The link between crime and genetics is so well-documented these days that even an undergraduate can write a term paper on it and cite scientific references. So I won't try to duplicate those efforts. But I just want to correct the misconception that "both genes and environment play a role in the criminality of the individual". It's not always true.
First, what is the environment? Your parents share your genes, so being raised in a violent family situation can't really be considered "environment". Why? Because the way your parents nurture you depends on how you ask to be raised. If you have a genetic condition like Conduct Disorder or Oppositional Defiance Disorder, it will put stress on your parents. Since they share your genes (or more specifically, your gene flavors), chances are greater that they'll also have a short temper and lack of impulse control, and so may respond with uncontrolled violence. So the shared genes a child has with his parents are responsible for both the incessant provocation, and the violent response. Family environment is thus genetic.
Second, psychologists often say things like "Families with poor communication and weak family bonds have been shown to have a correlation with children's development of aggressive/criminal behavior". Or "children are at a fifty percent greater risk of engaging in criminal acts, if they were neglected or abused". But what's the real cause? If you're born with Attention Deficit Hyperactivity Disorder (ADHD), that later develops into Antisocial Personality Disorder (ASPD), of course you're going to be a bad parent -- remote and distant and uncaring. Since the parent shares the same genes as the child, poor parental nurturing can't really be considered "environment" at all; it's genes reacting to genes.
So why can't we intervene, and teach a child how to control his aggressive behaviors? Why is it so certain that genes will deterministically lead to the effect? It's the same reason that you can predict with 100% certainty that a loaded mousetrap in a room full of mice will get sprung. The world is full of chances to respond, but it only takes one time, and you can't completely control the environment. If you have a high genetic tendency to react aggressively, even the most peaceful environment will set you off once in a while, and you'll kill someone or impulsively steal their belongings.
January 01, 2009 in Nature vs Nurture