called it in their secret language. They were heavy girls and very dark-skinned, and made to feel bad about it, especially when they passed through Soul Corner, the yellow brick corridor at Fort Hunt High School where the tough kids hung out. The daily initiation rite on that patch of turf provided a clue on how the twins would confront life's adversities. Sherry would go out of her way to avoid the taunts of Soul Corner. Sharon plunged right through, elbows at the ready, giving as good as she got. "Sharon," recalls Sherry, "would go to bed with her fists balled up in case anybody messed with her in her dreams." Sherry, on the other hand, was not as resilient. At age 13, she attempted suicide by swallowing pills. And a year later, after their father died suddenly of heart failure, Sharon became Sherry's protector. If the Clark twins both had exactly the same DNA, how was it that from infancy their personalities were so distinct? Was it a subtle shading of parental affection that accounted for Sharon's brashness and Sherry's anxiety? Or something that happened in the womb? Was it the kind of invisible, indelible influence that behavior geneticists, who try to divide influences on personality into neat categories -- genes or environment -- simply throw up their hands about and call "noise," which is the scientific equivalent of a shrug? Although the Clarks are one of 15,000 twin pairs in an ongoing study of human behavior at Virginia Commonwealth University, the scientists who mail Sherry and Sharon questionnaires every several years don't presume to know the women's whys and wherefores. The biological history of any individual is a Tower of Babel in which nature and nurture are virtually inseparable. "Science," in the words of the study's co-director, Lindon J. Eaves, "generally does a good job of dealing with the statistical properties of aggregates. It does a lousy job of dealing with the behavior of individual particles." But individual particles, of course, are what interest most of us in our individualistic culture. And at the level of particles, the nature-nurture debate, a scientific controversy that has been raging for more than a century, is all about our limits -- about whether they come from outside or within. Whether we are what we are because our genes programmed us that way, or because environment -- be it womb, home, neighborhood or nation -- shaped us. It is a debate that has often been tipped one way or another by shifts in public opinion. There was a time early this century when scientists tried to create a kind of animal husbandry for humans that aimed, in the words of its founder, Sir Francis Galton, to "check the birth rate of the unfit" and "improve the race" by promoting "early marriages of the best stock." The Nazis took Galton's idea, which was called eugenics, to its notorious extreme, and in the horrific failure of their master race rendered the idea of tinkering with the human gene pool unfit for a generation. In the postwar era, bolstered by the ideas of anthropologist Franz Boas and psychologist B.F. Skinner, among others, nurture took over. People weren't intrinsically good or bad stock; it was how mama raised the kids, and the social context she raised them in, that accounted for whether they became bums or angels. Now the pendulum has swung again. Nurture is out, nature is back. And science is largely the reason why. Every week, it seems, comes a new revelation about who we are and why we are, all attributable to our genetic code. So amazing are these discoveries that they seem in need of exclamation points and screaming headline type: We've isolated the gene that determines sexual orientation! We've isolated the gene that causes breast cancer! We've isolated the gene that determines whether we will become substance-addicted! It's as if we are on the brink of being able to alter fate through genetic engineering, one person at a time. In truth, the discoveries have proved to be more complicated than they might appear at first glance, but they portend an ability to unlock in the laboratory some of the mysteries of life. Well before molecular genetics gained center stage, however, scientists had -- and continue to have -- another way of approaching these mysteries, and that's through the mysteries that are twins. For behavior geneticists like Eaves, who do the nitty-gritty work in the nature-nurture debate, twins are the perfect people on whom to test hypotheses about what is molded by life's pressures and what is inborn. But such scientists wage the battle from a distance, using statistics to describe the behavior of populations -- of aggregates, in other words -- rather than individuals. These statistics have shown that on average, identical twins tend to be around 80 percent the same in everything from stature to health to IQ to political views. The similarities are partly the product of similar upbringing. But evidence from the comparison of twins raised apart points rather convincingly to genes as the source of a lot of that likeness. In the most widely publicized study of this type, launched in 1979, University of Minnesota psychologist Thomas Bouchard and his colleagues have chronicled the fates of about 60 pairs of identical twins raised separately. Some of the pairs had scarcely met before Bouchard contacted them, and yet the behaviors and personalities and social attitudes they displayed in lengthy batteries of tests were often remarkably alike. The first pair Bouchard met, James Arthur Springer and James Edward Lewis, had just been reunited at age 39 after being given up by their mother and separately adopted as 1-month-olds. Springer and Lewis, both Ohioans, found they had each married and divorced a woman named Linda and remarried a Betty. They shared interests in mechanical drawing and carpentry; their favorite school subject had been math, their least favorite, spelling. They smoked and drank the same amount and got headaches at the same time of day. Equally astounding was another set of twins, Oskar Stohr and Jack Yufe. At first, they appeared to be a textbook case of the primacy of culture in forming individuals -- just the opposite of the Lewis-Springer pair. Separated from his twin six months after their birth in Trinidad, Oskar was brought up Catholic in Germany and joined the Hitler Youth. Jack stayed behind in the Caribbean, was raised a Jew and lived for a time in Israel. Yet despite the stark contrast of their lives, when the twins were reunited in their fifth decade they had similar speech and thought patterns, similar gaits, a taste for spicy foods and common peculiarities such as flushing the toilet before they used it. Bouchard's collection of twins-raised-apart is unique in American behavior genetics. In most twin studies, including Eaves's research, scientists are comparing the similarities between identical twins and fraternal twins; in other words, they compare comparisons. To test the assumption that genes play a role in IQ, for example, scientists ask whether the IQs of identical twins (who share all their genes) are more similar than those of fraternal twins (who share an average of 50 percent). To have any statistical validity, such studies must examine thousands of twin pairs. But enough studies have been done to show that identical twins are roughly 85 percent similar for IQ, fraternal twins about 60 percent. Crunching the numbers, behavior geneticists say about half the variation in IQ, whether among twins or non-twins, may be due to genes. It was this figure that provided the grist for the controversial 1994 book The Bell Curve, whose conservative authors argued that little could be done to help the poor because they suffered from low IQs that were fixed, for the most part, by their genes. The book has been disputed by many critics, including those who deny that IQ is a worthy measure of intelligence. Even if it is, though, the genetic component of IQ that The Bell Curve trumpets is not an overwhelming factor: Even if half of IQ variation is due to genes, that leaves room for plenty of average kids to be born to brilliant parents, and vice versa. And when children of smart parents are smart, it is nearly impossible to know whether this is due to the "smart" genes they inherited, or the "smart" environment their parents provided. When it comes to social policy, genetics provides no worthy pretext for neglecting the disadvantaged, as geneticist and child development expert Sandra Scarr has pointed out. Scarr, a former University of Virginia professor, is considered a hard-liner for nature; she believes that if a child's basic needs are met, genes become the dominant control on how far the child will go in life. But she also has called for massive intervention to help underprivileged children. Infants and toddlers lacking the basics in food, shelter and affection, she says, are likely to be stunted in ways that outweigh genetic considerations. When journalists first began interviewing Bouchard's twins-raised-apart, they focused on the spectacularly similar pairs, like the Springer-Lewis twins. But those twins turned out to be outliers in the Minnesota study. Most of the other twins weren't nearly as alike. Furthermore, since no one is claiming there is a gene for flushing the toilet before you use it, or a gene for marrying women named Betty, such coincidences are statistical anomalies, as Bouchard is quick to acknowledge. The quirky cases strengthen our sense of the power of nature, but they don't provide enough data to make a scientific case. "There probably are genetic influences on almost all facets of human behavior," Bouchard says today, "but the emphasis on the idiosyncratic characteristics is misleading. On average, identical twins raised separately are about 50 percent similar -- and that defeats the widespread belief that identical twins are carbon copies. Obviously, they are not. Each is a unique individual in his or her own right." Geneticist humor: A little joke is posted on the bulletin board of the clinical genetics department at Johns Hopkins Hospital in Baltimore. It's a diagram of the X chromosome, with the names and descriptions of imaginary genes scribbled at intervals along it: "visa" -- the gene for shopping addiction; "klutz" -- the inability-to-manipulate-mechanical-objects gene; "blab" -- the gene for prolonged telephone conversation; "eek" -- the fear-of-bugs gene. There's even a gene for emotional instability -- "shrill" -- and one for learned helplessness: "honey . . ." Variations on this joke can be found at labs around the country, pushpinned next to the job notices and 401(k) plan announcements and postcards from colleagues at the beach in Aruba. Like the "You want it when?!" posters on the walls of auto repair shops, the joke map parodies popular perceptions of the profession but contains a kernel of truth. Scientists have made some pretty remarkable claims recently about the impact of particular molecules on behavior. In journals such as Science and Nature Genetics, they publish news of genes for neuroticism and thrill-seeking and risk-taking, genes for alcoholism and aggression and anxiety. In 1993, a National Cancer Institute researcher reported the location -- on chromosome X, as it happens -- of a gene that seemed to cause homosexuality. It must be said, however, that the meanings of such discoveries shed their precision as they travel from the scientific literature to the popular culture. Genes don't really make homosexuals or violent kids or depressed adults, and no reputable scientist would claim that they do. Genes make proteins that contribute to chemical pathways that play a role in complex neurological and existential events. But that's a long story, so spare us the details. Something inside us -- a "fatalism" gene, perhaps? -- makes us want to believe that the genetic blueprint holds the secrets of who we are. Something of this fatalism imbues the folklore of twins. In the books and magazine articles and Web sites about their lives, twins tell uncanny stories of wordless understandings, of moments of grief or joy communicated at a distance without benefit of a phone, by some kind of genetic magnetism. On one Web site, a woman named Gilia Angell recalls wandering into the St. Patrick's Cathedral gift shop in New York and buying a postcard of an airbrushed Jesus, which she mailed to her twin sister in Olympia, Wash. A few days later, she says, a letter postmarked the same day arrived from Olympia. Enclosed was a refrigerator magnet "with the same filmy airbrushed picture of Jesus!" Then there are those twin pranks, adding to our general sense of wonderment over their doubleness -- duped boyfriends and confused motor vehicles officials, cheating on SAT tests -- and the good twin/bad twin dichotomies, exemplified by Jeen Han, a California 23-year-old who was recently convicted of trying to kill her twin. Yet for all of that synergy stuff, natural-born clones don't have to be told they are separate individuals -- they know it. "You could take 50 cells from my leg and make 50 other people who look and sound like me, but they won't be me," says Richard Bausch, a novelist, short-story writer and George Mason University writing instructor. "To any twin, the idea that human clones would be the same is absurd. When you're a twin you know that. People are much too complicated to be replicated, no matter how many genes they discover." On the face of it, Bausch's convictions seem undercut by his biology. His identical twin, Robert, is also a fiction writer, and also a writing instructor, at Northern Virginia Community College in Woodbridge. But these are writers, of course, whose job is to be finely aware of nuances. Robert is the more intellectual one; Richard is more religious. Their take on cloning is similar, but Robert's analogies are Jesuitic. "If everything that we call the will is just genetics and chemistry," he asks, "then who in the hell are we talking to when we try to remember something that's on the tip of our tongue?" At 51, the Bausch twins see less of each other than they used to. They each have their own work and their own children to worry about. They have been profiled too often to really enjoy being a novelty act anymore. Neither feels that being a clone is what defines him. But Richard will say this: "How people react to life is determined by their nature, but I don't think nature is biological. I still believe in good and evil, and that there is such a thing as sin. " Genes' is just the word we use to describe God." Anne Clark, born Anne Collins, believes in God and free will, but also that chance can affect how you exercise free will. How else to explain her own history? Anne and her identical twin, Lynne Weindel, grew up middle-class in Fairfax County, like Sharon and Sherry Clark. They were quiet and mild-mannered girls with freckles and red corkscrew hair, neither deeply reflective nor entirely untroubled -- average students who played together and shared friends and looked very similar. There was no good twin/bad twin dichotomy that anyone noticed, and the proper young ladies didn't stage any mistaken-identity pranks because they didn't think they looked alike. "We thought we'd get caught," says Lynne, mildly. When the girls were 8, their father died suddenly from the complications of a gallstone operation. Lynne's memory of the event is faint. So is Anne's, but she is convinced it was a defining moment, perhaps the defining moment, of her life. "I remember waiting for my mom at our aunt's house, playing in their basement," she says. "I remember my brother crying, the emptiness and fear. It was numbing." The next year the girls were split into separate classes at Laurel Ridge Elementary in Fairfax. Lynne loved her second-grade teacher; Anne recalls being lonely. Together they attended Shepherd College in West Virginia, but Anne dropped out after the second year to marry a part-time DJ named Mike Clark, whom she'd met when he was lip-synching a Van Halen song at a talent show. Lynne got her degree in education and became a teacher at a day-care center. Anne had a daughter, broke up with Mike after two years, cobbled together jobs and living arrangements until Mike and she got back together in 1995. Their daughter, now 7, has been diagnosed with attention deficit disorder, and Anne blames herself. "They say ADD is genetic, but it could be the environment, too. They just don't know much about it." She fiddles with her hair. She's sitting in the living room of her twin's Herndon town house. Where she stayed for an entire year once, while she was getting her life back together. What accounts for the different choices? "I think I was always more impulsive than Lynne. More rebellious." Was her rebelliousness native or learned? "In school, it seemed like she always had a friend and I was pretty much on my own. I needed a male in my life more than Lynne did." Genes? Environment? A distorted mirror of the past? Anne Clark shrugs. "Those were the choices I made in my life," she says. "My choices." When it comes to sorting through the intricacies of environment, genes and free will, one could have no better guide than Victor McKusick, whose cluttered office at Johns Hopkins is down the hall from the bulletin board with the satirical chromosome map. At 76, McKusick is the father of medical genetics, a biological categorizer extraordinaire whose contribution to genetics recently won him the highest honor in American science, the 1997 Albert Lasker Award. McKusick, as an admirer noted recently, seems to have had his career mapped out "from the very instant he became unicellular." McKusick is not only a cloner of DNA, but also, a clone. He's an identical twin. A few years ago, McKusick was on a government ethics panel that met to discuss The Bell Curve. The book troubled many scientists, who felt it misused behavior genetics data in the interest of making a political point. McKusick signed a statement condemning the book, but in the preceding discussion he disagreed with panel members who wanted to say there was no genetic basis for intelligence. "That's like denying gravity," McKusick says. His own experience contributed to this conviction. Born on a Maine dairy farm in 1921, Victor and his twin, Vincent, both displayed remarkable brain power that enabled them to climb to the peaks of their respective professions. At 13, Victor was treated at Massachusetts General Hospital for an infected arm, and emerged several weeks later with the certainty that he would make his career in medicine. Vincent, who managed to stay out of the hospital as a kid, went on to Harvard Law School and eventually became chief justice of the Maine Supreme Court. In both men one senses a polite but adamantine intelligence and a genius for organization. They are similar in height and weight -- about six feet, 180 pounds; similar in their soft-spoken but intense speech patterns and mannerisms. There are the usual share of serendipities: On a recent summer vacation in Nova Scotia, Victor developed a dental abscess. His brother, sailing up from Maine to join him, had to make an emergency shore call after developing the same problem. Still, "there were differences in the two of us right from the beginning," says Victor. All through their childhood, Vincent usually took the lead, Victor usually followed. As to what accounts for that, Vincent defers to his brother, and Victor is too careful a scientist to venture a guess. Perhaps it had to do with the random movement of the neurons as they migrated to the brain stems of the weeks-old McKusick fetuses. Perhaps it was a "runting effect" -- the tendency of one identical twin to suck up womb nutrition to the detriment of the other. Or perhaps the differences in identical twins can be explained by the expectations of parents and siblings, desperate to find some way to tell the twins apart. In any case, "a geneticist is really as much interested, if not more interested, in the differences between identical twins as they are in the similarities," says McKusick. "The differences give you information on non-genetic factors." Behavior genetics -- concerned as it is with differences in sensitive matters such as intelligence, personality and sexuality -- is still a controversial field, even if its taint-by-association with eugenics has faded. Over the years, behavior geneticists have grown more careful and conscious of their biases. They are increasingly interested not so much in highlighting the power of biology as in studying the interaction of genes and environment -- the hyphen in the nature-nurture debate. They puzzle, for example, over issues like this: Since it's well known that there is a large genetic component to schizophrenia, why is it that only 50 percent -- and not all -- of the identical twins of schizophrenics are schizophrenics themselves? If they are clones, what makes them so different? On a recent afternoon in the auditorium of the Virginia Institute for Psychiatric and Behavioral Genetics, a research center at Virginia Commonwealth University in downtown Richmond, psychiatric geneticist Kenneth Kendler is training 30 new interviewers for a survey on stress and coping. In 1979, the same year Bouchard began his path-breaking work in Minnesota, researchers at VCU began assembling some 15,000 fraternal and identical twin pairs in the Virginia Twin Registry, one of the world's largest data banks of its kind. Kendler is co-director of the institute along with Lindon Eaves. For the current study, some 5,500 twins will be interviewed for several hours each about such dicey topics as religious convictions, family tragedies, drug use and mental illness. The researchers will measure the similarity of the answers each twin gives, then compare the degree of sameness in identical twins with that of fraternal twins. Using complex statistical models, they intend to assess the genetic component of different behaviors and environmental risk factors that can send one twin over the edge into clinical depression, for example, while the other copes. Critics of such studies contend that behavior geneticists confuse correlations with causation and underestimate the effects of environment. Plant grass seed in a desert and a meadow, these critics say, and the seed in one will thrive while the seed in the other perishes, whether or not the seeds are equally good. "Two twins who look like Christie Brinkley are going to have more similar life experiences than two who look like Roseanne," says Joseph L. Graves, a developmental biologist at Arizona State University West in Phoenix. Some critics also contend that the behavior geneticists bend statistics toward nature over nurture. "There's an old joke about statistics that says with two points I can draw you an elephant, with three points I can make it sing," says Andrew Futterman, a psychologist at Holy Cross College in Massachusetts. The practitioners of behavior genetics say their aim is to discern differences, not to prove an ideological point. In the current survey, Kendler and Carol Prescott, another geneticist, are trying to get at some of the vagaries of genetic and social stress underlying particular types of drug abuse. "Why does somebody use alcohol and somebody else marijuana, and somebody else cocaine?" asks Kendler, a wiry man with a long gray beard that he strokes absent-mindedly while speaking. "Is it a matter of economics? Of drug availability? Of very specific brain receptors for certain drugs?" One of the most surprising findings of behavior genetics has been that, statistically speaking, family environment plays no consistent role in determining personality or intelligence. Some family environments, in fact, tend to exaggerate the differences among siblings -- even identical twins -- rather than making them more the same. "In my more cavalier mode I would have said that all that stuff psychology and sociology are made of -- you know, parents influencing their kids -- doesn't amount to squat," says Eaves, a bluff Briton who speaks in the clotted burr of his native Birmingham. Upon further reflection, Eaves found it hard to characterize the boundary between genetics and environment. "If a parent is saying to the kid, For Christ's sake stop peeling the wallpaper and go outside and play,' you might find a correlation between parental shouting and conduct disorder. But where is the causality? Was it the parent yelling at the kid, or the kid's intrinsic need to peel wallpaper?" Kendler spells out the gene-environment interplay like this: A person with a predisposition to depression may be hard to get to know. When something bad happens to that person -- say, the death of a parent -- he or she probably won't have supportive people to turn to, and this social isolation deepens the depression. The lack of friends is a "genetically selected" environment, which ends up deepening the original genetic effect. The scientists in Richmond have shown that correlations for marijuana and cocaine use are nearly the same between fraternal and identical twins, suggesting that social environment is the main cause of drug experimentation. But the identical twin of a drug addict is far more likely to be an addict than is the fraternal twin of an addict. That suggests that genes play a major role when it comes to the biochemistry of drug dependence. In other words, "environment" may lead you to fool around with drugs, but genes hook you. Neurological studies point in the same direction -- to the idea that addicts' brains are wired differently. All the same, Kendler is wary of psychiatrists who disregard environment altogether. "Over the last 15 years we've suddenly switched to a biological emphasis, so that some of my colleagues now really doubt that stressful life events have anything to do with depression," Kendler says. "From our data, that's simply absurd. It's as bad as it used to be when people blamed mothers for everything." Some of this complexity is reflected in a substance-abuse study that Charlene Woodard, a clinical psychologist on Eaves's team, is conducting with a National Institute on Drug Abuse grant. Previous studies have shown that while African Americans tend to get into drug abuse later than whites, they end up suffering greater health consequences. Woodard's study, which reflects her experience as a counselor for drug addicts, incorporates factors such as a sense of racial identity, religiosity and perceived discrimination as correlates of drug abuse among African American adolescents. "There may be genetic contributions to alcoholism, but the protective and risk factors are different depending on cultural factors," she says. Woodard's hope is that once she finds attitudes that correlate with low drug abuse, the knowledge can be incorporated into prevention programs. An African American researcher like Woodard treads a delicate path in behavior genetics. After all, leading behavior geneticists such as Arthur Jensen and the late Hans Eysenck -- a German-born Briton with whom Eaves worked -- have been accused of racism for suggesting that genes accounted for the lower IQ scores of blacks (and the Irish). The Bell Curve drew heavily on the work of these scholars. "Using the term genetics in the same sentence with African American is very sensitive," says Woodard. "All I can do as a researcher is consciously recognize that there is some scientific value in answering the question, Why are we dying at an earlier age than whites? Why is our health worse?' " Adds Carol Prescott, "Traditionally, the What good is it?' of twin studies was to convince the people who thought everything in human behavior was nurture. I think we've come beyond that and maybe are swinging in the other direction, to everything being genetic." From her perspective, once scientists can determine that a genetic disposition puts someone at risk for a problem, it enables society to attack environmental causes that push that person over the edge. "If you look at the co-twin of someone who has a history of depression, what predicts whether or not they will get it? What might protect them? What increases the risk?" says Prescott. One of the Richmond group's studies is looking at identical twins in which one is an alcoholic and the other is not. The idea is to understand what differences in environment may have protected the sober twin. If we knew which individual genes were responsible for psychiatric ailments, environments presumably wouldn't matter as much. But molecular techniques have thus far failed to find the genetic causes for any complex mental illness or condition. "There's been lots of talk and lots of hope," says Kendler, "but no concrete widely replicated results." Lots of hope -- and lots of hype. One striking trend in molecular genetics at the moment is the frustration scientists are encountering in isolating genetic components of traits such as schizophrenia. After the identification of one-gene defects such as cystic fibrosis and Huntington's disease, scientists had expected to quickly identify the more complex genetic components of psychiatric conditions. Some widely publicized discoveries have been announced, but few have been reproduced. After the 1996 discovery of a "novelty seeking" gene was splashed on the front page of the New York Times, three follow-up studies failed to duplicate the finding. If there is such a gene, geneticist David Cox of Stanford prophesies, its overall contribution to personality is likely to be small. Which is why some scientists, like Evan Balaban of the Neurosciences Institute in San Diego, argue that hunts for behavioral genes are almost a waste of time. It's not that Balaban doesn't think behavior has biological roots -- he's a scientist who manipulates behavior for a living, transplanting brain cells from quails to chickens in such a way that makes the chickens sing like quails. Behavior originates in the brain -- he knows that -- but he also knows what else goes on in the brain: a random movement of particles that can have unpredictable results, particularly during fetal development, when the odd molecular bounce can cause a lifetime's worth of effects. "Labeling something genetic doesn't explain the process by which it happens," says Balaban. "You have to see how the genes interact. And there are so damn many and they interact in so many ways. As a science this is going to take us a long time to sort out." So many mysteries, and Lindon Eaves knows them as well as anyone. Not only does he oversee twin studies, he also once designed a computer program to test a biological application of chaos theory -- the theory that small changes can lead to enormous changes in complex systems. In his case, the complex system was a pair of twins, and by the time he finished running his program he had seen that a tiny alteration in the developing embryos of identical twins would end up making them different in half their characteristics by the time they were adults. As intriguing as that result was, though, Eaves has even bigger mysteries on his mind one Sunday evening as he heads not to his office, but to his church. On this night he is wearing the black shirt and collar of the Rev. Eaves, weekend Episcopalian minister, and he is preaching evensong at St. James's Episcopal Church in Richmond. The reading for the evensong sermon comes from the seventh chapter of Luke, in which Jesus's touch revives a dead man in the town of Nain. But those who have come to church this evening expecting a pious service from a minister instead get a sermon from a minister who is part scientist as well. God talk is mostly "mumbo-jumbo" and "gibberish," he thunders. Did the man in Nain really rise from the dead? "That's too far off the wall," he says. Yet there was a miracle, and it was this: Jesus reached out to the man in the coffin. "Jesus approached while others shrank away. Jesus touched the untouchable. He went into the space where the rule book ends, the place where science fails." Later, at a small gathering of students in a church meeting house, a plate of potluck stew balanced on his stomach, Eaves expands on the topic of genes and religion. "Science deals with humans in an essentially deterministic way," he says. "Whether we talk about genes or environment, we're talking about people being caused. And that's a little bit different than when we talk about ourselves as centered, acting, creating beings. How do we reconcile that with the idea that we're carrying around a code that when it unpacks has a strong impact on us? I mean, I have a paper out there that shows there may be some genetic effects on whether we go to church." But if going to church is just a biological imperative, an impulse selected by evolution, one student asks, "How do you explain Mother Teresa?" After all, Mother Teresa and her genes apparently sacrificed their future so that the genes of others might live. That didn't make sense from an evolutionary viewpoint, because if it were so, the genes for saintly sacrifice would never have survived. "The trouble with Mother Teresa -- I mean God love her," says Eaves, chewing a mouthful of food, "is that she may have been a mutant." "And what about Jesus?" asks the student. "Whoa! Whoa!" says Eaves. "Excellent question." Sherry Clark's answer: "I was very angry at God when my father died," she is saying one night, sitting in a kitchen in somebody else's Gaithersburg house where she's been a live-in housekeeper for a few months. She's talking not only about God and her dear departed father, but also her genes. "I felt betrayed," she says. "But eventually I realized that God isn't a murderer. People make certain choices in their lives and you get points taken off for being stupid. Father had a bad heart. Not a smoker or drinker but he didn't eat well. Part of it was genetic, and part of it was his emotions and attitudes. He used food for a purpose it wasn't intended to be used for, for comfort. He abused food. That's where I get it from, and my twin, too." Maybe yes, and maybe no. In fact, the only definitive truth about identical twins is that they're not identical at all. On the subject of nature and nurture, it turns out that Sharon and Sherry Clark disagree. "I would say 75 percent of personality is genetic and 25 percent is environment," says Sherry. "Right down the line." "What you are comes from your parents, other siblings and their perceptions of you," says Sharon. "The first three years of a child's life are so important in terms of whether they become secure or insecure." Whatever created the Clarks' separate natures -- nature, nurture or some blend -- it has held true through their lives so far. At age 16, Sharon won a statewide singing contest, and two years later she enrolled at the Berklee School of Music in Boston. Sherry, also a talented singer but not quite as gifted as her sister, went off with a friend to North Hollywood, Calif., where she worked the odd job and partied a lot. A year later they were back together, living with their mother at the family home on Dade Lane in Alexandria. Sherry was an international operator for AT&T for seven years. Later she went to culinary school and started housekeeping, saving money to start a catering business. In November she got fired from the Gaithersburg gig, just as she was about to quit. Sharon has held a series of retail jobs while attending the academy of the clubs -- listening to hundreds of hours of music at D.C.-area bars. In September, she finally cut the CD she hoped would set her career on fire. "Finally," it was called. Success led to more success: After a Blues Alley performance in November she met some Democratic Party officials, and got invited to sing at the White House in the spring. For all the good fortune, untimely death gnaws at the Clark family. The twins' oldest sister, Sylvia, succumbed to leukemia in 1980, leaving two sons, Curtis and Israel. Rejected by their father, the boys grew up essentially parentless. After a short life that involved several years in prison, Curtis Lamont Clark was slain over a drug deal on Mother's Day 1996, age 21. He was found in D.C. in a car, shot so full of holes that Sharon, who identified the body, barely recognized him. "He was a big guy, maybe 230 pounds, but he looked tiny in that coffin," she says. Maybe Curtis's life carries its own small point to the nature-nurture debate. Sharon thinks so. "He didn't get the adult love he needed to form himself without going to the streets to be accepted," she says. "And the street whipped him." And maybe the larger point was this: Nothing in the genetic blueprint puts you out on the street, a young man shot full of holes, just as nothing in the blueprint arranges for you to sing at the White House. Genes, in other words, aren't everything. Just ask any identical twin. Arthur Allen last wrote for the Magazine about medical ethics and the use of painkillers. CAPTION: Identical twins Sherry, left, and Sharon Clark have had distinctly non-identical lives. CAPTION: Psychologist Charlene Woodard and geneticists Kenneth Kendler and Carol Prescott are part of a Virginia Commonwealth University research team that tracks some 15,000 pairs of twins. CAPTION: Geneticists Lindon J. Eaves, above, of Virginia Commonwealth University, and Victor McKusick of Johns Hopkins Hospital study twins to answer the nature-nurture question.