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Whenever we want to find a genetic implication for ANYTHING (whether it's criminality, or intelligence), we look at separate people who have the exact same genes.
We look at twins.
Identical twins are believed to share the exact same genes.
Fraternal twins share about 50% of the same genes.
Let's pretend that a violent criminal has a twin.
If we see that the violent criminal's twin ALSO has a history of violent crime,
then we can safely assume that genes play at least SOME kind of role in criminality.
And it happens to be true.
Psychologists and geneticists in Denmark discovered that if a violent criminal had a twin,
then that twin was much more likely to have had a criminal history, compared to other people.
It's not true that if one twin is a criminal, then the other twin will DEFINITELY also be a criminal.
It's only more LIKELY that they will also be a criminal.
Sometimes, the other twin is also a criminal.
Sometimes, they're not.
What this means is that genetics plays role in criminality,
but genetics isn't the ONLY thing that determines whether or not someone will turn out to be a true criminal.