One of the subjects of my joint honours BSc was in biology (with a focus on genetics), and my first MSc was in anthropology of oppression, and my second MSc was in social policy and mental health care. One of the things that's most crucial to understand when studying any kind of science - and this is why I don't believe that Polly has ever studied genetics - is that science is never neutral, and is always prone to political and other forms of bias. Actual scientists, even science students, are taught from day one of the dangers of applying scientific research to ideology.
Which is to say, all the existing body of research into genetic traits, epigenetics, concepts such as "is there a gene for criminality", nature vs nurture; all this research is very much in its infancy, and has limited application to politics or real world social policy.
Genetics and other concepts from the biological sciences have been abused all throughout modern scientific history to oppress people. For example, there was a very famous case in the 1800s where a scientist illegally collected hundreds of skulls and measured them, and provided "scientific proof" that black people had smaller crania than white people, and were therefore scientifically less intelligent and more brutish. During slavery, there was a great deal of "scientific research" done with the intention of providing scientific proof that black people are inherently inferior and thus need to be controlled by white people: science with a clear agenda to support and justify slavery.
in my first term as an undergrad, my tutor assigned the book called 'The Bell Curve.' This is a non-peer reviewed "scientific" work exploring the alleged link between race and intelligence, which was extremely controversial and widely accused of being racist. My tutor taught this book in the context of how both biology and anthropology have historically been used to justify oppression of minorities. Yet it was a published "science" book and to a lot of people, that makes it the Gospel.
(I actually wrote my MSc thesis on how scientific research has historically been used to justify oppression of women - think things like "hysteria", wandering wombs, etc. Mumsnet tends to be very pro-feminist, and I wonder how different the response would be if posters were using "science" to demonise women, rather than demonising people from low socioeconomic backgrounds.)
The research on the nature versus nurture debate is still in infancy, anyone confidently stating that everything is genetic, that there's a gene for being a child abuser, that babies born to abusive parents are doomed even if they're removed from their abusive homes in infancy and adopted by loving families because they've already inherited the child abuser gene - I'm sorry but there just is no substantial body of peer-reviewed evidence to be able to state any of that as fact, and anyone claiming otherwise either has an agenda, or has never taken a genetics class. Anyone with even a small amount of genetic knowledge knows that genetics is highly complex; there are rarely single genes that govern behaviour but many different genes working and influencing each other. The punnet square genetics we all learned at GCSE level is highly over-simplified; the reality is that even something as seemingly straight forward as eye colour is controlled by many different genes.
I've read a few peer-reviewed journal articles on the CDH13 gene (which controls neural connectivity) and the MAOA gene (which is related to dopamine) - these are the two genes proudly declared by the tabloid press to be the "crime genes." In reality, dopamine production is very very complex and is linked with Tourette's Syndrome and schizophrenia, as well as conditions like Parkinsons and tic disorders, to name but a few. That's why L-Dopa was such an astonishing discovery. (And dopamine production is linked with physical injury or disfunction of the basal ganglia; the relationship between genetics and neuroanatomy is not yet all that well understood.)
Even if we had solid evidence that there was a gene for child abuse (which we don't, and no actual geneticist would ever claim otherwise), it's clearly dangerous to base social policy on lab work. One of Polly's posts upthread, for example, suggests that babies born to abusive homes are more likely to grow up to be abusers since they have the gene for child abuse, regardless of whether they are removed from the abusive home in infancy or not. That actually isn't true and there's no statistically evidence to support it - there is no large scale study of babies removed from abusive homes in infancy which studied those babies into adulthood. That's a very worrying application of poorly understood junk science, since it would be very easy to twist that into "well those babies are doomed because they have bad genes anyway, so why bother to remove them from abusive homes" and once again, exploit science as a way to justify oppression.
Please, please go to PubMed or JStor and actually read peer reviewed studies yourself if you're interested, don't just blindly believe everything you read online.
You can perhaps start here: