There's a lot of research (similar to the gay sheep thingy) which points to differences in brain physiology which distinguish addicts and sociopaths and violent people from the normal population.
So you may say "oh they are born criminals."
But there are other studies which show that neglect and abuse of infants and children can cause measurable physical changes in the structure of the brain. Likewise, positive stimulation of infants can enhance structures in the brain. None of this is completely proven, but the number of studies which show effects like this is increasing all the time.
So it seems that maybe "nurture" can alter "nature", at least in children. Stimulation during early development can
But it's also known that the brain's physical development pretty much stops after childhood. After that the only thing that can change it greatly is drugs.
So I would say that if someone is born with bad "nature", they can probably be helped by "nurture" as a child. But beyond a certain point the concrete is set, and "nurture" will have a very low percentage of success.
PS There's a related (at least to the "hardened concrete" notion) issue with ADHD and medication. In ADHD children, medication with stimulants (what is sold on the street as speed, aka amphetamines) causes them to calm down. They don't get the normal "speedy" effect, and they frequently get a great functional improvement from the drug. But if you have an untreated ADHD person who reaches adulthood, they have a high probability of becoming an alcoholic or other type of addict. And if you try to treat them with the medication used for children, it will often kickstart them into full-blown addiction. This is just anectdotal stuff from a doctor with a lot of experience, but it makes you think.
PPS There is a famous case study (they made a PBS special out of it) of a little girl who was locked in a closet for the first few years of her life. The severe neglect (she did not suffer a head injury) resulted in her being retarded and unable to learn language. She wasn't just developmentally delayed, she was developmentally broken.