Thinking about this recently, this seems to be a very odd way of expressing behaviour (especially in light of my recent reading of "figments of Reality" with its emphasis on co-evolution of culture and the mind).
For me to say that only 30% of my behaviour is based around my experiences seems to be ludicrous - after all I got dressed this morning, got some food out of the fridge, caught a bus to work, signed myself in and started programming a new system on the computer - taking occasional breaks to check LJ. All of these acts are learned - there's not a single thing there which would be instantly picked up by someone without background knowledge (what a fridge does, how buses work, computing skills, etc.).
The same is true for most of the things people do - and I've long been a firm believer in the ability of people to adapt to new situations, learn and overcome their limitations and mentally reprogram themselves to use more of their potential. So why do I still believe that nature has a large effect on people's lives?
It boils down to this - if we take the closest non-human animals and nurtured/raised them as people, how well would they manage? Half as well as people? One tenth as well? Not at all? I think the answer should be fairly obvious. now, I'm obviously going a bit over the top here - but the basic point is that the basic makeup of the brain is important in defining what can be done by that brain. No amount of training or experience will ever teach calculus to a brain that isn't equipped to deal with it. No matter how long you spend going over the background and teaching them the language, you're not going to get a monkey to appreciate Shakespeare.
But, the argument goes, people are all equally equipped to deal with whatever life throws at them - any differences between them are down to post-birth training. I'll put forward a few reasons why I think this is incorrect:
1) Going back to gestation - it's been shown that alcohol levels, low oxygen supply (caused by smoking) and diet can all affect mental ability in later life. As the environment of that gestation is just as important as the genes themselves in producing the foetus and the genetics of the mother will effect the workings of the womb, this will definitely have an effect. Some studies have shown that high levels of testosterone in the womb affect development in later life and this would definitely be the aegis of the mother's genetics.
2) The development of the brain from pre-human creatures has obviously involved various changes in our genetics. Our physical bodies have a variety of differences in them due to our differening genetic structures, it makes sense that our brains would be no different. To simplify - our brains are grown according to genetic designs, genes vary from person to person, therefore our brains will differ.
3) There are vast numbers of brain structures that are very similar across the whole human race - down to very small details. If these areas were formed based purely by the effects of the outside world on a homogenous mass, you'd expect these areas to differ very widely as our early experiences vary.
4) There are a variety of genetic defects that cause syndromes that affect how well people's brains work. These seem to affect how well the aforementioned brain areas work. Autism seems to be one of these, affecting (for instance) how the Amygdala works in mediating emotional response.
5) The vast majority of the structures in our brains are the same as those in other mammals and we can see that some of them are the same in reptiles. As reptiles (and to a lesser extent mammals) do have a lot of their behaviour pre-programmed it would seem very odd if that behaviour wasn't also extant in people.
So, having outlined my reasons for thinking there is a strong genetic component and having said that there’s also a strong cultural component, how can I make any statement about the amount of a person’s personality that each makes up?
I think it comes down to different levels of personality (what Leary called Circuits) – the first few levels are largely instinctual, and these levels seem to refer to intrinsic parts of the brain. Here are where our similarities are hard-coded because they are (nearly) universally found to be useful – fight/flight reflexes, hierarchical thinking, reactions towards human young (and other mammal young too), anger, fear, love, hate, smiling, mathematics, spatial reasoning.
However, we also have higher circuits (higher, in this case, meaning more recent, not better). These ones are more flexible and allow for more feedback – effectively they learn. At the top of this hierarchy are the frontal lobes where abstraction and logic lie. This is what truly differentiates our minds from those of other animals (although some other primates do appear to be able to conceptualise simply) – we have these higher levels that observe the lower ones and provide feedback to them.