Twins Focus the Nature vs. Nurture Mystery

Can "bad genes" destine people to violence and criminality? As a mystery fan, I've never liked that kind of characterization because I want to unravel not only the puzzle of who-done-it but also the why. A genetically predetermined monster just isn't as interesting in terms of motive and plotting. Plus, the "bad genes" theory tends to drift into offensive racial, ethnic and social stereotypes. And, finally, I prefer mystery solutions that deliver justice, and that means supposing free will rather than genetic determinism. Last year, a Boston Globe article by two associate professors of criminal justice approached the controversy head-on, arguing that a genetic basis for crime has not been adequately explored both because scientists do not have the practical or ethical ability to perform randomized controlled trials and because political headwinds cause them to avoid the issue. In the meantime, the best crime-relevant data untangling heritable personality traits (such as aggression) from environmental factors (such as bad parenting) come from identical twin studies. Overall, research finds about 50% of personality traits are heritable and 50% due to environmental factors--so no simple behavior answers from the twins. Perhaps the mystery of genetics-vs.-environment is why twins are an old theme in literature, often using a good one vs. evil one trope. For the curious, here's some mystery fiction with a twin twist: The Thirteenth Tale by Diane Setterfield, where the gothic history of a mysterious family includes the deeply bonded identical twins Adeline (the violent one) and Emmeline (the calm one); John Hart's The Last Child about a 13-year-old boy who embarks on a dangerous quest for his missing twin sister, with help and hindrance from a local detective; The Nightspinners by Lucretia Grindle, in which a teenage girl with a telepathic connection to her brutally murdered twin sister faces an unknown killer; and Lives of the Twins by Rosamond Smith (aka Joyce Carol Oates), the tale of a shallow woman who takes a dangerous path by initiating affairs with identical twin brothers, one gentle and one sadistic, who are estranged by a terrible secret. For more twin-themed books, see https://www.abebooks.com/books/identical-evil-siamese-lamb-niffenegger/twins-in-literature.shtml