On December 14, 2012, 20-year-old Adam Lanza killed 20 children at a Connecticut elementary school, as well as 6 school staffers, his mother, and himself. Within two weeks, the Connecticut Medical Examiner commissioned a group of geneticists to screen Lanza’s DNA.

And for what, exactly? Who knows. There are any number of genetic variants the scientists could zero in on — variants that have been linked to a propensity for violence, aggression, psychopathy, or psychiatric disorders. One thing I’d bet on: The screen will find something. Each of us carries genetic mutations somewhere along our 3-billion-letter DNA code. Some mutations are benign, some are not; some have huge effects, others tiny. But there’s no way to know how (or whether)