I blame my childhood friends

Forget home and school. Look on the streets - Magnus Linklater
Both (Blair and Cameron) agree on one thing: all trouble starts with the family. The more children grow up in a stable environment, the more they are likely to turn into model citizens. Restore family life and you restore the well-adjusted child. But what if that theory is wrong? What if the family has nothing to do with the way the young turn out, and the influences, for bad or good, lie elsewhere? What, in short, if the nurture part of the nature/nurture debate has been looking in the wrong place all this time?That is the subversive argument that Judith Rich Harris, the American psychologist, has been pursuing for the past ten years or so, and which she has built into two books, The Nurture Assumption and No Two Alike. The theory she advances is that what influences behaviour is not so much the home or the family, or even the genetic make-up of a child, but the peer group in which they grow up. The survival instinct, which teaches the young either to conform with their contemporaries or to become their leader, kicks in early on and can result in huge variations in behaviour. One child may turn into a model conformist, while another, brought up in the same household, becomes a tearaway. To explain why, you have to look outside the family not inside it.
Harris has an article in the latest Prospect:
JRH: The surprising result had to do with the environment. Since genetic effects account for only about half of the differences among us, the other half has to be the result of environmental effects, right? Well, that was the assumption. But researchers still haven't been able to pin down which aspects of the environment are important. All they've been able to determine is which aspects of the environment are not important. The aspects of the environment that don't seem to matter are all those that are shared by all the children who grow up in a given family—which includes most of the things the word "home" makes you think of. Whether the home is headed by one parent or two, whether the parents are happily married or constantly rowing, whether they believe in pushing their children to succeed or leaving them to find their own way in life, whether the home is filled with books or sports equipment, whether it is orderly or messy, a city flat or a farmhouse—the research shows, counterintuitively, that none of these things makes much difference. The child who grows up in the orderly, well-run home is, on average, no more conscientious as an adult than the one who grows up in the messy one. Or rather, he or she will be more conscientious only to the extent that this characteristic is inherited.
There was a short 'nature/nurture discussion in relation to adoption on Woman's Hour earlier this week where Professor Steve Jones was rather dismissive of some of the claims for genetic influence on behaviour. He clarified some of the confusion over genes versus environment with this telling observation:

'If everbody smoked, lung cancer would be a genetic disease'